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Automated, Low-Power Chamber System for Measuring Nitrous Oxide Emissions
Abstract
Continuous measurement of soil NO emissions is needed to constrain NO budget and emission factors. Here, we describe the performance of a low-power Teledyne NO analyzer and automated chamber system, powered by wind and solar, that can continuously measure soil NO emissions. Laboratory testing of the analyzer revealed significant temperature sensitivity, causing zero drift of -10.6 nmol mol °C. However, temperature-induced span drift was negligible, so the associated error in flux measurement for a typical chamber sampling period was on the order of 0.016 nmol m s. The 1-Hz precision of the analyzer over a 10-min averaging interval, after wavelet decomposition, was 1.5 nmol mol, equal to that of a tunable diode laser NO analyzer. The solar/wind hybrid power system performed well during summer, but system failures increased in frequency in spring and fall, usually at night. Although increased battery storage capacity would decrease down time, supplemental power from additional sources may be needed to continuously run the system during spring and fall. The hourly flux data were numerically subsampled at weekly intervals to assess the accuracy of integrated estimates derived from manually sampling static chambers. Weekly sampling was simulated for each of the five weekdays and for various times during each day. For each weekday, the cumulative N emissions estimate using only morning measurements was similar (within 15%) to the estimate using only afternoon measurements. Often, weekly sampling partially or completely missed large episodic NO emissions that continuous automated chamber measurements captured, causing weekly measurements to underestimate cumulative N emissions for 9 of the 10 sampling scenarios.
... Crops were harvested at the end of September by cutting the stover five inches above the soil. Hourly N 2 O fluxes (mg N m −2 h −1 ) and CO 2 fluxes (g C m −2 h −1 ) were measured using non-steady-state flux chambers with a CO 2 analyzer (LI-10820 for 2016 and LI-7000 for 2017 and 2018, LI-COR Biosciences, Lincoln, NE) and a N 2 O analyzer (Teledyne M320EU, Teledyne Technologies International Corp, Thousand Oaks, CA) (a detail method can be retrieved from Fassbinder et al., 2012Fassbinder et al., , 2013. We also collected soil moisture at 15 cm depth (VWC as abbreviation of volumetric water content, m 3 m −3 ), weekly 0-15 cm depth soil NO 3 − + NO 2 − concentration (NO 3 − for short in the following text, g N Mg −1 ), soil NH 4 + concentration (NH 4 + , g N Mg −1 ) and related environment variables including air temperature, radiation, humidity, and soil and crop properties from three growing seasons during 2016-2018 and six mesocosm chambers ( Fig. S1 in the Supplement). ...
... We also collected soil moisture at 15 cm depth (VWC as abbreviation of volumetric water content, m 3 m −3 ), weekly 0-15 cm depth soil NO 3 − + NO 2 − concentration (NO 3 − for short in the following text, g N Mg −1 ), soil NH 4 + concentration (NH 4 + , g N Mg −1 ) and related environment variables including air temperature, radiation, humidity, and soil and crop properties from three growing seasons during 2016-2018 and six mesocosm chambers ( Fig. S1 in the Supplement). The magnitude of N 2 O flux and NO 3 − soil concentration and their responses following fertilizer application from this mesocosm experiment are slightly higher than several field studies of agricultural soils (Fassbinder et al., 2013;Grant et al., 1999Grant et al., , 2006Grant et al., , 2008Grant et al., , 2016Hamrani et al., 2020;Venterea et al., 2011). More details about the mesocosm facility and experimental design can be found in the thesis of Miller (2021). ...
... First, the KGML-ag models in this study are limited by the available observed data. The mesocosm measurements of N 2 O fluxes (16.9 ± 11.7 mg N m −2 d −1 during days 45-60; Highest value is 71 mg N m −2 d −1 ) and NO 3 − soil concentrations (59.3 ± 20.7 g N Mg −1 during days 45-60; Highest value is 95.2 g N Mg −1 ) are at the high end of the range that has been observed by field studies (Fassbinder et al., 2013;Grant et al., 1999Grant et al., , 2006Grant et al., , 2008Grant et al., , 2016Hamrani et al., 2020;Venterea et al., 2011). Some IMVs with high feature importance scores (e.g., O 2 flux, N 2 flux) or at different depths (e.g., soil NO 3 − at 5 cm depth, VWC at 5 cm depth), and data out of growing seasons are not included. ...
Agricultural nitrous oxide (N2O) emission accounts for a non-trivial
fraction of global greenhouse gas (GHG) budget. To date, estimating
N2O fluxes from cropland remains a challenging task because the related
microbial processes (e.g., nitrification and denitrification) are controlled
by complex interactions among climate, soil, plant and human activities.
Existing approaches such as process-based (PB) models have well-known
limitations due to insufficient representations of the processes or
uncertainties of model parameters, and due to leverage recent advances in
machine learning (ML) a new method is needed to unlock the “black box” to
overcome its limitations such as low interpretability, out-of-sample failure
and massive data demand. In this study, we developed a first-of-its-kind
knowledge-guided machine learning model for agroecosystems (KGML-ag) by
incorporating biogeophysical and chemical domain knowledge from an advanced PB
model, ecosys, and tested it by comparing simulating daily N2O fluxes with
real observed data from mesocosm experiments. The gated recurrent unit (GRU)
was used as the basis to build the model structure. To optimize the model
performance, we have investigated a range of ideas, including (1) using
initial values of intermediate variables (IMVs) instead of time series as
model input to reduce data demand; (2) building hierarchical structures to
explicitly estimate IMVs for further N2O prediction; (3) using multi-task
learning to balance the simultaneous training on multiple variables; and (4)
pre-training with millions of synthetic data generated from ecosys and fine-tuning
with mesocosm observations. Six other pure ML models were developed using
the same mesocosm data to serve as the benchmark for the KGML-ag model.
Results show that KGML-ag did an excellent job in reproducing the mesocosm
N2O fluxes (overall r2=0.81, and RMSE=3.6 mgNm-2d-1
from cross validation). Importantly, KGML-ag always outperforms
the PB model and ML models in predicting N2O fluxes, especially for
complex temporal dynamics and emission peaks. Besides, KGML-ag goes beyond
the pure ML models by providing more interpretable predictions as well as
pinpointing desired new knowledge and data to further empower the current
KGML-ag. We believe the KGML-ag development in this study will stimulate a
new body of research on interpretable ML for biogeochemistry and other
related geoscience processes.
... Chambers have been paired with analyzers to measure other trace gases, including N 2 O and CH 4 , by utilizing methods such as gas chromatography (GC), photoacoustic infrared detection, tunable diode laser (TDL), or cavity ring-down laser spectroscopy (Ambus and Robertson, 1998;Breuer et al., 2000;Courtois et al., 2019;Papen and Butterbach-Bahl, 1999;Pihlatie et al., 2005). Fassbinder et al. (2013) provide a detailed summary of the advantages and limitations of commonly used analyzers that we briefly summarize here. GC systems with electron capture detectors (ECDs) have often been used to measure N 2 O from automated chambers (Breuer et al., 2000;Papen and Butterbach-Bahl, 1999). ...
... Interference from water vapor and other gases potentially limits the use of photoacoustic analyzers in the field (Rosenstock et al., 2013). Laser-based analytical approaches are capable of rapid (e.g., 10 Hz) and precise N 2 O measurements, but these analyzers are considerably more expensive (> USD 70 000) and often have relatively high power requirements for autonomous field deployment (Fassbinder et al., 2013;Pihlatie et al., 2005). We sought to implement a lower-cost, solar-powered, soil gas flux measurement system capable of operating unattended in a harsh field environment and where analyzers could feasibly be replaced if stolen or damaged. ...
... We sought to implement a lower-cost, solar-powered, soil gas flux measurement system capable of operating unattended in a harsh field environment and where analyzers could feasibly be replaced if stolen or damaged. For these reasons, we utilized a gas filter correlation (GFC) infrared N 2 O analyzer in our study (∼ USD 16 000), similar to that described previously by Fassbinder et al. (2013), along with an infrared gas analyzer for CO 2 and H 2 O measurement (∼ USD 4000). However, other analyzers could be readily employed with the chamber and manifold system described below. ...
Soils play an important role in Earth's climate system through their regulation of trace greenhouse gases. Despite decades of soil gas flux measurements using manual chamber methods, limited temporal coverage has led to high uncertainty in flux magnitude and variability, particularly during peak emission events. Automated chamber measurement systems can collect high-frequency (subdaily) measurements across various spatial scales but may be prohibitively expensive or incompatible with field conditions. Here we describe the construction and operational details for a robust, relatively inexpensive, and adaptable automated dynamic (steady-state) chamber measurement system modified from previously published methods, using relatively low cost analyzers to measure nitrous oxide (N2O) and carbon dioxide (CO2). The system was robust to intermittent flooding of chambers, long tubing runs (>100 m), and operational temperature extremes (−12 to 39 ∘C) and was entirely powered by solar energy. Using data collected between 2017 and 2019 we tested the underlying principles of chamber operation and examined N2O diel variation and rain-pulse timing that would be difficult to characterize using infrequent manual measurements. Stable steady-state flux dynamics were achieved during 29 min chamber closure periods at a relatively low flow rate (2 L min−1). Instrument performance and calculated fluxes were minimally impacted by variation in air temperature and water vapor. Measurements between 08:00 and 12:00 LT were closest to the daily mean N2O and CO2 emission. Afternoon fluxes (12:00–16:00 LT) were 28 % higher than the daily mean for N2O (4.04 vs. 3.15 nmol m−2 s−1) and were 22 % higher for CO2 (4.38 vs. 3.60 µmolm-2s-1). High rates of N2O emission are frequently observed after precipitation. Following four discrete rainfall events, we found a 12–26 h delay before peak N2O flux, which would be difficult to capture with manual measurements. Our observation of substantial and variable diel trends and rapid but variable onset of high N2O emissions following rainfall supports the need for high-frequency measurements.
... The ThermoScientific 46i (ThermoFisher Scientific) and the Teledyne models M320EU2 (Teledyne Instruments API) analyzers use GFC technology. For the Tedelyne M320EU2, it was used with an automatic chamber system developed by the University of Minnesota (Fassbinder et al. 2013). For the Thermo Scientific 46i analyzer, it was previously used by different French INRA laboratories outfitting with same auto-chamber system (Bessou et al. 2010;Laville et al. 2011;Vermue et al. 2013;Klumpp et al. 2011). ...
... For the Thermo Scientific 46i analyzer, it was previously used by different French INRA laboratories outfitting with same auto-chamber system (Bessou et al. 2010;Laville et al. 2011;Vermue et al. 2013;Klumpp et al. 2011). Laboratory testing with the Tedelyne carried out by Fassbinder et al. (2013) revealed significant temperature sensitivity causing zero drift but negligible for the span drift. This zero drift was also observed with the Thermo-Scientific 46i analyzer during the IPNOA experiment and seems to be therefore common to the two GFC methodologies. ...
... This good MDF was achieved thanks to a very sensitivity GC gas analysis with a mean precision of about ± 1.4 ppb. For the model M320EU2 of Teledyne using same Gas Filter Correlation methodology as the Thermo-Scientific 46i, Fassbinder et al. (2013) found a similar detection threshold that of the IPNOA device, with a noise for the analyzer of 7.5 ppb (for a 15 min of integrated time) and a MDF of 4.5 ng N m −2 s −1 (with a chamber height of 0.4 m). Thus, resolution in the gas analysis is a very determining factor in particular to conclude on the validity of negative fluxes corresponding to a net N 2 O uptake by soils (Neftel et al. 2007). ...
The assessment of nitrous oxide (N2O) fluxes from agricultural soil surfaces still poses a major challenge to the scientific community. The evaluations of integrated soil fluxes of N2O are difficult owing to their lower emissions when compared with CO2. These emissions are also sporadic as environmental conditions act as a limiting factor. A station prototype was developed to integrate annual N2O and CO2 emissions using an automatic chamber technique and infrared spectrometers within the LIFE project (IPNOA: LIFE11 ENV/IT/00032). It was installed from June 2014 to October 2015 in an experimental maize field in Tuscany. The detection limits for the fluxes were evaluated up to 1.6 ng N-N2O m² s⁻¹ and 0.3 μg C-CO2 m² s⁻¹. A cross-comparison carried out in September 2015 with the “mobile IPNOA prototype”; a high-sensibility transportable instrument already validated provided evidence of very similar values and highlighted flux assessment limitations according to the gas analyzers used. The permanent monitoring device showed that temporal distribution of N2O fluxes can be very large and discontinuous over short periods of less than 10 days and that N2O fluxes were below the detection limit of the instrumentation during approximately 70% of the measurement time. The N2O emission factors were estimated to 1.9% in 2014 and 1.7% in 2015, within the range of IPCC assessments.
... Automated systems include robotic chambers, designed to collect air samples in glass vials for chromatographic analysis on site or in laboratory [3,23,30,31,32], as well as a vast array of continuous flow-through systems based on the spectroscopic N 2 O and CO 2 detection techniques [1,8,16]. Versatility of N 2 O flow-through analyzers due to increased number of N 2 O data points, commonly measured at the frequency of 1 s versus approximately 90s for onsite gas chromatography (GC) systems, as well as their reduced calibration requirements compared to GC systems resulted in their wider acceptance for in situ measurements. ...
... Despite conducting the calibration and warm-up procedures, and placing the analyzer in the air-conditioned instrumentation trailer with internal temperature at 18-25°C, we observed a drift of ±0.5 ppm, which occurred over the course of several hours, and was likely the result of using the Teledyne T320 in the field environment. Previous research utilized a more precise model of the Teledyne N 2 O analyzer (0.05 ppm N 2 O detection limit compared to 0.20 ppm detection limit for T320) and reported a temperature-induced drift [8], however our observed drift was 4 times higher than the values reported by Fassbinder et al. [8]. Regression analysis for effects of airflow moisture and temperature on N 2 O levels measured by the Teledyne T320 over the course of 5 days during the times when dry warm weather was followed by additions of water, demonstrated a strong negative effect of moisture (R 2 = 0.76) on the N 2 O readings (Fig. 4, A). ...
... Despite conducting the calibration and warm-up procedures, and placing the analyzer in the air-conditioned instrumentation trailer with internal temperature at 18-25°C, we observed a drift of ±0.5 ppm, which occurred over the course of several hours, and was likely the result of using the Teledyne T320 in the field environment. Previous research utilized a more precise model of the Teledyne N 2 O analyzer (0.05 ppm N 2 O detection limit compared to 0.20 ppm detection limit for T320) and reported a temperature-induced drift [8], however our observed drift was 4 times higher than the values reported by Fassbinder et al. [8]. Regression analysis for effects of airflow moisture and temperature on N 2 O levels measured by the Teledyne T320 over the course of 5 days during the times when dry warm weather was followed by additions of water, demonstrated a strong negative effect of moisture (R 2 = 0.76) on the N 2 O readings (Fig. 4, A). ...
