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Soil CO2 Emissions as Affected by 20-Year Continuous Cropping in Mollisols
Abstract
Long-term continuous cropping of soybean (Glycine max), spring wheat (Triticum aesativum) and maize (Zea mays) is widely practiced by local farmers in northeast China. A field experiment (started in 1991) was used to investigate the differences in soil carbon dioxide (CO2) emissions under continuous cropping of the three major crops and to evaluate the relationships between CO2 fluxes and soil temperature and moisture for Mollisols in northeast China. Soil CO2 emissions were measured using a closed-chamber method during the growing season in 2011. No remarkable differences in soil organic carbon were found among the cropping systems (P>0.05). However, significant differences in CO2 emissions from soils were observed among the three cropping systems (P<0.05). Over the course of the entire growing season, cumulative soil CO2 emissions under different cropping systems were in the following order: continuous maize ((829±10) g CO2 m−2)>continuous wheat ((629±22) g CO2 m−2)>continuous soybean ((474±30) g CO2 m−2). Soil temperature explained 42–65% of the seasonal variations in soil CO2 flux, with a Q10 between 1.63 and 2.31; water-filled pore space explained 25–47% of the seasonal variations in soil CO2 flux. A multiple regression model including both soil temperature (T, °C) and water-filled pore space (W, %), log(f)=a+bT log(W), was established, accounting for 51–66% of the seasonal variations in soil CO2 flux. The results suggest that soil CO2 emissions and their Q10 values under a continuous cropping system largely depend on crop types in Mollisols of Northeast China.
... Steppe Chernozems of Mongolia, Russia (Kursk oblast), Canada, and USA emitted 15 to 28 g CO 2 /(m 2 day) on average for the warm season [4,19,31,49], which is comparable to our results. However, long-term plowing of Chernozems resulted indecreasing the soil СО 2 emission in comparison with native analogs [4,49,54]. Arable Chernozems in our study was presented by bare fallow, which soil СО 2 emission of was about six times lower than in the virgin steppe. Moreover, plowing fallow promoted activation of mineralization processes and intense short-time carbon emission into the atmosphere (almost 88 g CO 2 /(m 2 day)). ...
... The soil CO 2 emissions obtained from field crop plantation of our study were in a similar range with other ecosystems. Meng-yang et al. (2014) reported soil CO 2 emissions as 829, 629 and 474 g CO 2 m -2 from July 1 to September 30 (921, 698 and 526 kg ha -1 week -1 , respectively) for fields under 20-year continuous maize, wheat and soybean cultivation in a similar semi arid area located in northeast China. Gu et al. (2016) reported the soil CO 2 emissions under wheat plantation ranging from 372 to 418 mg m -2 h -1 (equals to 625-702 kg ha -1 week -1 ). ...
... They found that manure might be a proper fertilization regime for promoting crop yield and maintaining soil fertility, but having low GHG emission. You et al. (2014) observed the significant differences in CO 2 emissions from three cropping systems in Mollisols of Northeast China. Soil CO 2 emission and Q 10 values largely depended on the crops of the cropping system. ...
Soil respiration is a major source of atmospheric carbon dioxide (CO2). Intensive exploring on soil respiration in farmland may provide important information for controlling CO2 emissions. However, some important meteorological factors, especially for the wind, have been neglected previously. In this two-season field observation, we measured soil total respiration (Rt) in maize farmland in an agro-pastoral transitional zone in northeastern China, including autotrophic (root) respiration (Ra) and heterotrophic respiration (Rh), using the root-exclusion method. The mean Rt was 4.0 ± 0.8 and 6.1 ± 1.4 μmol·m⁻²·s⁻¹ in the two growing seasons. The root contribution to the total soil respiration rate (Rc) was 55.5 and 44.1 %, respectively, and both followed an approximately unimodal curve as a function of day of year. Ra and Rh were both positively correlated with the soil temperature. Ra decreased with increasing wind speed, but Rh was not significantly affected; thus, Rc was negatively correlated with the wind speed. The decrease of Rc caused by increased wind speed differed among the growth stages. Significant suppression of Rc was detected at the jointing stage and at maturity. Responses of Rt and its components to meteorological factors differed significantly in magnitude between the day and night. However, Rc decreased with increasing wind speed both during the day and at night. In summary, the decrease of farmland soil respiration by the wind resulted from decreased root respiration and could be regulated by the modifying of artificial windbreaks or by the expanding of row space of the maize planting, thereby reducing carbon dioxide emission. Indeed, this study is only the observation and analysis of some traditional factors, more intensive observations, especially evidence from physiological traits, morphological anatomy and other perspectives are required to confirm this relationship in further studies.