Continuous flow through chamber systems are recognized for their superiority in obtaining high resolution seasonal greenhouse gas (GHG) emissions data. In the current study, we combine Li-Cor 8100A CO2 flux system with the infrared N2O analyzer (Teledyne API, San Diego, CA) and a laser spectroscopic analyzer (Los Gatos Research Inc, Mountain View, CA) to design a robust setup for reliable concurrent measurements of N2O and CO2 emissions from automatic chambers and to obtain flux gradient N2O data from agricultural land. Field testing revealed considerable interference of moisture and temperature with the infrared N2O analyzer data, which showed a negative drift of up to 0.5ppm at increased humidity (R²=0.76). Addition of desiccant column on line in an attempt to improve infrared analyzer N2O readings due to removal of moisture affected the N2O and CO2 baselines, likely due to flow modulation by the Li-Cor 8100A air pump. High precision output from the laser spectroscopic analyzer and stability of its baseline at variable temperature and moisture were preferable for combined automatic chamber and flux tower measurements. The total N2O produced at the site fertilized with 100 kg N ha⁻¹ was 1.8±0.7 kg N2O-N ha⁻¹, and the total CO2 emissions were 4350±1173 kg CO2-C ha⁻¹ during the measurement period in the fall and spring. The high frequency of sampling and low labor intensity of the N2O and CO2 measurement setup allows for comparison of GHG emissions from short term events and across seasons, and establishes the basis for better understanding of emissions sources in agricultural systems.
... Measurements were taken at approximately weekly intervals for 42d immediately after fertilization (DOY 125). Previous experiments in this field indicate that N 2 O fluxes are highest in the 20 to 50-d following fertilization Fassbinder et al., 2013). Beyond this time frame, N 2 O fluxes decline Turner et al., 2016a) and as a result, the cumulative emission budget is most sensitive to loss during this brief period. ...
... These hourly mean flux density observations are comparable in strength to a previous study that used 6 automated chambers to estimate the annual N 2 O budget at this field site during the 2010 corn phase (Fassbinder et al., 2013). Those investigators determined that N 2 O emissions were elevated for 20 to 50 days after fertilization, but then losses declined precipitously and the average hourly standard deviation fell 14-fold, suggesting relatively low temporal measurement uncertainty beyond this brief period (Fassbinder et al., 2013). ...
... These hourly mean flux density observations are comparable in strength to a previous study that used 6 automated chambers to estimate the annual N 2 O budget at this field site during the 2010 corn phase (Fassbinder et al., 2013). Those investigators determined that N 2 O emissions were elevated for 20 to 50 days after fertilization, but then losses declined precipitously and the average hourly standard deviation fell 14-fold, suggesting relatively low temporal measurement uncertainty beyond this brief period (Fassbinder et al., 2013). ...
... Agricultural N 2 O emissions arise from direct emissions from fertilized soils, and through two indirect pathways: (i) from the deposition of NH 3 and NO x volatilized from synthetic fertilizer and manure; and (ii) from the leaching and runoff of fertilizer and manure N, mainly as nitrate (NO 3 -). Previous studies from various agricultural fields within Minnesota indicate that direct N 2 O emission is about 1.3% of applied synthetic N and in excellent agreement with the IPCC direct emission factor [Fassbinder et al., 2013;Griffis et al., 2013]. A recent meta-analysis has shown that this emission factor is well constrained but increases nonlinearly as N addition exceeds crop demand [Shcherbak et al., 2014]. ...
... [2013] and Fassbinder et al. [2013]. Using an independent approach, our analyses indicate that the both direct and indirect emissions are underestimated in the IPCC inventories, and provide further support that the large disparity between top-down and bottom-up methodologies is likely due to poor constraints on indirect emissions. ...
... where Γ is the direct/indirect scaling factors. Short-term heavy precipitation is also an important factor contributing to wetter and anaerobic soil conditions and heavier surface runoff, and therefore larger direct and indirect emissions [Fassbinder et al., 2013] 3. Inverse modeling analyses support that the indirect emission factor associated with runoff and leaching ranges from 0.014 to 0.035 for the US Corn Belt. This represents an upward adjustment of 1.9-to 4.6-fold relative to the Intergovernmental Panel on Climate Change and is in excellent agreement with recent bottom-up field studies. ...
Nitrous oxide (N2O) emissions within the US Corn Belt have been previously estimated to be 200-900% larger than predictions from emission inventories, implying that one or more source categories in bottom-up approaches are underestimated. Here we interpret hourly N2O concentrations measured during 2010 and 2011 at a tall tower using a time-inverted transport model and a scale factor Bayesian inverse method to simultaneously constrain direct and indirect agricultural emissions. The optimization revealed that both agricultural source categories were underestimated by the Intergovernmental Panel on Climate Change (IPCC) inventory approach. However, the magnitude of the discrepancies differed substantially, ranging from 42–58% and 200–525% for direct and indirect components, respectively. Optimized agricultural N2O budgets for the Corn Belt were 319 ± 184 (total), 188 ± 66 (direct), and 131 ± 118 Gg-N yr-1 (indirect) in 2010, versus 471 ± 326, 198 ± 80, and 273 ± 246 Gg-N yr-1 in 2011. We attribute the inter-annual differences to varying moisture conditions, with increased precipitation in 2011 amplifying emissions. We found that indirect emissions represented 41–58% of the total agricultural budget, a considerably larger portion than the 25–30% predicted in bottom-up inventories, further highlighting the need for improved constraints on this source category. These findings further support the hypothesis that indirect emissions are presently underestimated in bottom-up inventories. Based on our results, we suggest an indirect emission factor for runoff and leaching ranging from 0.014–0.035 for the Corn Belt, which represents an upward adjustment of 1.9–4.6 times relative to the IPCC and is in agreement with recent bottom-up field studies.
... This type of equilibrator has a fast response time [Santos et al., 2012;Yoon et al., 2016] and has been used in oceanic N 2 O campaigns [Bange et al., 1996;Arévalo-Martínez et al., 2013O'Reilly et al., 2015] but has never been used to measure N 2 O in freshwater systems. Equilibrator headspace air was drawn through a desiccant tube before entering a Teledyne gas filter correlation N 2 O analyzer (Model M320EU2; Teledyne Instruments, City of Industry, CA, USA) [Fassbinder et al., 2013;Turner et al., 2015]. A circular loop was created by returning sample air back to the equilibrator device. ...
... The sensor responsivity of the N 2 O analyzer was estimated using an equilibrator time constant τ (61.6 s) that describes the time required for a 63% step change to occur. A wavelet denoising technique was applied to the N 2 O concentration data to improve the signal-to-noise ratio of the signal [Fassbinder et al., 2013]. For convenience of data processing and statistical analyses, all of the data streams were subjected to 30 s block averaging resulting in a data series of n = 3667. ...
... The analyzer was zeroed and spanned using analytical grade standards (Specialty Gases of America, Toledo, OH, USA) before each measurement campaign. The concentration precision after wavelet denoising is 1.5 nmol mol À1 [Fassbinder et al., 2013]. These two metrics, [N 2 O] and [N 2 O amb ], are water temperature dependent. ...
The U.S. Corn Belt is one of the most intensive agricultural regions of the world and is drained by the Upper Mississippi River (UMR), which forms one of the largest drainage basins in the U.S. While the effects of agricultural nitrate (NO3-) on water quality in the UMR have been well documented, its impact on the production of nitrous oxide (N2O) has not been reported. Using a novel equilibration technique, we present the largest dataset of freshwater dissolved N2O concentrations (0.7 to 6-times saturation) and examine the controls on its variability over a 350-km reach of the UMR. Driven by a supersaturated water column, the UMR was an important atmospheric N2O source (+68 mg N2O-N m-2 yr-1) that varies non-linearly with the NO3- concentration. Our analyses indicated that a projected doubling of the NO3- concentration by 2050 would cause dissolved N2O concentrations and emissions to increase by about 40%.
... The analyzer was powered in the field by deep cycle 12 V batteries wired in parallel to a DC-to-AC inverter. The chamber system has a minimum detectable flux of 0.028 nmol N 2 O·m −2 ·s −1 (53). The analyzer was calibrated at the beginning of the season using an analytical grade standard and zeroed two times per month using N 2 gas. ...
... The analyzer was calibrated at the beginning of the season using an analytical grade standard and zeroed two times per month using N 2 gas. The concentration precision of the analyzer was 1.5 nmol·mol −1 , and the flux measurement precision was 0.003 nmol·m −2 ·s −1 (53). ...
... where ρ (mol·m −3 ) is the molar density of dry air, A (m 2 ) is the surface area enclosed by the chamber, V (m 3 ) is the chamber volume, and Δ (nmol N 2 O·mol −1 ·s −1 ) is the rate of change of N 2 O concentration in the chamber headspace determined from linear regression (53). Before calculating the chamber N 2 O fluxes, a wavelet denoising technique was applied to the raw concentration data. ...
Significance
N 2 O emissions from riverine systems are poorly constrained, giving rise to highly uncertain indirect emission factors that are used in bottom-up inventories. Using a non–steady-state flow-through chamber system, N 2 O fluxes were measured across a stream order gradient within the US Corn Belt. The results show that N 2 O emissions scale with the Strahler stream order. This information was used to estimate riverine emissions at the local and regional scales and demonstrates that previous bottom-up inventories based on the Intergovernmental Panel on Climate Change default values have significantly underestimated these indirect emissions.
... where dC/dt is the rate of change of N 2 O concentration over time or slope of the linear equation (µg N 2 O g −1 air s −1 ), V is the headspace volume of the chamber (m 3 ), A is the land surface area covered by the chamber (m 2 ), p is the barometric pressure (Pa = J m −3 ), M is the molar mass of N 2 O (44.013 g mol −1 ), R is the universal gas constant (8.314 J mol −1 K −1 ), T is the absolute temperature (K) around sampling chamber and K is a conversion factor (864 mg µg −1 m 2 ha −1 s h −1 ) to convert values to g ha −1 d −1 [63,64]. Soil and air temperature were monitored at the same time as gas sampling at four places around each sampling chamber. ...
... Daily N 2 O fluxes were calculated using Microsoft Office Excel 2016 (Microsoft, Inc., Washington, WA, USA). It was assumed that N 2 O emissions between 0900 and 1200 h on each sampling date were the average of the daily N 2 O flux [62,64]. Cumulative N 2 O emissions were calculated by multiplying the average fluxes of two successive determinations by the length of the period between samplings and adding that amount to the previous cumulative total [26,66] using following Equation (2). ...
Topography affects soil hydrological, pedological, and biochemical processes and may influence nitrous oxide (N2O) emissions into the atmosphere. While N2O emissions from agricultural fields are mainly measured at plot scale and on flat topography, intrafield topographical and crop growth variability alter soil processes and might impact N2O emissions. The objective of this study was to examine the impact of topographical variations on crop growth period dependent soil N2O emissions at the field scale. A field experiment was conducted at two agricultural farms (Baggs farm; BF and Research North; RN) with undulating topography. Dominant slope positions (upper, middle, lower and toeslope) were identified based on elevation difference. Soil and gas samples were collected from four replicated locations within each slope position over the whole corn growing season (May–October 2019) to measure soil physio-chemical properties and N2O emissions. The N2O emissions at BF ranged from −0.27 ± 0.42 to 255 ± 105 g ha−1 d−1. Higher cumulative emissions were observed from the upper slope (1040 ± 487 g ha−1) during early growing season and from the toeslope (371 ± 157 g ha−1) during the late growing season with limited variations during the mid growing season. Similarly, at RN farm, (emissions ranged from −0.50 ± 0.83 to 70 ± 15 g ha−1 d−1), the upper slope had higher cumulative emissions during early (576 ± 132 g ha−1) and mid (271 ± 51 g ha−1) growing season, whereas no impact of slope positions was observed during late growing season. Topography controlled soil and environmental properties differently at different crop growth periods; thus, intrafield variability must be considered in estimating N2O emissions and emission factor calculation from agricultural fields. However, due to large spatial variations in N2O emissions, further explorations into site-specific analysis of individual soil properties and their impact on N2O emissions using multiyear data might help to understand and identify hotspots of N2O emissions.
... We used either a LI-COR 830 (or subsequently, LI-COR 850) Infrared Gas Analyzer to measure CO2 concentrations by infrared absorbance. Downstream, a 215 Teledyne 320U gas filter correlation analyzer measured N2O concentration via infrared absorbance by frequently comparing the sample to a reference gas in a rotating filter (Fassbinder et al., 2013). Instantaneous gas concentrations, as well as the air temperature, inlet flow, outlet flow, and sample flow were measured every 10 seconds and recorded on a datalogger (Campbell CR3000). ...
... These values correspond to approximately 0.5 ppm difference in CO2 and 0.08 ppm difference in N2O at the high and low temperature range observed. Taken together, we found that the N2O instrument had a -0.006 ppm °C -1 340 sensitivity, in close agreement to the -0.009 ppm °C -1 found by Fassbinder et al. (2013). As detailed above, standards were measured every two hours to account for instrument sensitivity to environmental conditions. ...
Abstract. Soils play an important role in Earth's climate system through their regulation of trace greenhouse gases. Despite decades of soil gas flux measurements using manual chamber methods, limited temporal coverage has led to high uncertainty in flux magnitude and variability, particularly during peak emission events. Automated chamber measurement systems can collect high-frequency (sub-daily) measurements across various spatial scales but may be prohibitively expensive or incompatible with field conditions. Here we describe the construction and operational details for a robust, relatively inexpensive, and adaptable automated dynamic (steady-state) chamber measurement system modified from previously published methods, using relatively low-cost analyzers to measure nitrous oxide (N<sub>2</sub>O) and carbon dioxide (CO<sub>2</sub>). The system was robust to intermittent flooding of chambers, long tubing runs (> 100 m), operational temperature extremes (−12–39 °C), and was entirely powered by solar energy. Using data collected between 2017–2019 we tested the underlying principles of chamber operation and examined N<sub>2</sub>O diel variation and rain-pulse timing that would be difficult to characterize using infrequent manual measurements. Stable steady-state dynamics were achieved during the 29-minute chamber closure periods at relatively low flow rate (2 L min<sup>-1</sup>). Instrument performance and calculated fluxes were minimally impacted by variation in air temperature and water vapor. Measurements between 08:00 and 12:00 were closest to the daily mean N<sub>2</sub>O and CO<sub>2</sub> emission. Afternoon fluxes (12:00–16:00) were 28 % higher than the daily mean for N<sub>2</sub>O (4.04 versus 3.15 nmol m<sup>-2</sup> s<sup>-1</sup>) and were 22 % higher for CO<sub>2</sub> (4.38 versus 3.60 umol m<sup>-2</sup> s<sup>-1</sup>). High rates of N<sub>2</sub>O emission are frequently observed after precipitation. Following four discrete rainfall events, we found an 12 to 26-hour delay before peak N<sub>2</sub>O flux, which would be difficult to capture with manual measurements. Our observation of substantial and variable diel trends and rapid but variable onset of high N<sub>2</sub>O emissions following rainfall support the need for high-frequency measurements.
... The manual chamber method is commonly used to measure GHG fluxes because it is relatively cheap and easy to operate in situ (Tallec et al., 2019). However, this method usually observes N 2 O fluxes from limited spaces at relatively low temporal frequencies and thereby may miss hotspots and/or hot moments of N 2 O fluxes (Fassbinder et al., 2013). For instance, daily N 2 O fluxes were estimated using measurements between 09:00 am to 11:00 am and performed every 10 days in this study. ...
... For instance, daily N 2 O fluxes were estimated using measurements between 09:00 am to 11:00 am and performed every 10 days in this study. This low measurement frequency may result in errors in representing the daily average N 2 O fluxes (Chen et al., 2017) and cannot completely capture high N 2 O fluxes that are often episodic, happening within 72 h of N fertilization or heavy rainfall events in agricultural soils (Fassbinder et al., 2013). Therefore, the discrepancies may partially result from the fact that the model predicted daily average N 2 O fluxes using average daily climate and soil properties at a given site while the observations cannot accurately represent the average fluxes (Molina-Herrera et al., 2016). ...
Covering soils using mulch can increase crop productivity in dryland agriculture. However, there remains large uncertainty regarding impacts of mulching on nitrous oxide (N2O) emissions, especially under climate change (increases in air temperature and atmospheric CO2 concentration and changes in precipitation). In this study, we applied a biogeochemical model, DeNitrification-DeComposition (DNDC), to predict impacts of different mulching practices on wheat (Triticum aestivum L.) and maize (Zea mays L.) yields and N2O emissions under future climate scenarios in the South Loess Plateau of China. When tested against the observed crop yields and N2O emissions under no-mulching (NM), straw mulching (SM), and plastic film mulching (PM), DNDC successfully simulated crop yields and annual N2O emissions under all treatments. Simulations and observations both suggested that applying SM or PM increased crop yields and N2O emissions in comparison with NM. Sensitivity analyses of crop yields and N2O emissions indicated that the crop yields were primarily influenced by precipitation and N2O emissions were sensitive to changes in air temperature, precipitation, soil organic carbon, and nitrogen application rate. Application of SM or PM reduced the sensitivity of the crop yield and N2O emissions to precipitation change. Compared with historical climate conditions, future climate from 2017 to 2100 significantly increased crop yields except during the 2090s for NM or SM and during the 2070s to 2090s for PM under the high emission scenario (RCP8.5), while N2O emissions were increased under all treatments. The positive impacts of PM on crop yields could be reduced under the RCP8.5 scenario. The DNDC predictions suggest that straw mulching might be an optimum mulching method to improve crop productivity and mitigate increasing N2O under future climate conditions in semi-arid to sub-humid areas such as the South Loess Plateau of China.
... Because of the intensive labor required for flux chamber measurements, several researchers have developed automated chamber systems (Smith and Dobbie, 2001;Parkin, 2008;Denmead et al., 2010;Jorgensen et al., 2012;Fassbinder et al., 2013;Barton et al., 2015). These automated chambers typically use pivoting or hinged chamber lids, such that the chamber remains open between sampling periods. ...
... Therefore, it was important that there be no air leaks between the chamber pan and the lid. Researchers have used a variety of sealing mechanisms, including foam gaskets, rubber membranes, weighted skirts, and channels that contain liquid (Parkin, 2008;Denmead et al., 2010;Jorgensen et al., 2012;Fassbinder et al., 2013). We evaluated three types of sealing mechanisms, including a small channel design filled with (1) water, (2) a cornstarch and water mixture, and (3) commercially available ballistic gelatin (i.e., a solution of gelatin and water that was originally developed to simulate the effects of firearm ammunition on human tissue). ...
ABSTRACT. Nitrous oxide (N2O) emission rates have traditionally been measured using non-flow-through (NFT), nonsteady-state (NSS) chambers, which rely on measuring the increase in N2O concentration in the sealed chamber headspace over time. These flux measurements are very labor- and time-intensive, requiring three to four gas samples collected over a 30 to 60 min period, followed by laboratory N2O measurement with a gas chromatograph (GC) and subsequent flux rate calculation. The objective of this research was to develop and evaluate improved, real-time flux chamber designs that rapidly quantify N2O emissions from manure and soil. The first chamber system consisted of six square 0.95 m2 chamber pans. The chamber pans were mounted on a rail system to facilitate controlled indoor/outdoor laboratory research at a pilot scale. An aluminum lid was moved among the chamber pans. A second portable chamber system with a circular footprint (0.49 m internal dia.) was designed for use in field measurements. With both systems, N2O concentrations were measured
each second with 0.1 ppb resolution by recirculating sample air through a real-time continuous N2O analyzer with return flow into the recirculating-flow-through (RFT-NSS) chamber. Performance and observational data are presented for different chamber vent designs, sealing mechanisms between the chamber pan and lid, recirculation pumps, and presence/absence of an internal fan that mixes headspace air within the sealed chamber. As examples of the repeatability and precision of the methodology, ten consecutive flux measurements were obtained using moist manure (32.6% wet basis water content, WCWB) within a 15 min period in which chamber pans were fitted with lids for 60 s and removed for 30 s. The mean calculated
N2O flux was 43.08 0.89 mg N2O m-2 h-1. Using dry manure (WCWB = 10.8%), five consecutive flux measurements showed a very low, but consistent, flux that averaged 0.025 0.0016 mg N2O m-2 h-1. Five case study experiments demonstrate the usefulness of these chamber systems and highlight discoveries and lessons learned to enhance future research efforts. Major discoveries and observations include: (1) installation of a small internal fan within the chamber lids decreased N2O fluctuation over small time periods, allowing precise measurement of manure N2O fluxes as low as 0.0073 mg N2O m-2 h-1 during a 60 s measurement period; (2) two distinct N2O peaks were observed at 1 and 21 d following the addition of water to manure (initial WCWB = 32.6%), with the second peak accounting for 83% of the total N2O emitted over 45 d; and (3) there was notable diurnal variation in N2O fluxes due to temperature variation, even when the manure was dry (WCWB = 10.8%). These flux chamber systems proved to be more rapid, precise, and repeatable than traditional flux chamber methods and offer promise for future greenhouse gas emissions research on manure and soil.
... Soil N 2 O fluxes were measured with automated soil chambers (Model LI8100-104, Li-Cor Inc.) controlled by a datalogger (Model 23X, Campbell Scientific) connected to a multiplexer controlling two sets of solenoids (Clippard Inc.) Fassbinder et al., 2013). All soil chambers (n = 8) were vented, finished with white enamel to minimize solar heating, and installed onto PVC collars. ...
... where P is air pressure (Pa), V is the chamber volume (0.004 m 3 ), A is the chamber footprint (0.032 m 2 ), R is the molar gas constant ( J mol -1 K -1 ), T is the air temperature at the time of measurement (K), and D is the slope of N 2 O concentration change over time in the chamber headspace. Before slope calculation, the raw N 2 O concentration data were passed through a wavelet denoising algorithm to improve the signal to noise ratio (Fassbinder et al., 2013). The slope was calculated from a 90-s window beginning 150 s after chamber closure. ...
Nitrous oxide (N 2 O), produced primarily in agricultural soils, is a potent greenhouse gas and is the dominant ozone‐depleting substance. Efforts to reduce N 2 O emissions are underway, but mitigation results have been inconsistent. The leguminous perennial kura clover ( Trifolium ambiguum M. Bieb.) (KC) can grow side‐by‐side with cash crops in rotational corn ( Zea mays L.)–soybean ( Glycine max L.) systems. With biological nitrogen fixation, KC provides land managers an opportunity to reduce external fertilizer inputs, which may diminish problematic N 2 O emissions. To investigate the effect of a KC living mulch on N 2 O emissions, automated soil chambers coupled to a N 2 O analyzer were used to measure hourly fluxes from April through October in a 2‐yr corn–soybean (CS) rotation. Emissions from the KC treatment were significantly greater than those from the conventional CS treatment despite the fact that the KC treatment received substantially less inorganic nitrogen fertilizer. A seasonal tradeoff was observed with the KC treatment wherein emissions before strip‐tillage were reduced but were surpassed by high losses after strip‐tillage and postanthesis. These results represent the first reported measurements of N 2 O emissions from a KC‐based living mulch. The findings cast doubt on the efficacy of KC for mitigating N 2 O loss in CS systems. However, if KC reduces nitrate leaching losses, as has been reported elsewhere, it may result in lower indirect (offsite) N 2 O emissions.
Core Ideas
Kura clover living mulch increased total N 2 O emissions.
Nitrogen scavenging by the kura clover living mulch may have reduced spring N 2 O emissions.
Emissions in the kura clover treatment were affected by soil disturbance and plant stress.
Corn and soybean yield were only marginally affected by kura clover living mulch.
... Unlike automatic chambers, manual chambers typically are used to sample once per day at maximum [15][16][17]. By contrast, automated systems with a higher frequency sampling allow to address the response of GHG emissions to weather conditions, rainfall events, irrigation or N fertilization [13,18], and to capture diurnal fluctuations [19], not practically possible with manual chambers. The use of automatic systems could therefore optimize the estimation of GHG emissions also contributing to improve mitigation strategies, and to offer a deeper and more precise insight of the dynamics that rule GHG emissions [20,21]. ...
OpenToolFlux is an open-source software to estimate soil gas fluxes from gas concentration time-series data generated by automatic chamber systems. This paper describes the physical equipment used as well as software design and workflow. The software is a command-line application that imports tabular time-series data from the analyzer following the instructions specified in a configuration file by the user, performs configurable data-cleaning operations, and outputs a data file with volumetric flux estimates as well as diagnostic plots. The software can be configured according to the specifics of physical equipment and experimental setup and is therefore applicable in a wide range of studies.
... However, 156 measurements, and thus knowledge, of soil-atmosphere gas-exchanges are often discontinuous and 157 biased toward 'dry' conditions(Scott et al. 1999;Ford et al. 2012). Although automated infrastructure 158 for monitoring gas efflux exists, it is expensive, logistically challenging, and spatially limited 159 (missing hotspots)(Fassbinder et al. 2013).160 Microbial activities associated with transient, storm-related niches are observable by scientists 161 who persist through the rain (Burgin et al. 2011). ...
Rainwater is a vital resource and dynamic driver of terrestrial ecosystems. Yet, processes controlling precipitation inputs and interactions during storms are often poorly seen, and poorly sensed when direct observations are substituted with technological ones. We discuss how human observations complement technological ones, and the benefits of scientists spending more time in the storm. Human observation can reveal ephemeral storm-related phenomena such as biogeochemical ‘hot moments’, organismal responses, and sedimentary processes which can then be explored in greater resolution using sensors and virtual experimentation. Storm-related phenomena trigger lasting, oversized impacts on hydrologic and biogeochemical processes, organismal traits/functions, and ecosystem services. We provide examples of phenomena in forests, across disciplines and scales, to inspire mindful, holistic observation of ecosystems during storms. We conclude that technological observations alone are insufficient to trace the process complexity and unpredictability of fleeting biogeochemical or ecological events without the “shower thoughts” produced by scientists’ human sensory and cognitive systems during storms.
... Crops were harvested at the end of September by cutting the stover five inches 175 above the soil. Hourly N2O fluxes (mg N m -2 h -1 ) and CO2 fluxes (g C m -2 h -1 ) were measured using non-steady-state flux chambers with a CO2 analyzer (LI-10820 for 2016 and LI-7000 for 2017 and 2018, LI-COR Biosciences, Lincoln, NE) and a N2O analyzer (Teledyne M320EU, Teledyne Technologies International Corp, Thousand Oaks, CA) (Detail method can be retrieved from Fassbinder et al., 2012Fassbinder et al., , 2013. We also collected soil moisture at 15 cm depth (VWC as abbreviation of volumetric water content, m 3 m -3 ), weekly 0-15 cm depth soil NO3 -+ NO2concentration (NO3for short in the following 180 text, g N Mg -1 ), soil NH4 + concentration (NH4 + , g N Mg -1 ), and related environment variables including air temperature, radiation, humidity and soil/crop properties from three growing seasons during 2016-2018 and six mesocosm chambers (Fig. S1). ...
Agricultural nitrous oxide (N2O) emission accounts for a non-trivial fraction of global greenhouse gases (GHGs) budget. To date, estimating N2O fluxes from cropland remains a challenging task because the related microbial processes (e.g., nitrification and denitrification) are controlled by complex interactions among climate, soil, plant and human activities. Existing approaches such as process-based (PB) models have well-known limitations due to insufficient representations of the processes or constraints of model parameters, and to leverage recent advances in machine learning (ML) new method is needed to unlock the “black box” to overcome its limitations due to low interpretability, out-of-sample failure and massive data demand. In this study, we developed a first of its kind knowledge-guided machine learning model for agroecosystems (KGML-ag), by incorporating biogeophysical/chemical domain knowledge from an advanced PB model, ecosys, and tested it by simulating daily N2O fluxes with real observed data from mesocosm experiments. The Gated Recurrent Unit (GRU) was used as the basis to build the model structure. To optimize the model performance, we have investigated a range of ideas, including: 1) Using initials of intermediate variables (IMVs) instead of time series as model input to reduce data demand; 2) Building hierarchical structures to explicitly estimate IMVs for further N2O prediction; 3) Using multitask learning to balance the simultaneous training on multiple variables; and 4) Pretraining with millions of synthetic data generated from ecosys and fine tuning with mesocosm observations. Six other pure ML models were developed using the same mesocosm data to serve as the benchmark for the KGML-ag model. Results show that KGML-ag did an excellent job in reproducing the mesocosm N2O fluxes (overall r2 = 0.81, and RMSE = 3.6 mg N m−2 day−1 from cross-validation). Importantly KGML-ag always outperforms the PB model and ML models in predicting N2O fluxes, especially for complex temporal dynamics and emission peaks. Besides, KGML-ag goes beyond the pure ML models by providing more interpretable predictions as well as pinpointing desired new knowledge and data to further empower the current KGML-ag. We believe the KGML-ag development in this study will stimulate a new body of research on interpretable ML for biogeochemistry and other related geoscience processes.
... Currently, the main techniques for measuring CH 4 flux are the closed chamber methods and the eddy covariance (EC) method. However, with continuing advancements in automation technologies and online monitoring, new measurement technologies have been applied in areas including field (Fassbinder et al. 2013;Davis et al. 2018), landfill (Izumoto et al. 2018), wetland (Glagolev et al. 2011), and industrial environments (Kannath et al. 2011), but there still are few applications for CWs. Online monitoring and automation measurement technologies may be the ideal method to achieve a continuous measure of CH 4 flux, but these methods remain expensive (Bansal et al. 2018). ...
CH4 flux measured by a portable chamber using an infrared analyzer was compared with the flux by static chamber measurement for CW at 13 different sites from May 2012 to May 2017 in the Living Water Garden (LWG) in Chengdu, Sichuan Province, China, over 4 timescales (daily, monthly, seasonal, and annual). During the measurement period, a total of 1443 data were collected. CH4 fluxes were measured using the portable chamber method and the results showed that the annual mean and median CH4 flux values in the LWG were 17.4 mg m−2 h−1 and 6.2 mg m−2 h−1, respectively, ranging from − 19.7 to 98.0 mg m−2 h−1. Cumulative CH4 emissions for LWG ranged from − 0.17 to 0.86 kg m−2 year−1. Global warming potential (GWP, 25.7 kg CO2eq m−2 year−1) was at a high level, which means that the LWG was a source of CH4 emissions. Significant temporal variations on the 4 timescales were observed. And the asymmetry of measurement uncertainty of CH4 flux increases with the timescale. Although the total mean CH4 flux measured by the portable chamber method was 42.1% lower than that of the static chamber method, the temporal variation trends of CH4 flux were similar. The uncertainty of CH4 flux measured in portable chamber was more symmetrical than that in static chamber. These results suggest that the portable chamber method has considerable value as a long-term measurement method for CH4 flux temporal variations.
... With manually operated static chambers, it is difficult to perfectly track all emission peaks, leading to underestimates of accumulated N 2 O values, but the daytime measurements tend to overestimate emissions compared to more frequent automated chambers or near continuous micro-meteorological techniques. However, the latter techniques are very costly and typically deployed over much shorter time periods than in this study, and are also less suitable for larger multi-treatment experiments (Rochette and Eriksen-Hamel, 2008;Fassbinder et al., 2013). To enhance the accuracy of our estimates, we took frequent samples during all expected high emission events which included N applications, dry/wet cycles and freeze/thaw cycles. ...