Humic acid originating from lignite is a popular resource of organic fertilizer. The effects of humic acid application on crop biomass and soil CO2 emission charged the regional agro-ecosystem carbon balance. Two kinds of humic acid, obtained from lignite via H2O2-oxidation (OHA) and KOH-activation (AHA), were applied in a wheat-maize rotation located field at three levels of 500 (OHA1; AHA1), 1000 (OHA2; AHA2), and 1500 kg hm⁻² (OHA3; AHA3), only chemical fertilizer treatment (CF) as control to investigate the change of soil CO2 emission, crop yield and ecosystem carbon balance in 2016–2019. During the four experimental years, the trend of cumulative efflux of soil CO2 was increasing in medium and high dosage humic acid treatments. The grain yield of wheat and maize had the same trend as the cumulative efflux of soil CO2 due to the increase of soil NO3⁻-N and soil available P directly affected by humic acid application. The main factor of cumulative soil CO2 efflux improvement was soil NO3⁻-N and soil available P in 2016, while soil available potassium became key factor in 2019 with the step regression. Net ecosystem productivity (NEP) was used to assess ecosystem carbon balance, which was positive values showed atmospheric CO2 sink under all the fertilization treatments and increased with the increase of humic acid use level. AHA2 and AHA3 treatments charged the higher NEP in 2019 than 2016. Meanwhile, AHA treatment presented a higher NEP average than OHA treatment with the same applied level. Crop yield and soil available P was the directly positive factor to NEP over four years under the fertilization by SEM analysis. It is recommended that AHA be applied at 1000 kg hm⁻² together with chemical fertilizers to achieve the higher crop yield and a sink of the atmospheric CO2 in agricultural fields in North China.
Conservation agriculture (CA) helps to mitigate climate change. Firstly, the modifications introduced by CA on the carbon dynamics in the soil directly result in an increase of the carbon (C) in the soil fraction. Secondly, CA drastically reduces C oxidation processes by diminishing the mechanical manipulation of the soil.Spain's position in relation to the Kyoto Protocol must be improved, as is one of the European countries in a non-compliance situation. With the aim of providing knowledge about the potential of CA as C sink in Spain, 29 articles on this subject were reviewed. According to 2010 CA uptake, the results demonstrated that conservation practices have the potential to promote the fixation in soil of about 2Ggyear -1 more C than traditional tillage (TT) systems. As indicated by Tebrügge (2001), 3.7Mg of CO 2 are generated from 1Mg of C through microbial oxidation processes taking place in the ground, meaning that through CA almost 7.5Gg of CO 2 could be sequestered from the atmosphere every year until the equilibrium is reached.C fixation was found to be irregular over time. C fixation rates were high in newly implemented systems during the first 10years, reaching top values of 0.85Mgha -1year -1 for no-tillage (NT) and 1.54Mgha -1year -1 for cover crops (CC) implemented in-between perennial tree rows. After those first 10years, it followed a period of lower but steady growth until equilibrium was reached. Nevertheless, C decreases of 0.16Mgha -1year -1 in the first 10years may be expected when practicing minimum tillage (MT). C sequestration rate resulted higher in case farmers do crop rotations in NT and MT rather than monoculture. In woody crops, studies reported higher C fixation values for native species when compared to sowed CC. Also, climate conditions seem to affect C sequestration rate in Spain. Although in NT differences observed between maritime and continental climates are not pronounced, as approximately 25% of the values recorded in both climates are equal, in the case of MT about 75% of maritime climate values result higher than the continental situation.