Dairy farms need to improve their environmental performance to justify continued consumption of dairy products. Previous studies have demonstrated improved re-use of dairy slurry nitrogen (N) and phosphorous (P) by separating solid and liquid fractions. This study was conducted to examine the long-term effects of applying whole dairy slurry (WS), separated liquid fraction (SLF), and mineral fertilizer on emission of nitrous oxide (N2O) from a grass sward in a moderate climate, and to identify methods to mitigate emissions. The results show that emission occurs in sharp spikes within a week of N application as well as after some wet/dry cycles and freeze/ thaw cycles even in this moderate climate. Greatest emissions occurred from late spring and summer applications. At the high application rate of total-N emissions were similar for WS and SLF, at the lower rate SLF exceeded WS, whereas at equivalent mineral-N, emissions were greater from WS than SLF. On the basis of crop N-uptake, SLF had similar or lower emissions than WS at comparable N rates, and the same was true on basis of yield except for lower emissions with WS than SLF at the low total-N rate (400 kg ha−1). By not finding pollution swapping, the study helps to support the use of slurry manure separation for its agronomic and environmental benefits. This multi-year field study supports current IPCC default values as cautious emission factors (EF) for dairy slurry and commercial fertilizer on intensively managed grass, though a lower EF may be justified for lower input grassland (<50 kg mineral-N ha−1 per dose). Our results suggest that N2O emissions can be reduced by transferring summer slurry application to early spring but on farms where this is not possible, mitigating summertime peaks should be targeted, possibly with the use of a nitrification inhibitor.
... The availability of and accessibility to an apparatus with automatic chambers make it feasible to sample gases daily, hourly, or more than once a week in long-term experiments, which enhances accuracy and decreases sampling errors (Fassbinder et al., 2013;Reeves & Wang, 2015). Only in this way, will it be possible to follow recommendations to increase sampling frequency after important events that alter gas emissions, such as rainfall, fertilization, and sowing, among other soil and cultural management practices (Parkin & Venterea, 2010;Reeves & Wang, 2015). ...
The objective of this work was to assess the influence of gas sampling frequency on the cumulative emissions of nitrous oxide (N2O) from the soil. Gas emissions were assessed over a period of two years (2014-2016), in four systems: eucalyptus forestry, crops, pasture, and native forest. The cumulative emissions of N2O were calculated at sampling intervals of 7, 14, and 21 days. The sampling intervals did not influence the final results of cumulative N2O emissions from the soil in the assessed systems.
... There are many studies that have recognized that the use of linear regression in flux calculation can cause significant underestimation of the flux (e.g., Healy et al., 1996;Hutchinson et al., 2000;Nakano et al., 2004;Livingston et al., 2005Livingston et al., , 2006Kutzbach et al., 2007;Kroon et al., 2008;Pedersen et al., 2010;Pihlatie et al., 2013). However, many studies have used linear regression (e.g., Laine et al., 2006;Alm et al., 2007;Jones et al., 2011;Bergier et al., 2013;Fassbinder et al., 2013), because under field conditions it is more robust to random measurement errors than nonlinear methods. Moreover, the use of linear regression is preferred when comparing measurement sites as it is not as sensitive as nonlinear models to small differences in soil properties (Venterea et al., 2009). ...
We measured methane
(CH4) exchange rates with automatic chambers at the forest floor of a
nutrient-rich drained peatland in 2011–2013. The fen, located in southern
Finland, was drained for forestry in 1969 and the tree stand is now a mixture
of Scots pine, Norway spruce, and pubescent birch. Our measurement system
consisted of six transparent chambers and stainless steel frames, positioned
on a number of different field and moss layer compositions. Gas
concentrations were measured with an online cavity ring-down spectroscopy gas
analyzer. Fluxes were calculated with both linear and exponential regression.
The use of linear regression resulted in systematically smaller CH4
fluxes by 10–45 % as compared to exponential regression. However, the
use of exponential regression with small fluxes
( < 2.5 µg CH4 m−2 h−1) typically resulted in
anomalously large absolute fluxes and high hour-to-hour deviations.
Therefore, we recommend that fluxes are initially calculated with linear
regression to determine the threshold for low fluxes and that higher
fluxes are then recalculated using exponential regression. The exponential
flux was clearly affected by the length of the fitting period when this
period was < 190 s, but stabilized with longer periods. Thus, we also
recommend the use of a fitting period of several minutes to stabilize the
results and decrease the flux detection limit. There were clear seasonal
dynamics in the CH4 flux: the forest floor acted as a CH4 sink
particularly from early summer until the end of the year, while in late
winter the flux was very small and fluctuated around zero. However, the
magnitude of fluxes was relatively small throughout the year, ranging mainly
from −130 to +100 µg CH4 m−2 h−1. CH4
emission peaks were observed occasionally, mostly in summer during heavy
rainfall events. Diurnal variation, showing a lower CH4 uptake rate
during the daytime, was observed in all of the chambers, mainly in the summer
and late spring, particularly in dry conditions. It was attributed more to
changes in wind speed than air or soil temperature, which suggest that
physical rather than biological phenomena are responsible for the observed
variation. The annual net CH4 exchange varied from −104 ± 30 to
−505 ± 39 mg CH4 m−2 yr−1 among the six chambers,
with an average of −219 mg CH4 m−2 yr−1 over the 2-year
measurement period.
... (R-Studio Team 2015). Measured N 2 O flux was considered representative of the average daily N 2 O flux, because the gas sampling was performed between 1000 and 1400 h (Fassbinder et al. 2013). Fluxes for the injected plots were corrected to account for potential overestimation of area-scaled fluxes due to placement of the chamber over the injection area. ...
Anaerobically digested dairy manure (AD) has been proposed as an alternative nitrogen source to reduce soil nitrous oxide (N2O) emissions compared to raw dairy manure (RM). The aim of this research was to compare soil N2O emissions associated with AD and RM according to three application methods: surface broadcasting (SB), incorporation (SBI) and injection (INJ). The field experiment was conducted on a loam soil at Elora, Ontario, from November 2012 to November 2014, using a randomized block design with 4 replications. Manure was applied in mid-November (fall) and corn (Zea mays) was planted in late-May of each year. Nitrous oxide flux was measured using non-steady state chambers sampled weekly or bi-weekly. Cumulative N2O emissions were significantly affected by the interaction between source and method (F=3.99, p<0.01), with the highest value for surface broadcast AD (6.4 kg N2O-N ha-1) and the lowest value for injected AD (2.6 kg N2O-N ha-1). Manure source affected cumulative N2O emissions (F=4.67, p<0.1), with the largest emissions for AD (4.8 kg N2O-N ha-1). Anaerobically digested manure was proven to reduce cumulative N2O emissions when it was fall injected to corn in cold climates; however, if AD is broadcasted or broadcasted and incorporated, it may result in greater N2O emissions than those produced by RM.
... Nitrous oxide flux calculations were performed using Microsoft Excel and RStudio (2015). It was assumed that N 2 O flux measured between 1000 and 1400 h on a sampling date was representative of the average daily N 2 O flux (Fassbinder et al., 2013). Emissions measured in the INJ plots were corrected to account for the placement of the chambers on top of the injection row. ...
Core Ideas
The year × timing interaction affects cumulative N 2 O emissions.
Injection of manure produces the highest cumulative N 2 O emissions.
Injection of manure produces the highest corn yields.
Manure application to agricultural soils enhances N 2 O emissions, but these emissions could be reduced by using improved application methods at the right time. We conducted a 3‐yr study on corn ( Zea mays L.) grown in Elora, ON, Canada, to test the effects of timing and method of liquid dairy manure application on year‐round N 2 O emissions. A randomized complete block design was set up every year evaluating two application times (fall vs. spring) and three methods of manure application (surface broadcasting, incorporation, and injection). Lower cumulative N 2 O emissions for fall than spring application (mean ± standard error = 1.2 ± 0.3 vs. 2.9 ± 0.3 kg N 2 O‐N ha ⁻¹ ) were found during the driest year (2012), whereas no differences in emissions occurred between application timing in near‐normal precipitation years (2013 and 2014). Nitrous oxide emissions were not affected by the timing × method of application interaction. Injected manure resulted in cumulative N 2 O emissions not different than surface broadcast manure (3.6 ± 0.5 vs. 3.0 ± 0.5 kg N 2 O‐N ha ⁻¹ ) but significantly higher than incorporated manure (2.2 ± 0.3 kg N 2 O‐N ha ⁻¹ ). Injection resulted in greater corn yields than the other two methods. Our results suggest that (i) method of application affects N 2 O emissions independently of timing; (ii) including N 2 O emissions for the non‐growing season avoided biased estimates for the fall application timing since 20 to 60% of total emissions occurred during this period; and (iii) incorporating manure is the best practice to mitigate N 2 O emissions, although if N rates are optimized, injection could potentially produce yields with low N 2 O intensity.
... Currently, the use of eddy covariance systems over lakes and reservoirs is relatively new and poses several challenges. These challenges include (a) high instrument cost, (b) poor sensor performance during wet conditions, and (c) difficulty associated with estimating measurement footprints, especially in small, heterogeneous areas (Fassbinder et al. 2013, Peltola et al. 2013. ...
Collectively, reservoirs created by dams are thought to be an important source of greenhouse gases (GHGs) to the atmosphere. So far, efforts to quantify, model, and manage these emissions have been limited by data availability and inconsistencies in methodological approach. Here, we synthesize reservoir CH 4 , CO 2 , and N 2 O emission data with three main objectives: (1) to generate a global estimate of GHG emissions from reservoirs, (2) to identify the best predictors of these emissions, and (3) to consider the effect of methodology on emission estimates. We estimate that GHG emissions from reservoir water surfaces account for 0.8 (0.5–1.2) Pg CO 2 equivalents per year, with the majority of this forcing due to CH 4. We then discuss the potential for several alternative pathways such as dam degassing and downstream emissions to contribute significantly to overall emissions. Although prior studies have linked reservoir GHG emissions to reservoir age and latitude, we find that factors related to reservoir productivity are better predictors of emission.
... However, many studies have used linear regression (e.g. Reth et al., 2005;Laine et al., 2006;Wang et al., 2006;Alm et al., 2007;Jones et al., 2011;Bergier et al., 2013;Fassbinder et al., 2013), because under field conditions it is more robust than nonlinear methods. Moreover, the use of linear regression is preferred 15 when comparing measurement sites as it is not as sensitive as non-linear models to small differences in soil properties (Venterea et al., 2009). ...
We measured methane (CH4) exchange rates with automatic chambers at the forest floor of a nutrient-rich drained peatland in 2011–2013. The fen, located in southern Finland, was drained for forestry in the 1970s and the tree stand is now a mixture of Scots pine, Norway spruce and pubescent birch. Our measurement system consisted of six transparent polycarbonate chambers and stainless steel frames, positioned on a number of different field and moss layer types. Flux rates were calculated with both linear and exponential regression. The use of linear regression systematically underestimated CH4 flux rates by 20–50 % when compared to exponential regression. However, the use of exponential regression with small fluxes (
... To eliminate bias associated with temporal variability, continuous automatic chamber systems have been developed and their use has increased within the last decade (Smith and Dobbie, 2001;Livesley et al., 2011;Scheer et al., 2012;Barton et al., 2013;Fassbinder et al., 2013;Kennedy et al., 2013;Van Der Weerden et al., 2013). However, the initial setup and ongoing costs involved with running an automatic system, the limited number of chambers available, the requirements for adequate monitoring and maintenance, and the risk of prolonged data loss in the event of instrument failure mean that the static manual chamber method would still be used extensively in the field. ...
Annual cumulative nitrous oxide (N2O) emissions from soil have historically been calculated from intermittent data measured manually via the static chamber method. The temporal variability in emissions, both diurnally and between days, introduces uncertainty into the up-scaling of static chamber data. This study assessed the most appropriate time of the day to sample and the best sampling frequency to ensure reliable estimates of annual cumulative emissions. Sub-daily N2O emissions were measured using automatic gas sampling chambers over three years in a sub-tropical cereal crop system. The sub-daily dataset was divided into eight time periods per day to assess the best sampling time of the day. Daily mean N2O emissions were subsampled from the dataset to simulate different sampling frequencies, including pre-set and rainfall-based scenarios. Annual cumulative N2O emissions were calculated for these scenarios and compared to the 'actual' annual cumulative emissions. The results demonstrated that manual sampling between mid-morning (09:00) and midday (12:00), and late evening (21:00) and midnight (24:00) best approximated the daily mean N2O emission. Factoring in the need to sample during daylight hours, gas sampling from mid-morning to midday was the most appropriate sampling time. Overall, triweekly sampling provided the most accurate estimate (±4% error) of annual cumulative N2O emissions, but was undesirable due to its labour intensive high sampling frequency. Weekly sampling with triweekly sampling in the two weeks following rainfall events was the most efficient sampling schedule, as it had similar accuracy (±5% error) to the triweekly sampling, the smallest variability in outcomes and approximately half the sampling times of triweekly sampling. Inter-annual rainfall variability affected the accuracy and variability of estimations of annual cumulative emissions, but did not affect the overall trends in sampling frequency accuracy. This study demonstrated that intermittent samplings are capable of estimating the annual cumulative N2O emissions satisfactorily when timed appropriately.
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... According to USEPA, agricultural systems contributed 75% of nitrous oxide (N 2 O) emissions in 2012 [15]. Also Fassbinder et al. reported up to 80% of N 2 O emissions from agricultural systems [16]. The N 2 O emissions have a warming ability that is 310 times that of carbon dioxide (CO 2 ) emissions [15]. ...
Current and future demand for food, feed, fiber, and energy require novel approaches to land management, which demands that multifunctional landscapes are created to integrate various ecosystem functions into a sustainable land use. We developed an approach to design such landscapes at a field scale to minimize concerns of land use change, water quality, and greenhouse gas emissions associated with production of food and bioenergy. This study leverages concepts of nutrient recovery and phytoremediation to place bioenergy crops on the landscape to recover nutrients released to watersheds by commodity crops. Crop placement is determined by evaluating spatial variability of: 1) soils, 2) surface flow pathways, 3) shallow groundwater flow gradients, 4) subsurface nitrate concentrations, and 5) primary crop yield. A 0.8 ha bioenergy buffer was designed within a 6.5 ha field to intercept concentrated surface flow, capture and use nitrate leachate, and minimize use of productive areas. Denitrification-Decomposition (DNDC) simulations show that on average, a switchgrass (Panicum Virgatum L.) or willow (Salix spp.) buffer within this catchment according to this design could reduce annual leached NO3 by 61 or 59% and N2O emission by 5.5 or 10.8%, respectively, produce 8.7 or 9.7 Mg ha−1 of biomass respectively, and displace 6.7 Mg ha−1 of corn (Zea mays L.) grain. Therefore, placement of bioenergy crops has the potential to increase environmental sustainability when the pairing of location and crop type result in minimal disruption of current food production systems and provides additional environmental benefits.
... It is possible that DAYCENT's underestimation of high N 2 O flux may be due to how the measurement data were integrated to determine cumulative emissions. However, it should be noted that a comparison of continuous versus discontinuous N 2 O emission measurements demonstrated a pattern of underestimation of cumulative emissions using discontinuous data, due to failure to capture transient peaks in the time interval between measurements [52]. In this latter case, DAYCENT simulations of N 2 O emissions would diverge even further from measured estimates in years with high flux. ...
Harvesting crop residue needs to be managed to protect agroecosystem health and productivity. DAYCENT, a process-based modeling tool, may be suited to accommodate region-specific factors and provide regional predictions for a broad array of agroecosystem impacts associated with corn stover harvest. Grain yield, soil C, and N2O emission data collected at Corn Stover Regional Partnership experimental sites were used to test DAYCENT performance modeling the impacts of corn stover removal. DAYCENT estimations of stover yields were correlated and reasonably accurate (adjusted r
2 = 0.53, slope = 1.18, p << 0.001, intercept = 0.36, p = 0.11). Measured and simulated average grain yields across sites did not differ as a function of residue removal, but the model tended to underestimate average measured grain yields. Modeled and measured soil organic carbon (SOC) change for all sites were correlated (adjusted r
2 = 0.54, p << 0.001), but DAYCENT overestimated SOC loss with conventional tillage. Simulated and measured SOC change did not vary by residue removal rate. DAYCENT simulated annual N2O flux more accurately at low rates (≤2-kg N2O-N ha−1 year−1) but underestimated when emission rates were >3-kg N2O-N ha−1 year−1. Overall, DAYCENT performed well at simulating stover yields and low N2O emission rates, reasonably well when simulating the effects of management practices on average grain yields and SOC change, and poorly when estimating high N2O emissions. These biases should be considered when DAYCENT is used as a decision support tool for recommending sustainable corn stover removal practices to advance bioenergy industry based on corn stover feedstock material.
Changing precipitation has the potential to alter nitrous oxide (N2O) emissions from agricultural regions. In this study, we applied the Coupled Model Intercomparison Project phase 5 (CMIP5) end‐of‐century RCP 8.5 (business as usual) precipitation projections for the Upper Midwest U.S. and examined the effects of mean precipitation changes, characterized by increased early season rainfall and decreased mid‐ to late‐season rainfall, on N2O emissions from a conventionally managed corn (Zea mays L.) cropping system grown in an indoor mesocosm facility over four growing seasons. We also assessed the response of N2O emissions to over 1000 individual rain events. N2O emissions were most strongly correlated with WFPS and soil nitrogen status. Following rain events, the change in N2O emissions, relative to pre‐rain emissions, was more likely to be positive when soil NO3– was > 40 mg N kg–1 soil and soil NH4+ was > 10 mg kg–1 soil, yet was more likely to be negative when soil NO3– was > 40 mg N kg–1 soil and soil NH4+ was < 10 mg N kg–1 soil. Similarly, hourly N2O emissions remained below 5 nmol m–2 s–1 when combined NH4+ + NO3– was < 20 mg N kg–1 soil or NH4+ and NO3– were < 5 mg N kg–1 and 20 mg N kg–1 soil, respectively. Rain event magnitude did not substantially affect the change in N2O flux. Finally, growing season N2O emissions, soil moisture, and inorganic N content were not impacted by the future precipitation (FP) pattern. It is near optimal soil WFPS combined with the soil nitrogen concentrations above the identified thresholds that favor higher N2O emissions. This article is protected by copyright. All rights reserved Increasing precipitation in the Midwest U.S. may enhance soil N2O emissions. N2O emissions were higher when WFPS was near optimal and soil N was above identified thresholds. Rain event magnitude did not substantially affect hourly N2O emissions. Total growing season N2O emissions were not impacted by the future precipitation (FP) treatment.
There are aspects in the collection, handling, storage and subsequent analysis of discrete air samples from non‐steady state flux chambers that are critical to generating accurate and unbiased estimates of N2O fluxes. The focus of this paper is on air sample collection and storage in small vials (<12 mL) primarily for Gas Chromatography (GC) analysis. Sample integrity is assured through following simple procedures including storage under pressure and analysis within a few months of collection. Concurrent storage of standards in identical manner to samples is recommended and allows the storage period to be reliably extended. In the laboratory, an autosampler is typically used in batch analysis of around two hundred sequentially analysed samples by GC with an Electron Capture Detector (ECD). Some comparisons are given between GC and alternatives including optical N2O detectors that are increasingly being used for high precision N2O measurement. The importance of calibration and traceability of gas standards is discussed, where high quality standards ensure the most accurate assessment of N2O concentration and comparability between laboratories. The calibration allows a consistent and best estimate of flux to be derived. This article is protected by copyright. All rights reserved
This study seeks to quantify the roles of soybean and corn plants and the cropland ecosystem in the regional N2O budget of the Upper Midwest, USA. The N2O flux was measured at three scales (plant, the soil-plant ecosystem, and region) using newly designed steady-state flow-through plant chambers, a flux-gradient micrometeorological tower, and continuous tall-tower observatories. Results indicate that the following. (1) N2O fluxes from unfertilized soybean (0.03 ± 0.05 nmol m(-2) s(-1)) and fertilized corn plants (-0.01 ± 0.04 nmol m(-2) s(-1)) were about one magnitude lower than N2O emissions from the soil-plant ecosystem (0.26 nmol m(-2) s(-1) for soybean and 0.95 nmol m(-2) s(-1) for corn), confirming that cropland N2O emissions were mainly from the soil. (2) Fertilization increased the corn plant flux for a short period (about 20 days), and late-season fertilization dramatically increased the soybean plant emissions. (3) The direct N2O emission from cropland accounted for less than 20 % of the regional flux, suggesting a significant influence by other sources and indirect emissions, in the regional N2O budget.
Corn stover removal, whether for silage, bedding, or bioenergy production, could have a variety of environmental consequences through its effect on soil processes, particularly N2O production and soil respiration. Because these effects may be episodic in nature, weekly snapshots with static chambers may not provide a complete picture. We adapted commercially available automated soil respiration chambers by incorporating a portable N2O analyzer, allowing us to measure both CO2 and N2O fluxes on an hourly basis through two growing seasons in a corn field in southern Minnesota, from spring 2010 to spring 2012. This site was part of a USDA multilocation research project for five growing seasons, 2008–2012, with three levels of stover removal: zero, full, and intermediate. Initially in spring 2010, two chambers were placed in each of the treatments, but following planting in 2011, the configuration was changed, with four chambers installed on zero removal plots and four on full removal plots. The cumulative data revealed no significant difference in N2O emission as a function of stover removal. CO2 loss from the full removal plots was slightly lower than that from the zero removal plots, but the difference between treatments was much smaller than the amount of C removed in the residue, implying loss of soil carbon from the full removal plots. This is consistent with soil sampling data, which showed that in five of six sampled blocks, the SOC change in the full removal treatments was negative relative to the zero removal plots. We conclude that (a) full stover removal may have little impact on N2O production, and (b) while it will reduce soil CO2 production, the reduction will not be commensurate with the decrease in fresh carbon inputs and, thus, will result in SOC loss.
[1] Nitrous oxide (N2O) is a greenhouse gas with a large global warming potential and is a major cause of stratospheric ozone depletion. Croplands are the dominant source of N2O, but mitigation strategies have been limited by the large uncertainties in both direct and indirect emission factors (EFs) implemented in “bottom-up” emission inventories. The Intergovernmental Panel on Climate Change (IPCC) recommends EFs ranging from 0.75% to 2% of the anthropogenic nitrogen (N) input for the various N2O pathways in croplands. Consideration of the global N budget yields a much higher EF ranging between 3.8% and 5.1% of the anthropogenic N input. Here we use 2 years of hourly high-precision N2O concentration measurements on a very tall tower to evaluate the IPCC bottom-up and global “top-down” EFs for a large representative subsection of the United States Corn Belt, a vast region spanning the U.S. Midwest that is dominated by intensive N inputs to support corn cultivation. Scaling up these results indicates that agricultural sources in the Corn Belt released 420±50 Gg N (mean ±1 standard deviation; 1 Gg =109 g) in 2010, in close agreement with the top-down estimate of 350±50 Gg N and 80% larger than the bottom-up estimate based on the IPCC EFs (230 ± 180 Gg N). The large difference between the tall tower measurement and the bottom-up estimate implies the existence of N2O emission hot spots or missing sources within the landscape that are not fully accounted for in the IPCC and other bottom-up emission inventories. Reconciling these differences is an important step toward developing a practical mitigation strategy for N2O.
Nitrous oxide (N2O) is an important greenhouse gas that is emitted from soil, but obtaining precise N2O source and sink strength estimates has been difficult due to high spatial and temporal flux variability and a poor understanding of the mechanisms controlling fluxes. Tools that improve our ability to quantify trace gas fluxes from soil and constrain annual budgets are therefore needed. Here we describe an improved chamber-based sampling system that continuously traps evolving soil gases onto molecular sieve thereby obtaining a single sample that integrates fluxes over extended periods (several weeks or more) and the use of stable isotopic methods to study microbial origins of N2O. We demonstrate that N2O can be trapped on molecular sieve within our chamber system with near 100% recovery and without isotopic fractionation. In field trials the site preference of N2O (the difference in δ15N between the central and outer N atoms) varied between −6 and 14.4‰, indicating that the majority of flux was derived from bacterial denitrification. Further development with automation would improve flux estimates by providing a system capable of capturing episodic flux events owing to long-term deployment. Further, an automated trapping chamber approach will also provide process-based understanding of N2O dynamics via stable isotopes and a new and affordable tool for evaluating the response of trace gas fluxes to land management practices.
The exchange of gases between soil and atmosphere is an important process that affects atmospheric chemistry and therefore climate. The static-chamber method is the most commonly used technique for estimating the rate of that exchange. We examined the method under hypothetical field conditions where diffusion was the only mechanism for gas transport and the atmosphere outside the chamber was maintained at a fixed concentration. Analytical and numerical solutions to the soil gas diffusion equation in one and there dimensions demonstrated that gas flux density to a static chamber deployed on the soil surface was less in magnitude than the ambient exchange rate in the absence of the chamber. This discrepancy, which increased with chamber deployment time and air-filled porosity of soil, is attributed to two physical factors: distortion of the soil gas concentration gradient (the magnitude was decreased in the vertical component and increased in the radial component) and the slow transport rate of diffusion relative to mixing within the chamber. Instantaneous flux density to a chamber decreased continuously with time; steepest decreases occurred so quickly following deployment and in response to such slight changes in mean chamber headspace concentration that they would likely go undetected by most field procedures. Adverse influences of these factors were reduced by mixing the chamber headspace, minimizing deployment time, maximizing the height and radius of the chamber, and pushing the rim of the chamber into the soil. 29 refs., 8 figs., 1 tab.
The vast majority of soil N2O flux data reported in the literature was obtained using non-flow-through non-steady-state (NFT-NSS) chambers. Considerable variation in chamber methodology may influence N2O flux measurements, however, raising concerns about the reliability and accuracy of these measurements. The objectives of this study were to determine criteria for assessing the quality of soil N2O flux measurements made using NFT-NSS chambers, to evaluate NFT-NSS chamber methodologies used in the scientific literature, and to propose a minimum set of criteria for NFT-NSS chamber design and deployment methodology. We identified 16 characteristics of chamber methodology and developed four factors contributing to the quality of N2O flux measurements made using NFT-NSS chambers. We compiled a data set of 356 studies and evaluated the quality of each study against the set of characteristics and factors to determine the confidence in the reported N2O flux. Confidence in the absolute flux values reported in about 60% of the studies was estimated to be very low or low due to poor methodologies or incomplete reporting. The confidence in flux measurements improved with time; however, there were still about 50% of recent studies (2005-2007) with low or very low confidence levels. This study has shown that the quality of sod N2O flux measurements reported in the literature is often poor. While the flux data obtained may be valid for comparisons between situations (e.g., treatments) within a given study, they are often biased estimates of actual fluxes. We propose a minimum set of criteria for reliable soil N2O flux measurements using NFT-NSS chambers.
Trace gas fluxes often show temporal variability on the order of hours and accurate quantification may be difficult without continuous or near-continuous measurements. We developed an automated near-continuous trace gas analysis system (NCTGAS) to measure soil-atmosphere gas fluxes on a several-times-per-day basis. In this system, air is circulated in a dosed sample loop between fully automated flow-through chambers and a photoacoustic infrared trace gas analyzer (TGA). The TGA quantifies infrared active gases at ambient levels within 2 to 3 min. We tested sensitivity, stability, and calibration of the TGA, and the ability of the NCTGAS to measure fluxes of CO2 and N2O. In addition to static tests, fluxes of CO2 and N2O were simulated by bleeding known quantities of these gases into a test chamber. Gas samples were simultaneously analyzed by TGA and removed for independent analysis of CO2 by conventional infrared gas analysis and for N2O by gas chromatography. The TGA-based flux measurements were statistically identical to the independent measurements of both CO2 and N2O. In situ fluxes of CO2 and N2O measured by the NCTGAS were 105 ± 6 and 93 ± 10%, respectively, of those measured from hand-drawn samples. The TGA was as or more stable than conventional means for measuring CO2 and N2O in air at ambient concentrations, and was equally sensitive across the range of concentrations normally encountered in field measurements. Fast response time and ease of use offers significant advantages over conventional gas chromatography.
The 2 commonly used methods were assessed by directly comparing the sodalime method with a static chamber technique using gas chromatographic analysis of CO2-concentration changes during short-term incubations. A paired-chamber sampling design was applied in 11 different forest stands across a range of soil-CO2 efflux rates. No consistent differences in measured rates of CO2 efflux were observed between the 2 methods, indicating that their method may be applied to the range of mean daily rates encountered in this study. A large number of chambers can be deployed with the soda-lime method, but no information on diel variations in efflux rates is obtained. The short-term incubation technique, on the other hand, can be utilized to simultaneously monitor several trace gases and diel trends in flux rates. -from Authors
Recent concern over N 2 O losses from soils has highlighted the need for new techniques for measuring N 2 O exchange in the field. Two systems are described, in which nondispersive infrared gas analysis is used to measure the change in N 2 O concentration of air passing through a cylindrical chamber driven 0.1 m into the ground. When interfering gases are removed from the air stream, the infrared analyzer has a resolution of ± 12 ppb N 2 O.
In one system, air circulates in a closed loop between the chamber air space and the gas analyzer; in the other, open system, outside air is drawn continuously through the chamber space and its N 2 O enrichment or depletion measured.
Two operational problems (common to many emission chambers) were encountered: the development of a low pressure in the chamber when air is withdrawn, which induces a mass flow of N 2 O from the soil in addition to the diffusive flow; and a readjustment of the N 2 O concentration in the soil air whenever the N 2 O concentration in the chamber air changes from ambient. These can lead to incorrect estimates of the flux. The former problem was overcome by chamber design. The latter appeared to be insurmountable in the closed system, but could be reduced to acceptable proportions in the open system by choice of air flow rate.
The open system permits relatively rapid equilibration, automatic and continuous operation, a discrimination less than 2 ng N m ⁻² sec ⁻¹ , and the maintenance of environmental conditions within the chamber close to those in the field.