To accurately evaluate the carbon sequestration potential and better elucidate the relationship between the carbon cycle and regional climate change, using eddy covariance system, we conducted a long-term measurement of CO2 fluxes in the rain-fed winter wheat field of the Chinese Loess Plateau. The results showed that the annual net ecosystem CO2 exchange (NEE) was (−71.6±5.7) and (−65.3±5.3) g C m−2 y−1 for 2008–2009 and 2009–2010 crop years, respectively, suggesting that the agro-ecosystem was a carbon sink (117.4–126.2 g C m−2 yr−1). However, after considering the harvested grain, the agro-ecosystem turned into a moderate carbon source. The variations in NEE and ecosystem respiration (Reco) were sensitive to changes in soil water content (SWC). When SWC ranged form 0.15 to 0.21 m3 m−3, we found a highly significant relationship between NEE and photosynthetically active radiation (PAR), and a highly significant relationship between Reco and soil temperature (Ts). However, the highly significant relationships were not observed when SWC was outside the range of 0.15–0.21 m3 m−3. Further, in spring, the Reco instantly responded to a rapid increase in SWC after effective rainfall events, which could induce 2 to 4-fold increase in daily Reco, whereas the Reco was also inhibited by heavy summer rainfall when soils were saturated. Accumulated Reco in summer fallow period decreased carbon fixed in growing season by 16–25%, indicating that the period imposed negative impacts on annual carbon sequestration.
Nitrous oxide (N2O) emissions from soil are characterized by strong emission pulses. Although several mechanisms are known to create them, pulses are difficult to predict. Currently there is no established systematic way to identify pulses from long-term static chamber measurement results. In this study we suggest a simple algorithm for pulse identification. The algorithm was applied on time series of N2O and carbon dioxide (CO2) fluxes from a field study on the long-term impact of fertilization and tillage practice. Between 4 and 9% of N2O values were pulse values; 20–60% of total emission was emitted as pulses. Minimum tillage resulted in more pulses than plowing. In contrast, long-term averages of N2O losses from nitrogen (N) fertilizer were similar (3–4%) for all management practices. N2O emissions per crop yield for increased fertilization practice were double the values for reduced fertilization practice independent of tillage practice. CO2 emission pulses were scarce and there was no significant effect of management practice on CO2 pulse probability.
Reducing emissions of greenhouse gases (GHG) from agriculture is related to increasing and protecting soil organic matter (SOM) concentration. Agricultural soils can be a significant sink for atmospheric carbon (C) through increase of the SOM concentration. The natural ecosystems such as forests or prairies, where C gains are in equilibrium with losses, lose a large fraction of the antecedent C pool upon conversion to agricultural ecosystems. Adoption of recommended management practices (RMPs) can enhance the soil organic carbon (SOC) pool to fill the large C sink capacity on the world's agricultural soils. This article collates, reviews, and synthesizes the available information on SOC sequestration by RMPs, with specific references to crop rotations and tillage practices, cover crops, ley farming and agroforestry, use of manure and biosolids, N fertilization, and precision farming and irrigation. There is a strong interaction among RMPs with regards to their effect on SOC concentration and soil quality. The new equilibrium SOC level may be achieved over 25 to 50 years. While RMPs are being adapted in developed economies, there is an urgent need to encourage their adoption in developing countries. In addition to enhancing SOC concentration, adoption of RMPs also increases agronomic yield. Thus, key to enhancing soil quality and achieving food security lies in managing agricultural ecosystems using ecological principles which lead to enhancement of SOC pool and sustainable management of soil and water resources.
To evaluate the response of soil respiration to soil moisture, temperature, and N fertilization, and estimate the contribution of soil and rhizosphere respiration to total soil CO2 emissions, a field experiment was conducted in the Fengqiu State Key Agro-Ecological Experimental Station, Henan, China. The experiment included four treatments: bare soil fertilized with 150 kg N ha(-1) (CK), and maize (Zea mays L.)-cropped soils amended with 0 (NO), 150 (N150), and 250 (N250) kg N ha-1. Mean seasonal soil CO2 emissions in the CK, NO, N150, and N250 treatments were estimated to be 294, 598, 541, and 539 g C m(-2), respectively. The seasonal soil CO2 fluxes were significantly affected by soil temperature, with the change in the rate of flux for each 10 degrees C increase in temperature (Q(10)) of 1.90 to 2.88, but not by soil moisture. Nitrogen fertilization resulted in a 10.5% reduction in soil CO 2 flux; however, it did not significantly increase the maize aboveground biomass but did increase maize yield. Soil respiration measurement using the root-exclusion technique indicated that soils fertilized with 150 kg N ha(-1) contributed 54% of the total soil CO 2 emission, or 8% of soil organic C down to a depth of 40 cm. An amount of C equivalent to 26% of the net assimilated C in harvested above- and belowground plant biomass was returned to the atmosphere by rhizosphere respiration.
A field experiment was conducted to examine the influences of 5-year no tillage without straw retaining on soil organic carbon (SOC) and carbon dioxide (CO2) emissions from Mollisols, and to relate soil CO2 effluxes to variations in soil temperature and moisture. A closed-chamber method was used to determine CO2 efflux during the maize growing season in 2011. Our results showed no remarkable increase (P>0.05) in SOC for no tillage without straw retaining (NT), although NT practice decreased cumulative CO2 emission during the growing season by 30% (P<0.05) compared with conventional tillage (CT). Annual soil CO2 emissions were estimated at 13.34 and 9.39 Mg CO2 ha(-1) for CT and NT, respectively. The amount of annual lost C through CO2 emission from NT soils could be roughly replenished by incorporation of maize straw. The log-transformed multiple regression model [log(f) = a + b T + c log(W)] including both soil temperature and moisture was established, which accounted for 68 and 74% of the season variations in soil CO2 effluxes in NT and CT, respectively (both P<0.01). The temperature sensitivity of soil respiration was 2.39-2.75 in CT, which was higher than 2.01-2.34 in NT; soil respiration was more sensitive t o soil temperature at 10 cm than at 5 cm depth. Compared with CT, NT also decreased cumulative N2O emissions by 50% and thus total global warming potential of CO2 and N2O emissions. Results suggest that, considering C sequestration and global warming effect, the practice of no tillage without straw retaining is feasible in Mollisols in northeast China.
Soil CO2 emissions are highly variable, both spatially and across time, with significant changes even during a one-day period. The
objective of this study was to compare predictions of the diurnal soil CO2 emissions in an agricultural field when estimated by ordinary kriging and sequential Gaussian simulation. The dataset consisted
of 64 measurements taken in the morning and in the afternoon on bare soil in southern Brazil. The mean soil CO2 emissions were significantly different between the morning (4.54μmol m−2s−1) and afternoon (6.24μmol m−2s−1) measurements. However, the spatial variability structures were similar, as the models were spherical and had close range
values of 40.1 and 40.0m for the morning and afternoon semivariograms. In both periods, the sequential Gaussian simulation
maps were more efficient for the estimations of emission than ordinary kriging. We believe that sequential Gaussian simulation
can improve estimations of soil CO2 emissions in the field, as this property is usually highly non-Gaussian distributed.
KeywordsSoil respiration–Geostatistics–Ordinary kriging–Sequential gaussian simulation
The effect of soil water content on efflux of CO2 from soils has been described by linear, logarithmic, quadratic, and parabolic functions of soil water expressed as matric potential, gravimetric and volumetric water content, water holding capacity, water-filled pore space, precipitation indices, and depth to water table. The effects of temperature and water content are often statistically confounded. The objectives of this study are: (1) to analyze seasonal variation in soil water content and soil respiration in the eastern Amazon Basin where seasonal temperature variation is minor; and (2) to examine differences in soil CO2 emissions among primary forests, secondary forests, active cattle pastures, and degraded cattle pastures. Rates of soil respiration decreased from wet to dry seasons in all land uses. Grasses in the active cattle pasture were productive in the wet season and senescent in the dry season, resulting in the largest seasonal amplitude of CO2 emissions, whereas deep-rooted forests maintained substantial soil respiration during the dry season. Annual emissions were 2.0, 1.8, 1.5, and 1.0 kg C m-2 yr-1 for primary forest, secondary forest, active pasture, and degraded pasture, respectively. Emissions of CO2 were correlated with the logarithm of matric potential and with the cube of volumetric water content, which are mechanistically appropriate functions for relating soil respiration at below-optimal water contents. The parameterization of these empirical functions was not consistent with those for a temperate forest. Relating rates of soil respiration to water and temperature measurements made at some arbitrarily chosen depth of the surface horizons is simplistic. Further progress in defining temperature and moisture functions may require measurements of temperature, water content and CO2 production for each soil horizon.