The effects of global climate change due to increased greenhouse gas emissions are a growing concern. It is important that technology-based solutions be evaluated in the event they need to be implemented. One method of mitigating anthropogenic CO2 emissions is through geologic storage or sequestration. Most efforts involving Carbon Capture and Storage (CCS) focus on CO2 injection in deep geologic formations. Two issues that arise with CCS technology are public acceptance and monitoring of the horizontal and vertical movements of the CO2 plume. With most CCS sites the risk for leak will be extremely low; however, it will be critical to assure the public of their safety. One way to mitigate public concern is through an integrated surface monitoring campaign. Natural surface background CO2 fluxes must be understood to characterize potential leak sites. LI-COR Biosciences has developed the technology to aid researchers in monitoring potential surface leak sites in areas where CCS is implemented. The LI-8100 Automated Soil CO2 Flux System measures diffusion of CO2 from the soil into the atmosphere, and the LI-7500 Open Path CO2/H2O Analyzer, when integrated with a sonic anemometer, can measure the CO2 flux over a large area. In this paper we discuss these two monitoring methods in detail and present data that illustrates the need for natural surface flux measurements in areas where CCS will be implemented. We will also provide examples of how this technology is currently being used for monitoring CO2 injections at geologic storage sites.
Highest rates of N2O emissions from fertilized as well as natural ecosystems have often been measured at spring thaw. But, it is not clear if management practices have an effect on winter and spring thaw emissions, or if measurements conducted over several years would reveal different emission patterns depending on winter conditions. In this study, we present N2O fluxes obtained using the flux-gradient approach over four winter and spring thaw periods, spanning from 1993 to 1996, at two locations in Ontario, Canada. Several agricultural fields (bare soil, barley, soybean, canola, grass, corn) subjected to various management practices (manure and nitrogen fertilizer addition, alfalfa ploughing, fallowing) were monitored. Nitrous oxide emissions from these fields from January to April over four years ranged between 0 and 4.8 kg N ha-1. These thaw emissions are substantial and should be considered in the nitrous oxide budgets in regions where thaw periods occur. Our study indicates that agricultural management can play a role in mitigating these emissions. Our data show that fallowing, manure application and alfalfa incorporation in the fall lead to high spring emissions, while the presence of plants (as in the case of alfalfa or grass) can result in negligible emissions during thaw. This presents an opportunity for mitigation of N2O emissions through the use of over-wintering cover crops.
Methane emissions measured at three subarctic fen sites by dynamic and static chambers were compared; in the dynamic chambers, the air was circulated at a wind speed of 1.9 m s. Emissions ranged from 7 to 214 mg CH4 m d and measurements from the two types of chamber were strongly correlated (r = 0.72), with no overall difference between the means (paired t‐test, p = 0.34) and with only 4 of the 14 comparisons showing statistically different means (t‐test, p < 0.25). The overall ratio of dynamic: static chamber fluxes was 1.24, but was highest (1.68) at the wettest, central site and lowest (0.74) at the driest, edge site. The coefficients of variation of chamber flux measurements at each site ranged from 0.13 to 1.77, with an overall average of 0.53; sampling with over 30 static chambers revealed approximately normal distributions at the edge and middle sites and a positively skewed distribution at the central site. Within both static and dynamic chambers, methane concentrations increased linearly through 24 h. These inexpensive, portable static chambers can be used to replicate methane emission measurements within a wide range of wetland sites.
The theory, applications, strengths and weaknesses of approaches commonly used for measuring trace gas fluxes are reviewed.
Chambers, representing the smallest scale (∼1m2), are the most common tools. Their operating principle is simple, they can be highly sensitive, the cost can be low and field
requirements small. Problems include leaks, stickiness of some gases, inhibition of fluxes through concentration build-up,
pressure effects and spatial and temporal variability in gas fluxes. Mass balance techniques are suitable for small, defined
source areas, typically tens to thousands of square metres in extent. Emissions are calculated from the difference in the
rates at which the gas is carried into a control volume above the source area by the wind and carried out. The required primary
data are profiles of gas concentration on the downwind boundaries as well as the wind speed profile, the wind direction and
the upwind background gas concentration. They have been used to measure gas emissions from landfills, treated fields and small
animal herds. Circular test areas make the method independent of wind direction. A newly developed technique based on a backward
Lagrangian stochastic dispersion model is also applicable to small, well-defined source areas of any shape. The surface flux
is calculated form measurements of atmospheric turbulence and stability and the gas concentration at any height downwind.
Implementation of the method is aided greatly by a software package WindTrax. The combination provides a powerful new tool
for measuring gas emissions from treated areas and intensive animal production systems. Finally, techniques suitable for measuring
gas emissions on large landscape scales (ha) are discussed. Eddy covariance is the micrometeorologist’s preferred technique
for this scale. The method uses fast response anemometers and gas sensors to make direct measurements of the vertical gas
flux at a point, several times a second. However, it is not feasible for many trace gases for a variety of reasons. These
are discussed. Relaxed eddy accumulation is an alternative technique that retains the attraction of eddy covariance by providing
a direct point measurement. It removes the need for a fast response gas sensor by substituting for it a fast solenoid valve
sampling system. Flux–gradient methods are in more common use. Fluxes are calculated as the product of an eddy diffusivity
and the vertical concentration gradient of the gas or the product of a transfer coefficient and the difference in gas concentration
between two heights. Assumptions of the method and precautions in its application are discussed.
Spring time soil nitrous oxide (N<sub>2</sub>O) fluxes were measured in an old beech ( Fagus sylvatica L.) forest with eddy covariance (EC) and chamber techniques. The aim was to compare the two techniques and to test whether EC can be used in the trunk-space of the forest to measure N<sub>2</sub>O. Mean N<sub>2</sub>O fluxes over the five week measurement period were 5, 10 and 16 ?g N m<sup>-2</sup>h<sup>-1</sup> from EC, automatic chamber and manual chambers, respectively. When data from one hot spot chamber was excluded the mean N<sub>2</sub>O flux of 8 ?g N m<sup>-2</sup>h<sup>-1</sup> from the soil chambers nearly equaled to the mean flux of 7 ?g N m<sup>-2</sup>h<sup>-1</sup> measured with EC from the direction were soil chambers located. Spatial variability in the N<sub>2</sub>O emissions was high in soil chamber measurements, while the EC integrated over this spatial variability and suggested that N<sub>2</sub>O emissions were uniform within the footprint area. The highest emissions measured with the EC occurred during the first week of May when the trees were leafing and when soil moisture content was at its highest. To our knowledge, this is the first study to demonstrate that the EC technique can be used to measure N<sub>2</sub>O fluxes in the trunk-space of a forest. If chamber techniques are used to estimate ecosystem level N<sub>2</sub>O emissions from forest soils, placing of the chambers should be considered carefully to cover the heterogeneity in the soil N<sub>2</sub>O emissions.
Eddy covariance (EC) flux measurements of nitrous oxide obtained by using a 3-D sonic anemometer and a tunable diode laser gas analyzer for N2O were investigated. Two datasets (Sorø, Denmark and Kalevansuo, Finland) from different measurement campaigns including sub-canopy flux measurements of energy and carbon dioxide are discussed with a focus on selected quality control aspects and flux error analysis. Although fast response trace gas analyzers based on spectroscopic techniques are increasingly used in ecosystem research, their suitability for reliable estimates of eddy covariance fluxes is still limited, and some assumptions have to be made for filtering and processing data. The N2O concentration signal was frequently dominated by offset drifts (fringe effect), which can give an artificial extra contribution to the fluxes when the resulting concentration fluctuations are correlated with the fluctuations of the vertical wind velocity. Based on Allan variance analysis of the N2O signal, we found that a recursive running mean filter with a time constant equal to 50~s was suitable to damp the influence of the periodic drift. Although the net N2O fluxes over the whole campaign periods were quite small at both sites (~5 μg N m−2 h−1 for Kalevansuo and ~10 μg N m−2 h−1 for Sorø), the calculated sub-canopy EC fluxes were in good agreement with those estimated by automatic soil chambers. However EC N2O flux measurements show larger random uncertainty than the sensible heat fluxes, and classification according to statistical significance of single flux values indicates that downward N2O fluxes have larger random error.
Soil N2O emissions from three corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] systems in central Iowa were measured from the spring of 2003 through February 2005. The three managements systems evaluated were full-width tillage (fall chisel plow, spring disk), no-till, and no-till with a rye (Secale cereale L. 'Rymin') winter cover crop. Four replicate plots of each treatment were established within each crop of the rotation and both crops were present in each of the two growing seasons. Nitrous oxide fluxes were measured weekly during the periods of April through October, biweekly during March and November, and monthly in December, January, and February. Two polyvinyl chloride rings (30-cm diameter) were installed in each plot (in and between plant rows) and were used to support soil chambers during the gas flux measurements. Flux measurements were performed by placing vented chambers on the rings and collecting gas samples 0, 15, 30, and 45 min following chamber deployment. Nitrous oxide fluxes were computed from the change in N2O concentration with time, after accounting for diffusional constraints. We observed no significant tillage or cover crop effects on N2O flux in either year. In 2003 mean N2O fluxes were 2.7, 2.2, and 2.3 kg N2O-N ha(-1) yr(-1) from the soybean plots under chisel plow, no-till, and no-till + cover crop, respectively. Emissions from the chisel plow, no-till, and no-till + cover crop plots planted to corn averaged 10.2, 7.9, and 7.6 kg N2O-N ha(-1) yr(-1), respectively. In 2004 fluxes from both crops were higher than in 2003, but fluxes did not differ among the management systems. Fluxes from the corn plots were significantly higher than from the soybean plots in both years. Comparison of our results with estimates calculated using the Intergovernmental Panel on Climate Change default emission factor of 0.0125 indicate that the estimated fluxes underestimate measured emissions by a factor of 3 at our sites.
METHANE and nitrous oxide are long-lived, radiatively active trace gases that account for approximately 20% of the total anticipated atmospheric warming 1. The atmospheric concentrations of both gases have increased dramatically over the past few decades, and continue to increase at a rate of approximately 1.1 and 0.25% yr-1 for CH4 (ref. 2) and N2O (ref. 3) respectively. Increased biospheric production is generally suggested as the reason for the increases, but decreases in global sinks may also be important. It has been suggested, for example, that nitrogen fertilization may decrease the rate at which tropical 4,5 and temperate forest soils 6 take up methane from the atmosphere. Furthermore, the recent extensive changes in land management and cultivation could be contributing to the observed increases in both atmospheric CH4 and N2O, as has been suggested for tropical soils 7. Little information exists on CH4 uptake in temperate grasslands (which currently occupy approximately 8% of the Earth's surface), its relation to N2O production, or the effect of land management or cultivation 8,9. Here we report measurements of CH4 uptake and N2O emissions in native, nitrogen-fertilized and wheat-growing prairie soils from spring to late autumn, 1990. We found that nitrogen fertilization and cultivation can both decrease CH4 uptake and increase N2O production, thereby contributing to the increasing atmospheric concentrations of these gases.
Eddy covariance and static chambers are different-scale methods for monitoring agricultural N(2)O that, when used together on heterogeneous agricultural landscapes, can help identify flux sources and sinks and evaluate the effect of management interventions on landscape-scale N(2)O emissions. This study compared the N(2)O flux data obtained by eddy covariance and static chambers during a short-term N(2)O measurement campaign from two adjacent agricultural treatments: alfalfa (Medicago sativa L.) and corn (Zea mays L.) fields. Wind direction data from micrometeorological observations were used to downscale the integrated eddy covariance N(2)O flux and estimate the treatment contributions. The N(2)O data from static chambers installed on each treatment were used to verify the partitioned eddy covariance fluxes. Both methods consistently showed greater emissions for the alfalfa field, which received more N fertilizer earlier in the growing season. Two methods were also compared with respect to the landscape-integrated N(2)O flux measured at the eddy covariance mast location. Upscaling the chamber N(2)O fluxes was performed by totaling the contributions from individual chambers weighted toward the source area share associated with their field locations using a simple footprint model. The comparison of the chambers' total to the measured eddy covariance emissions showed a difference of 7 to 33% between the methods. The best agreement was observed when the integrated eddy covariance flux was associated with uniform wind direction and a homogeneous source area. The results suggest that localization of the flux source using wind directions and footprint information can help in comparing different-scale N(2)O emissions.
Temporal trends of N2O fluxes across the soil–atmosphere interface were determined using continuous flux chamber measurements over an entire growing season of a subsurface aerating macrophyte (Phalaris arundinacea) in a nonmanaged Danish wetland. Observed N2O fluxes were linked to changes in subsurface N2O and O2 concentrations, water level (WL), light intensity as well as mineral‐N availability. Weekly concentration profiles showed that seasonal variations in N2O concentrations were directly linked to the position of the WL and O2 availability at the capillary fringe above the WL. N2O flux measurements showed surprisingly high temporal variability with marked changes in fluxes and shifts in flux directions from net source to net sink within hours associated with changing light conditions. Systematic diurnal shifts between net N2O emission during day time and deposition during night time were observed when max subsurface N2O concentrations were located below the root zone. Correlation (P O2 concentrations and incoming photosynthetically active radiation highlighted the importance of plant‐driven subsoil aeration of the root zone and the associated controls on coupled nitrification/denitrification. Therefore, P. arundinacea played an important role in facilitating N2O transport from the root zone to the atmosphere, and exclusion of the aboveground biomass in flux chamber measurements may lead to significant underestimations on net ecosystem N2O emissions. Complex interactions between seasonal changes in O2 and mineral‐N availability following near‐surface WL fluctuations in combination with plant‐mediated gas transport by P. arundinacea controlled the subsurface N2O concentrations and gas transport mechanisms responsible for N2O fluxes across the soil–atmosphere interface. Results demonstrate the necessity for addressing this high temporal variability and potential plant transport of N2O in future studies of net N2O exchange across the soil–atmosphere interface.
Changes in the hydrological cycle, as predicted and currently observed, are expected to significantly impact the water and carbon balance of water-limited forest ecosystems. However, differences in the water-sensitivity of component processes make carbon balance predictions challenging. To examine responses of ecosystem components to water limitations, we conducted a study of tree, soil and ecosystem-level processes in a young ponderosa pine stand under natural summer drought (control) and increased soil water conditions (watered). Weekly-averaged tree transpiration (Ttree), gross ecosystem photosynthesis (GPP) and soil CO2 efflux (Rstree; nearby trees) were related with soil water content (SWC; polynomial form: TtreeR2 = 0.98 and RstreeR2 = 0.91, logarithmic form: GPP R2 = 0.86) and declined rapidly when relative extractable soil water (REW) was <50%. The sensitivity of daily variations in canopy conductance (Gs) to vapor pressure deficit was affected by SWC (R2 = 0.97; logarithmic function), decreasing at REW <50%. Watering maintained REW at about 70% in July and August but positively affected tree carbon and water dynamics only at the end of summer when fluxes in the control treatment were strongly water-limited. A tight coupling of above- and belowground fluxes became apparent. In the control treatment, root-rhizosphere respiration (Rr) decreased along with GPP and Ttree (R2 = 0.58) as drought progressed, while watering maintained Rr, Ttree and Gs at a significantly higher level than those of the unwatered trees in late summer. In contrast, microbial respiration responded instantaneously and strongly to the watering compared to the control treatment. The net effect was that increased soil water availability during the typical dry growing season has a negative effect on the short-term seasonal ecosystem C balance due to a larger increase in decomposition than photosynthesis. However, longer-term effects remain uncertain. In summary, our study highlights that understanding the dissimilar response of tree dynamics and soil decomposition to water availability is a key component in predicting future C sequestration in water-limited forest ecosystems.