Wetland stores substantial amount of carbon and may contribute greatly to global climate change debate. However, few researches
have focused on the effects of global climate change on carbon mineralization in Zoigê alpine wetland, Qinghai-Tibet Plateau,
which is one of the most important peatlands in China. Through incubation experiment, this paper studied the effects of temperature,
soil moisture, soil type (marsh soil and peat soil) and their interactions on CO2 and CH4 emission rates in Zoigê alpine wetland. Results show that when the temperature rises from 5°C to 35°C, CO2 emission rates increase by 3.3–3.7 times and 2.4–2.6 times under non-inundation treatment, and by 2.2–2.3 times and 4.1–4.3
times under inundation treatment in marsh soil and peat soil, respectively. Compared with non-inundation treatment, CO2 emission rates decrease by 6%–44%, 20%–60% in marsh soil and peat soil, respectively, under inundation treatment. CO2 emission rate is significantly affected by the combined effects of the temperature and soil type (p < 0.001), and soil moisture and soil type (p < 0.001), and CH4 emission rate was significantly affected by the interaction of the temperature and soil moisture (p < 0.001). Q
10 values for CO2 emission rate are higher at the range of 5°C–25°C than 25°C-35°C, indicating that carbon mineralization is more sensitive
at low temperature in Zoigê alpine wetland.
Keywordsalpine wetland–carbon mineralization–marsh soil–peat soil–soil moisture–Qinghai-Tibet Plateau
We assessed the effect of biochar incorporation into the soil on the soil-atmosphere exchange of the greenhouse gases (GHG)
from an intensive subtropical pasture. For this, we measured N2O, CH4 and CO2 emissions with high temporal resolution from April to June 2009 in an existing factorial experiment where cattle feedlot
biochar had been applied at 10tha−1 in November 2006. Over the whole measurement period, significant emissions of N2O and CO2 were observed, whereas a net uptake of CH4 was measured. N2O emissions were found to be highly episodic with one major emission pulse (up to 502μg N2O-N m−2 h−1) following heavy rainfall. There was no significant difference in the net flux of GHGs from the biochar amended vs. the control
plots. Our results demonstrate that intensively managed subtropical pastures on ferrosols in northern New South Wales of Australia
can be a significant source of GHG. Our hypothesis that the application of biochar would lead to a reduction in emissions
of GHG from soils was not supported in this field assessment. Additional studies with longer observation periods are needed
to clarify the long term effect of biochar amendment on soil microbial processes and the emission of GHGs under field conditions.
KeywordsDenitrification–Nitrification–Improved pasture–Biochar–Nitrogen–Carbon
Soil respiration rates vary significantly among major plant biomes, suggesting that vegetation type influences the rate of soil respiration. However, correlations among climatic factors, vegetation distributions, and soil respiration rates make cause-effect arguments difficult. Vegetation may affect soil respiration by influencing soil microclimate and structure, the quantity of detritus supplied to the soil, the quality of that detritus, and the overall rate of root respiration. At the global scale, soil respiration rates correlate positively with litterfall rates in forests, as previously reported, and with aboveground net primary productivity in grasslands, providing evidence of the importance of detritus supply. To determine the direction and magnitude of the effect of vegetation type on soil respiration, we collated data from published studies where soil respiration rates were measured simultaneously in two or more plant communities. We found no predictable differences in soil respiration between cropped and vegetation-free soils, between forested and cropped soils, or between grassland and cropped soils, possibly due to the diversity of crops and cropping systems included. Factors such as temperature, moisture availability, and substrate properties that simultaneously influence the production and consumption of organic matter are more important in controlling the overall rate of soil respiration than is vegetation type in most cases. However, coniferous forests had 10% lower rates of soil respiration than did adjacent broad-leaved forests growing on the same soil type, and grasslands had, on average, 20% higher soil respiration rates than did comparable forest stands, demonstrating that vegetation type does in some cases significantly affect rates of soil respiration.
The relationship between soil structure and the ability of soil to stabilize soil organic matter (SOM) is a key element in soil C dynamics that has either been overlooked or treated in a cursory fashion when developing SOM models. The purpose of this paper is to review current knowledge of SOM dynamics within the framework of a newly proposed soil C saturation concept. Initially, we distinguish SOM that is protected against decomposition by various mechanisms from that which is not protected from decomposition. Methods of quantification and characteristics of three SOM pools defined as protected are discussed. Soil organic matter can be: (1) physically stabilized, or protected from decomposition, through microaggregation, or (2) intimate association with silt and clay particles, and (3) can be biochemically stabilized through the formation of recalcitrant SOM compounds. In addition to behavior of each SOM pool, we discuss implications of changes in land management on processes by which SOM compounds undergo protection and release. The characteristics and responses to changes in land use or land management are described for the light fraction (LF) and particulate organic matter (POM). We defined the LF and POM not occluded within microaggregates (53–250 m sized aggregates as unprotected. Our conclusions are illustrated in a new conceptual SOM model that differs from most SOM models in that the model state variables are measurable SOM pools. We suggest that physicochemical characteristics inherent to soils define the maximum protective capacity of these pools, which limits increases in SOM (i.e. C sequestration) with increased organic residue inputs.
In view of the significance of agricultural soils in affecting global C balance, the impact of manipulation of the quality of exogenous inputs on soil CO2–C flux was studied in rice–barley annual rotation tropical dryland agroecosystem. Chemical fertilizer, Sesbania shoot (high quality resources), wheat straw (low quality resource) and Sesbania + wheat straw (high + low quality), all carrying equivalent recommended dose of N, were added to soil. A distinct seasonal variation in CO2–C flux was recorded in all treatments, flux being higher during rice period, and much reduced during barley and summer fallow periods. During rice period the mean CO2–C flux was greater in wheat straw (161% increase over control) and Sesbania + wheat straw (+129%) treatments; however, during barley and summer fallow periods differences among treatments were small. CO2–C flux was more influenced by seasonal variations in water-filled pore space compared to soil temperature. In contrast, the role of microbial biomass and live crop roots in regulating soil CO2–C flux was highly limited. Wheat straw input showed smaller microbial biomass with a tendency of rapid turnover rate resulting in highest cumulative CO2–C flux. The Sesbania input exhibited larger microbial biomass with slower turnover rate, leading to lower cumulative CO2–C flux. Addition of Sesbania to wheat straw showed higher cumulative CO2–C flux yet supported highest microbial biomass with lowest turnover rate indicating stabilization of microbial biomass. Although single application of wheat straw or Sesbania showed comparable net change in soil C (18% and 15% relative to control, respectively) and crop productivity (32% and 38%), yet they differed significantly in soil C balance (374 and −3 g C m−2 y−1 respectively), a response influenced by the recalcitrant and labile nature of the inputs. Combining the two inputs resulted in significant increment in net change in soil C (33% over control) and crop yield (49%) in addition to high C balance (152 g C m−2 y−1). It is suggested that appropriate mixing of high and low quality inputs may contribute to improved crop productivity and soil fertility in terms of soil C sequestration.
Changes in agricultural management can potentially increase the accumulation rate of soil organic C (SOC), thereby sequestering CO2 from the atmosphere. This study was conducted to quantify potential soil C sequestration rates for different crops in response to decreasing tillage intensity or enhancing rotation complexity, and to estimate the duration of time over which sequestration may occur. Analyses of C sequestration rates were completed using a global database of 67 long-term agricultural experiments, consisting of 276 paired treatments. Results indicate, on average, that a change from conventional tillage (CT) to no-till (NT) can sequester 57 ± 14 g C m-2 yr-1, excluding wheat (Triticum aestivum L.)-fallow systems which may not result in SOC accumulation with a change from CT to NT. Enhancing rotation complexity can sequester an average 20 ± 12 g C m-2 yr-1, excluding a change from continuous corn (Zea mays L.) to corn-soybean (Glycine max L.) which may not result in a significant accumulation of SOC. Carbon sequestration rates, with a change from CT to NT, can be expected to peak in 5 to 10 yr with SOC reaching a new equilibrium in 15 to 20 yr. Following initiation of an enhancement in rotation complexity, SOC may reach a new equilibrium in approximately 40 to 60 yr. Carbon sequestration rates, estimated for a number of individual crops and crop rotations in this study, can be used in spatial modeling analyses to more accurately predict regional, national, and global C sequestration potentials.