Automated measurements of forest soil CO2 efflux (F) using non-steady-state chamber systems are necessary to study the short- and long-term responses of soil respiration to temporal variations in abiotic and biotic variables. Increased use of automated chamber systems in regional flux networks results in large data sets that demand an efficient and reliable protocol to ensure good quality measurements, efficient and robust calculations, and post-processing data-quality control.Using half-hourly measurements and simulations with a process-based model, we show that underestimation of efflux due to disturbance of the soil CO2 diffusion gradient arising from chamber closure for periods of up to 3 min is much less (<4%) than is often assumed. Also, we found that use of simple linear regression for calculating the rate of change in the chamber headspace concentration is the best method in comparison to non-linear models; it is robust and, for lid closure periods <3% of the chamber–soil system time constant, results in <2% underestimation of the efflux, which is smaller than the overestimation using some non-linear methods.The effective volume of a chamber is significantly higher than its geometric volume and varies markedly seasonally so its determination is important for accurate efflux measurements. A procedure is described for determining the effective volume, which we recommend should be followed at least once a day. We also describe various steps to ensure accurate measurements, including the use of a seasonal threshold value of the ratio of root mean square error of the linear fit to headspace concentration versus time to its slope for rejecting questionable measurements, and demonstrate the use of a procedure combining the use of automated and manual quality assurance/quality control in removing questionable measurements.
The large temporal variation in nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) flux rates is a major source of error when estimating cumulative fluxes of these radiative active trace gases. We developed an automated system for near-continuous, long-term measurements of N2O, CH4 and CO2 fluxes from cropland soils and used it to study the temporal variation of N2O and CH4 fluxes from potato (Solanum tuberosum L.) fields during the crop periods of 1997 and 1998, and also to determine the effects of management practices and weather. Additionally, we evaluated the error of other common methods, namely, weekly or monthly measurements, used for estimating cumulative fluxes. The fluxes were quantified separately for the ridges, uncompacted interrows and tractor-compacted interrows. Total N2O–N emission from the potato field during the growing period (end of May to September) was 1.6 kg ha−1 in 1997 and 2.0 kg ha−1 in 1998; emissions were highest for the tractor-compacted soil. Periods of increased N2O losses were induced by heavy precipitation (in particular in compacted soil) and by the killing of potato tops (on the ridges) by herbicide application. The total CH4–C uptake in the potato field during the growing period was 295 g ha−1 in 1997 and 317 g ha−1 in 1998. The major fraction of the total CH4 uptake (≈86%) occurred on the ridges. Weekly measurements of N2O fluxes complemented by additional event-related flux determinations provided accurate estimates of total emissions. The monthly flux determination was not adequate for determining the temporal variation of the N2O emission rates. Weekly measurements were sufficient to provide reliable estimates of the cumulative CH4 uptake.
The stable carbon isotope ratio, CO132/CO122, is a valuable tracer for studying the processes controlling the autotrophic (FRa) and heterotrophic (FRh) contributions to ecosystem respiration (FR) and the influence of photosynthesis on FR. There is increasing interest in quantifying the temporal variability of the carbon isotope composition of ecosystem respiration (δR) because it contains information about the sources contributing to respiration and is an important parameter used for partitioning net ecosystem CO2 exchange using stable isotope methods. In this study, eddy covariance, flux gradient, automated chambers, and stable carbon isotope techniques were used to quantify and improve our understanding of the temporal variability in FR and δR in a C3/C4 agricultural ecosystem. Six years (2004–2009) of isotope flux-gradient measurements indicated that δR had a very consistent annual pattern during both C3 (soybean) and C4 (corn) growing seasons due to significant contributions from FRa, which was strongly influenced by the isotope composition of the recent photosynthate. However, in the spring, δR exhibited a C3 signal regardless of the crop grown in the previous season. One hypothesis for this anomaly is that at these low soil temperatures microbial activity relied predominantly on C3 substrates. Automated chamber measurements of soil respiration (FRs) and its isotope composition (δRs) were initiated in the early corn growing season of 2009 to help interpret the variability in δR. These measurements showed good agreement with EC measurements of FR (within 0.5μmolm2s−1) and isotope flux gradient measurements of δR (within 2‰) at nighttime for near-bare soil conditions (LAI
Three different tropical rain forest sites (Kauri Creek, Lake Eacham, and Massey Creek) on the Atherton Tablelands, Queensland, Australia, were investigated for the magnitude of N2O emission from soils during different seasons, that is, wet season, dry season, and transition periods. Highest mean N2O emission rates were observed for soils derived from granite at the Kauri Creek site with 74.5+/-25.2mug N2O-Nm-2h-1, whereas for soils derived from Metamorphics (Lake Eacham site) mean N2O emission rates were much lower (13.1+/-1.1mug N2O-Nm-2h-1). For the Massey Creek site, with soils derived from Rhyolite, a mean annual N2O emission rate of 46.2+/-1.1mug N2O-Nm-2h-1 was calculated. The mean annual N2O emission rate calculated for all three sites over the entire observation period was 39.0mug N2O-Nm-2h-1 and thus at the high end of reports from tropical rain forest soils. N2O emission rates showed at all sites pronounced temporal as well as spatial variability. The magnitude of N2O emissions was strongly linked to rainfall events; that is, N2O emissions strongly increased approximately 6-8 hours after precipitation. Correlation analysis confirmed the strong dependency of N2O emissions on changes in soil moisture, whereas changes in soil temperature did not mediate considerable changes in N2O fluxes. Spatial variability of N2O fluxes on a site scale could be explained best by differences in water-filled pore space, CO2 emission, and C/N ratio of the soil. On the basis of all published N2O flux rates from tropical rain forest soils we recalculated the contribution of such forests to the global atmospheric N2O budget and come up with a figure of 3.55Tg N2O-Nyr-1, which is approximately 50% higher than reported by others.
A novel quantum cascade laser absorption spectrometer (QCL-AS) was tested to monitor N2O exchange fluxes over an intensively managed grassland using the eddy covariance approach. The instrument employs a continuous wave quantum cascade laser to scan over the absorption features of N2O, CH4 and water vapor at 7.8μm. The precision of the N2O flux measurements was determined to be 0.2nmolm−2s−1 but the accuracy can easily be affected by water vapor interferences twice as large.These water vapor interferences are not only due to the respective gas dilution effect but also due to an additional cross-sensitivity of the N2O analyzer to water vapor (0.3ppb N2O/% H2O). Both effects cause a negative bias of similar magnitude (0.3nmolm−2s−1 N2O flux/mmolm−2s−1 H2O flux) in the flux measurements. While the dilution (or density) correction is a well known and routinely applied procedure, the magnitude of the analyzer cross-talk may depend on the specific instrumental setup and should be empirically determined. The comparison with static chamber measurements shows the necessity of the cross-talk correction; otherwise the QCL-AS based eddy covariance system would yield unrealistically large uptake of N2O.
For 3 years we followed the complete annual cycles of N2O emission rates with 2-hour resolution in spruce and beech plantations of the Höglwald Forest, Bavaria, Germany, in order to gain detailed information about seasonal and interannual variations of N2O emissions. In addition, microbiological process studies were performed for identification of differences in N turnover rates in the soil of a spruce and a beech site and for estimation of the contribution of nitrification and denitrification to the actual N2O emission. Both pronounced seasonal and extreme interannual variations of N2O emissions were identified. During long-term frost periods, while the soil was frozen, and during soil thawing, extremely high N2O emissions occurred, contributing up to 73% to the total annual N2O loss. The enormous N2O releases during the long-term frost period were due to high microbial N turnover rates (tight coupling of ammonification, nitrification, denitrification) in small unfrozen water films of the frozen soil at high concentrations of easily degradable substrates derived from the enormous pool of dead microbial biomass produced during the long-term frost period. Liming of a spruce site resulted in a significant increase in ammonification, nitrification, and N2O emissions as compared with an untreated spruce control site. The beech control site exhibited 4-5 times higher N2O emissions than the spruce control site, indicating that forest type itself is an important modulator of N2O release from soil. At all sites, nitrification contributed ~70% to the N2O flux, whereas denitrification contributed markedly less (~30%). There was a significant positive correlation between amount of in situ N input by wet deposition and magnitude of in situ N2O emissions. At the beech site, 10% of the actual N input was released from the soil in form of N2O, whereas at the spruce site the fraction was 0.5%. N2O emission rates were positively correlated with net nitrification rates. The results demonstrate the need for long-term measurements over several years for more precise estimates of annual N2O losses from forest ecosystems. On the basis of our results we conclude that the importance of temperate and boreal forests for the global N2O source strength may have been significantly underestimated in the past and that these forests contribute most likely >>1.0TgN2ON.
The long-term effects of conservation management practices on greenhouse gas fluxes from tropical/subtropical croplands remain to be uncertain. Using both manual and automatic sampling chambers, we measured N2O and CH4 fluxes at a long-term experimental site (1968–present) in Queensland, Australia from 2006 to 2009. Annual net greenhouse gas fluxes (NGGF) were calculated from the 3-year mean N2O and CH4 fluxes and the long-term soil organic carbon changes. N2O emissions exhibited clear daily, seasonal and interannual variations, highlighting the importance of whole-year measurement over multiple years for obtaining temporally representative annual emissions. Averaged over 3 years, annual N2O emissions from the unfertilized and fertilized soils (90 kg N ha−1 yr−1 as urea) amounted to 138 and 902 g N ha−1, respectively. The average annual N2O emissions from the fertilized soil were 388 g N ha−1 lower under no-till (NT) than under conventional tillage (CT) and 259 g N ha−1 higher under stubble retention (SR) than under stubble burning (SB). Annual N2O emissions from the unfertilized soil were similar between the contrasting tillage and stubble management practices. The average emission factors of fertilizer N were 0.91%, 1.20%, 0.52% and 0.77% for the CT-SB, CT-SR, NT-SB and NT-SR treatments, respectively. Annual CH4 fluxes from the soil were very small (−200–300 g CH4 ha−1 yr−1) with no significant difference between treatments. The NGGF were 277–350 kg CO2-e ha−1 yr−1 for the unfertilized treatments and 401–710 kg CO2-e ha−1 yr−1 for the fertilized treatments. Among the fertilized treatments, N2O emissions accounted for 52–97% of NGGF and NT-SR resulted in the lowest NGGF (401 kg CO2-e ha−1 yr−1 or 140 kg CO2-e t−1 grain). Therefore, NT-SR with improved N fertilizer management practices was considered the most promising management regime for simultaneously achieving maximal yield and minimal NGGF.
Drained organic soils contribute substantial amounts of nitrous oxide to the global atmosphere, and we should be able to estimate this contribution. We have investigated when the fluxes of N2O from drained forested or cultivated organic soils could be determined by calculating the fluxes from the concentration gradients of the gas in soil or snow according to Fick's law of diffusion. A static chamber method was applied as a control technique for the gas gradient method. Concentrations of N2O in soil varied from 296 nl l−1 to 8534 nl l−1 during the snow-free periods and were greatest in the early summer. Our results suggest that the gas gradient method can be used to estimate N2O emissions from drained organic soils. There was some systematic difference in the N2O fluxes measured with these two methods, which we attributed to the differences in weather between years 1996 and 1997. In the wet summer of 1996 the chamber method gave greater flux rates than the gas gradient method, and the reverse was true in the dry summer of 1997. In the forest the N2O fluxes measured with the two methods agreed well. The gas gradient is convenient and fast for measuring N2O emissions from fairly dry organic unfrozen soil. In winter the diffusion calculation based on the N2O gradients in snow and the chamber method gave fairly similar flux rates and provided adequate estimates of the fluxes of N2O in winter.
An automated closed-chamber system was developed to measure N2O fluxes in the field. It was deployed at two N-fertilized grassland sites in two successive years, together with replicated manual chambers, to investigate the spatial and temporal variability in fluxes, and the likely impact of sampling frequency on cumulative flux values. The automated system provided flux data at 8-h intervals, while manual sampling was conducted at intervals of 3–7 days. The autochambers showed fluctuations in emissions not detected by manual sampling. However, integrated flux values based on the more intensive measurements were on average no more than 14% greater than those based on data from the autochambers that were obtained at the same time as manual sampling. This difference was not significant and well within the spatial variability determined with manual chambers. If daily sampling intervals were used immediately after fertilization, the agreement was closer still, increasing the confidence that can be placed in manual procedures. Diurnal variations in temperature and flux were small, and results from sampling at mid-day were not significantly different from those based on early morning or evening sampling. Where diurnal fluctuations in temperature and flux are likely to be much larger, the autochamber/sampler system could prove very useful to quantify the effect.
Understanding nitrous oxide (N2O) emissions from agricultural soils in semi-arid regions is required to better understand global terrestrial N2O losses. Nitrous oxide emissions were measured from a rain-fed, cropped soil in a semi-arid region of south-western Australia for one year on a sub-daily basis. The site included N-fertilized (100 kg N ha−1 yr−1) and nonfertilized plots. Emissions were measured using soil chambers connected to a fully automated system that measured N2O using gas chromatography. Daily N2O emissions were low (−1.8 to 7.3 g N2O-N ha−1 day−1) and culminated in an annual loss of 0.11 kg N2O-N ha−1 from N-fertilized soil and 0.09 kg N2O-N ha−1 from nonfertilized soil. Over half (55%) the annual N2O emission occurred from both N treatments when the soil was fallow, following a series of summer rainfall events. At this time of the year, conditions were conducive for soil microbial N2O production: elevated soil water content, available N, soil temperatures generally >25 °C and no active plant growth. The proportion of N fertilizer emitted as N2O in 1 year, after correction for the ‘background’ emission (no N fertilizer applied), was 0.02%. The emission factor reported in this study was 60 times lower than the IPCC default value for the application of synthetic fertilizers to land (1.25%), suggesting that the default may not be suitable for cropped soils in semi-arid regions. Applying N fertilizer did not significantly increase the annual N2O emission, demonstrating that a proportion of N2O emitted from agricultural soils may not be directly derived from the application of N fertilizer. ‘Background’ emissions, resulting from other agricultural practices, need to be accounted for if we are to fully assess the impact of agriculture in semi-arid regions on global terrestrial N2O emissions.
Climatic conditions and cultural practices in the sub-tropical and tropical high-rainfall regions in which sugarcane is grown in Australia are conducive to rapid carbon and nitrogen cycling. Previous research has identified substantial exchanges of methane (CH4) and nitrous oxide (N2O) between sugarcane soils and the atmosphere. However, that research has been mostly short-term. This paper describes recent work aimed at quantifying exchanges of CH4 and N2O from fertilised sugarcane soils over whole growing seasons. Micrometeorological and chamber techniques provided continuous measurements of gas emissions in whole-of-season studies in a burnt-cane crop on an acid sulfate soil (ASS) that was fertilised with 160 kg nitrogen (N) ha−1 as urea in the south of the sugarcane belt (Site 1), and in a crop on a more representative trash-blanketed soil fertilised with 150 kg urea-N ha−1 in the north (Site 2). Site 1 was a strong source of CH4 with a seasonal emission (over 342 days) of 19.9 kg CH4 ha−1. That rate corresponds to 0.5–5% of those expected from rice and wetlands. The many drains in the region appear to be the main source. The net annual emission of CH4 at Site 2 over 292 days was essentially zero, which contradicts predictions that trash-blankets on the soil are net CH4 sinks. Emissions of N2O from the ASS at Site 1 were extraordinarily large and prolonged, totalling 72.1 kg N2O ha−1 (45.9 kg N ha−1) and persisting at substantial rates for 5 months. The high porosity and frequent wetting with consequent high water filled pore space and the high carbon content of the soil appear to be important drivers of N2O production. At Site 2, emissions were much smaller, totalling 7.4 kg N2O ha−1 (4.7 kg N ha−1), most of which was emitted in less than 3 months. The emission factors for N2O (the proportion of fertiliser nitrogen emitted as N2O–N) were 21% at Site 1 and 2.8% at Site 2. Both factors exceed the default national inventory value of 1.25%. Calculations suggest that annual N2O production from Australian sugarcane soils is around 3.8 kt N2O, which is about one-half a previous estimate based on short-term measurements, and although ASS constitute only about 4% of Australia's sugarcane soils, they could contribute about 25% of soil emissions of N2O from sugarcane. The uptake of 50–94 t CO2 ha−1 from the atmosphere by the crops at both sites was offset by emissions of CH4 and N2O to the atmosphere amounting to 22 t CO2-e ha−1 at Site 1 and 2 t CO2-e ha−1 at Site 2.
In the absence of, or between, fertilization events in agricultural systems, soils are generally assumed to emit N2O at a small rate, often described as the ‘background’ flux. In contrast, net uptake of N2O by soil has been observed in many field studies, but has not gained much attention. Observations of net uptake of N2O form a large fraction (about half) of all individual flux measurements in a long-term time series at our temperate fertilized grassland site. Individual uptake fluxes from chamber measurements are often not statistically significant but mean values integrated over longer time periods from days to weeks do show a clear uptake. An analysis of semi-continuous chamber flux data in conjunction with continuous measurements of the N2O concentration in the soil profile and eddy covariance measurements suggests that gross production and gross consumption of N2O are of the same order, and as consequence only a minor fraction of N2O molecules produced in the soil reaches the atmosphere.
Nitrous oxide (N2O) fluxes from soil under mown grassland were monitored using static chambers over three growing seasons in intensively and extensively managed systems in Central Switzerland. Emissions were largest following the application of mineral (NH4NO3) fertilizer, but there were also substantial emissions following cattle slurry application, after grass cuts and during the thawing of frozen soil. Continuous flux sampling, using automatic chambers, showed marked diurnal patterns in N2O fluxes during emission peaks, with highest values in the afternoon. Net uptake fluxes of N2O and subambient N2O concentrations in soil open pore space were frequently measured on both fields. Flux integration over 2.5 years yields a cumulated emission of +4.7 kgN2O-N ha−1 for the intensively managed field, equivalent to an average emission factor of 1.1%, and a small net sink activity of −0.4 kg N2O-N ha−1 for the unfertilized system. The data suggest the existence of a consumption mechanism for N2O in dry, areated soil conditions, which cannot be explained by conventional anaerobic denitrification. The effect of fertilization on greenhouse gas budgets of grassland at the ecosystem level is discussed.
Conventional cropping systems rely on targeted short-term fertility management, whereas organic systems depend, in part, on long-term increase in soil fertility as determined by crop rotation and management. Such differences influence soil nitrogen (N) cycling and availability through the year. The main objective of this study was to compare nitrous oxide (N2O) emissions from soil under winter wheat (Triticum aestivum L.) within three organic and one conventional cropping system that differed in type of fertilizer, presence of catch crops and proportion of N2-fixing crops. The study was replicated in two identical long-term crop rotation experiments on sandy loam soils under different climatic conditions in Denmark (Flakkebjerg—eastern Denmark and Foulum—western Denmark). The conventional rotation received 165–170 kg N ha−1 in the form of NH4NO3, while the organic rotations received 100–110 kg N ha−1 as pig slurry. For at least 11 months, as from September 2007, static chambers were used to measure N2O emissions at least twice every calendar month. Mean daily N2O emissions across the year ranged from 172 to 438 μg N m−2 d−1 at Flakkebjerg, and from 173 to 250 μg N m−2 d−1 at Foulum. A multiple linear regression analysis showed inter-seasonal variations in emissions (P < 0.001), but annual N2O emissions from organic and conventional systems were not significantly different despite the lower N input in organic rotations. The annual emissions ranged from 54 to 137 mg N m−2, which corresponded to 0.5–0.8% of the N applied in manure or mineral fertilizer. Selected soil attributes were monitored to support the interpretation of N2O emission patterns. A second multiple linear regression analysis with potential drivers of N2O emissions showed a negative response to soil temperature (P = 0.008) and percent water-filled pore space (WFPS) (P = 0.052) at Foulum. However, there were positive interactions of both factors with NO3-N, i.e., high N2O emissions occurred during periods when high soil nitrate levels coincided with high soil temperature (P = 0.016) or high soil water content (P = 0.056). A positive effect (P = 0.03) of soil temperature was identified at Flakkebjerg, but the number of soil samplings was limited. Effects of cropping system on N2O emissions were not observed.
System power reliability under varying weather conditions and the corresponding system cost are the two main concerns for designing hybrid solar–wind power generation systems. This paper recommends an optimal sizing method to optimize the configurations of a hybrid solar–wind system employing battery banks. Based on a genetic algorithm (GA), which has the ability to attain the global optimum with relative computational simplicity, one optimal sizing method was developed to calculate the optimum system configuration that can achieve the customers required loss of power supply probability (LPSP) with a minimum annualized cost of system (ACS). The decision variables included in the optimization process are the PV module number, wind turbine number, battery number, PV module slope angle and wind turbine installation height. The proposed method has been applied to the analysis of a hybrid system which supplies power for a telecommunication relay station, and good optimization performance has been found. Furthermore, the relationships between system power reliability and system configurations were also given.
Continuous half-hourly measurements of soil CO2 efflux made between January and December 2001 in a mature trembling aspen stand located at the southern edge of the boreal forest in Canada were used to investigate the seasonal and diurnal dependence of soil respiration (Rs) on soil temperature (Ts) and water content (θ). Daily mean Rs varied from a minimum of 0.1 μmol m−2 s−1 in February to a maximum of 9.2 μmol m−2 s−1 in mid-July. Daily mean Ts at the 2-cm depth was the primary variable accounting for the temporal variation of Rs and no differences between Arrhenius and Q10 response functions were found to describe the seasonal relationship. Rs at 10 °C (Rs10) and the temperature sensitivity of Rs (Q10Rs) calculated at the seasonal time scale were 3.8 μmol m−2 s−1 and 3.8, respectively. Temperature normalization of daily mean Rs (RsN) revealed that θ in the 0–15 cm soil layer was the secondary variable accounting for the temporal variation of Rs during the growing season. Daily RsN showed two distinctive phases with respect to soil water field capacity in the 0–15 cm layer (θfc, ∼0.30 m3 m−3): (1) RsN was strongly reduced when θ decreased below θfc, which reflected a reduction in microbial decomposition, and (2) RsN slightly decreased when θ increased above θfc, which reflected a restriction of CO2 or O2 transport in the soil profile.
Nitrous oxide emissions and selected soil properties in a high and a low yielding area of a maize field were monitored weekly over a 1-year period. In both the high and the low yielding area, N2O emissions from a treatment subject to site-specific N-fertilization were compared to a conventionally fertilized control.Emission peaks were measured following N fertilization, rainfall, harvest, tillage and freeze-thaw cycles from all treatments in conditions favorable for denitrification. Between 80 and 90% of annual emissions were released between April and September. A value of 60% WFPS was identified as a threshold for the induction of elevated N2O emissions (>50 μg N2O-N m−2 h−1). A significant relationship (r2=0.41) between N2O flux rates and WFPS was found when neither soil nitrate contents nor temperature were limiting for microbial denitrification.Mean cumulative N2O emissions from the control treatments in the high yielding area, located in a footslope position and thus receiving lateral water and nutrient supply, more than doubled those from the control treatments in the low yielding area in a shoulder position (8.7 and 3.9 kg N2O-N ha−1, respectively). Higher average WFPS in the high yielding area was identified as responsible for this difference.The site-specific fertilized treatments in the low yielding area were supplied with 125 kg N fertilizer ha−1 as compared to 150 kg N fertilizer ha−1 (control treatments). This reduction resulted in 34% less N2O released in roughly 10 months following differentiated fertilization while crop yield remained the same. In the high yielding area, N fertilizer supply in the site-specific fertilized treatment was 175 kg N ha−1 as compared to 150 kg N ha−1 in the control. Neither crop yield nor N2O emissions were significantly affected by the different fertilizer rates.
An intercomparison that involved a standards intercomparison, interferant spiking tests and simultaneous ambient measurements was carried out between two CO measurement systems: a tunable diode laser absorption spectrometer (TDLAS) and a gas filter correlation, non-dispersive infrared absorption instrument (GFC). Both the TDLAS and the GFC techniques responded to CO. No major interferences were found for the TDLAS system; tested species included H2O, O3 and OCS. The GFC instrument exhibited no interference from H2O or O3, but only a relatively high upper limit could be placed on the O3 interference. For CO measurements in ambient air at levels from 100 to 1500 ppbv, the results from the two instruments agreed within their combined uncertainties. On average the GFC technique was 6% higher than the TDLAS system, and there was no systematic, constant offset. The precision of the GFC instrument was about 10%, and the precision of the TDLAS system was better than 4%.
Key uncertainties remain in accurately measuring soil respiration, including how the commonly-used technique
of collar insertion affects measured soil and root-derived CO2 fluxes. We hypothesized that total soil respiration
is frequently under-estimated because soil collar insertions sever surface roots, which coupled with the
preferential practice of taking daytime measurements, leads to the autotrophic (root-derived) component
frequently being missed. We measured root distribution and soil CO2 efflux in three contrasting ecosystems:
a Lodgepole pine (Pinus contorta) plantation, an upland heather-dominated peatland and a lowland sheep-
grazed grassland, where we combined shallow surface collars with collars at different soil insertion depths for
occasional and continuous hourly flux measurements. Collar insertion by only a few centimetres reduced total
soil CO2 efflux in all three ecosystems by an average of 15% but at times by up to 30–50%, and was directly
proportional to the quantity of cut fine roots. Most reduction occurred in the shallow-rooted peatland system
and least in the deep-rooted grassland. In the forest and grassland, soil temperatures explained most of the
deep-collar (largely heterotrophic) variation and did not relate to the root-derived (largely autotrophic) flux
component, whilst the opposite was true for the peatland site. For the forest, the autotrophic flux component
peaked at night during moist periods and was drought-limited. Mean flux estimates differed between sampling
time and insertion depth. Our results suggest strongly that accurate measurement and modelling of soil
respiration needs explicitly to consider collar insertion, and the root-derived flux component, with its own
temperature sensitivity and potential time-lag effects.
Agricultural management practices that enhance C sequestration, reduce greenhouse gas emission (nitrous oxide [N₂O], methane [CH₄], and carbon dioxide [CO₂]), and promote productivity are needed to mitigate global warming without sacrificing food production. The objectives of the study were to compare productivity, greenhouse gas emission, and change in soil C over time and to assess whether global warming potential and global warming potential per unit biomass produced were reduced through combined mitigation strategies when implemented in the northern U.S. Corn Belt. The systems compared were (i) business as usual (BAU); (ii) maximum C sequestration (MAXC); and (iii) optimum greenhouse gas benefit (OGGB). Biomass production, greenhouse gas flux change in total and organic soil C, and global warming potential were compared among the three systems. Soil organic C accumulated only in the surface 0 to 5 cm. Three-year average emission of N₂O and CH was similar among all management systems. When integrated from planting to planting, N₂O emission was similar for MAXC and OGGB systems, although only MAXC was fertilized. Overall, the three systems had similar global warming potential based on 4-yr changes in soil organic C, but average rotation biomass was less in the OGGB systems. Global warming potential per dry crop yield was the least for the MAXC system and the most for OGGB system. This suggests management practices designed to reduce global warming potential can be achieved without a loss of productivity. For example, MAXC systems over time may provide sufficient soil C sequestration to offset associated greenhouse gas emission.
An automated dynamic closed chamber system for CO(2) sampling and analysis was developed for the measurement of soil respiration under laboratory conditions. The system is composed by a gas chromatograph linked to a fully computerised sampling system composed by 16 sample jars and 2 multiposition valves. Besides CO(2), the system can automatically and simultaneously measure CH(4), N(2)O and other gases of environmental interest. The detection limits of the system for CO(2), N(2)O and CH(4) were 2, 1 and 4ppmv, respectively. The accuracy of the system, expressed as percent bias, was -0.88, -0.94 and -3.17% for CO(2), N(2)O and CH(4), respectively, with relative standard deviation of 0.42, 0.68 and 0.61%. Measurement of CO(2) evolved following acidification of a known amount of reagent grade CaCO(3) showed a standard recovery of 96.8+/-2.5% reached within 30s after acidification. A linear response of CO(2) respiration was obtained for a wide range of operative conditions (5-60min accumulation time, 10-80g soil sample size, 10-60mLmin(-1) air flow rate, 15-25 degrees C temperature of incubation) demonstrating the flexibility of the system, which allows for the measurement of soil samples characterised by different rates of gas evolution. Moreover, the results obtained with soil samples showed that within the above conditions the proposed system is not affected by potential limitations of static closed chamber systems such as CO(2) dissolution in the soil solution, reduced rate of CO(2) diffusion from soil to headspace and CO(2) inhibition of microbial activity. The system was also capable to detect significant changes in N(2)O emissions from soil amended with different amounts of glutamic acid. The automatic and frequent measurements provided by the system make possible an accurate description of the dynamics of gas evolution from soil samples under laboratory conditions.
An ir analyzer employing gas-filter correlation techniques has been designed and constructed to measure the concentrations of CO, NO, SO(2), HCl, and HF in the stacks or ducts of stationary pollutant sources. Use of a retroreflector allows the stack to be double passed, and no sample is extracted. For each gas, small interchangeable fixed-position grating polychromators are used as narrow (~1.5-cm(-1)) multiband spectral filters with the bands corresponding to locations of selected absorption lines. The approximate useful ranges (in parts per million-meters) over which this analyzer operates are 10-4000 for NO, 10-1500 for CO, 50-40,000 for SO(2), 10-2000 for HC1, and 5-200 for HF. The discrimination against other gases and particulates is excellent. The analyzer has been tested in the laboratory and on a variety of pollutant sources.
It is generally recognized that soil N(2)O emissions can exhibit pronounced day-to-day variations; however, measurements of soil N(2)O flux with soil chambers typically are done only at discrete points in time. This study evaluated the impact of sampling frequency on the precision of cumulative N(2)O flux estimates calculated from field measurements. Automated chambers were deployed in a corn/soybean field and used to measure soil N(2)O fluxes every 6 h from 25 Feb. 2006 through 11 Oct. 2006. The chambers were located in two positions relative to the fertilizer bands-directly over a band or between fertilizer bands. Sampling frequency effects on cumulative N(2)O-N flux estimation were assessed using a jackknife technique where populations of N(2)O fluxes were constructed from the average daily fluxes measured in each chamber. These test populations were generated by selecting measured flux values at regular time intervals ranging from 1 to 21 d. It was observed that as sampling interval increased from 7 to 21 d, variances associated with cumulative flux estimates increased. At relatively frequent sampling intensities (i.e., once every 3 d) N(2)O-N flux estimates were within +/-10% of the expected value at both sampling positions. As the time interval between sampling was increased, the deviation in estimated cumulative N(2)O flux increased, such that sampling once every 21 d yielded estimates within +60% and -40% of the actual cumulative N(2)O flux. The variance of potential fluxes associated with the between-band positions was less than the over-band position, indicating that the underlying temporal variability impacts the efficacy of a given sampling protocol.
Nitrous oxide emissions from a beech forest floor measured by eddy covariance and soil enclosure techniques
- Pihlatie