CO<sub>2</sub> efflux at the soil surface is the result of respiration in different depths that are subjected to variable temperatures at the same time. Therefore, the temperature measurement depth affects the apparent temperature sensitivity of field-measured soil respiration. We summarize existing literature evidence on the importance of this effect, and describe a simple model to understand and estimate the magnitude of this potential error source for heterotrophic respiration. The model is tested against field measurements. We discuss the influence of climate (annual and daily temperature amplitude), soil properties (vertical distribution of CO<sub>2</sub> sources, thermal and gas diffusivity), and measurement schedule (frequency, study duration, and time averaging). Q <sub>10</sub> as a commonly used parameter describing the temperature sensitivity of soil respiration is taken as an example and computed for different combinations of the above conditions. We define conditions and data acquisition and analysis strategies that lead to lower errors in field-based Q <sub>10</sub> determination. It was found that commonly used temperature measurement depths are likely to result in an underestimation of temperature sensitivity in field experiments. Our results also apply to activation energy as an alternative temperature sensitivity parameter.
The carbon sink capacity of the world's agricultural and degraded soils is 50 to 66% of the historic carbon loss of 42 to
78 gigatons of carbon. The rate of soil organic carbon sequestration with adoption of recommended technologies depends on
soil texture and structure, rainfall, temperature, farming system, and soil management. Strategies to increase the soil carbon
pool include soil restoration and woodland regeneration, no-till farming, cover crops, nutrient management, manuring and sludge
application, improved grazing, water conservation and harvesting, efficient irrigation, agroforestry practices, and growing
energy crops on spare lands. An increase of 1 ton of soil carbon pool of degraded cropland soils may increase crop yield by
20 to 40 kilograms per hectare (kg/ha) for wheat, 10 to 20 kg/ha for maize, and 0.5 to 1 kg/ha for cowpeas. As well as enhancing
food security, carbon sequestration has the potential to offset fossilfuel emissions by 0.4 to 1.2 gigatons of carbon per
year, or 5 to 15% of the global fossil-fuel emissions.
Among greenhouse gases, carbon dioxide (CO(2)) is one of the most significant contributors to regional and global warming as well as climatic change. A field study was conducted to (i) determine the effect of soil characteristics resulting from changes in soil management practices on CO(2) flux from the soil surface to the atmosphere in transitional land from perennial forages to annual crops, and (ii) develop empirical relationships that predict CO(2) flux from soil temperature and soil water content. The CO(2) flux, soil temperature (T(s)), volumetric soil water content (theta(v)) were measured every 1-2 weeks in no-till (NT) and conventional till (CT) malt barley and undisturbed soil grass-alfalfa (UGA) systems in a Lihen sandy loam soil (sandy, mixed, frigid Entic Haplustoll) under irrigated and non-irrigated conditions in western North Dakota. Soil air-filled porosity (epsilon) was calculated from total soil porosity and theta(v) measurements. Significant differences in CO(2) fluxes between land management practices (irrigation and tillage) were observed on some measurement dates. Higher CO(2) fluxes were detected in CT plots than in NT and UGA treatments immediately after rainfall or irrigation. Soil CO(2) fluxes increased with increasing soil moisture (R(2)=0.15, P<0.01) while an exponential relationship was found between CO(2) emission and T(s) (R(2)=0.59). Using a stepwise regression analysis procedure, a significant multiple regression equation was developed between CO(2) flux and theta(v), T(s) (CO(2) flux = e(-3.477+0.123T(s)+6.381theta)(v); R(2)=0.68, P <or= 0.01). Not surprisingly, soil temperature was a driving factor in the equation, which accounted for approximately 59% in variation of CO(2) flux. It was concluded that less intensive tillage, such as no-till or strip tillage, along with careful irrigation management will reduce soil CO(2) evolution from land being converted from perennial forages to annual crops.
The reuse of treated wastewater in agricultural systems could partially help alleviate water resource shortages in developing countries. Treated wastewater differs from fresh water in that it has higher concentrations of salts, Escherichia coli and presence of dissolved organic matter, and inorganic N after secondary treatment, among others. Its application could thus cause environmental consequences such as soil salinization, ammonia volatilization, and greenhouse gas emissions. In an incubation experiment, we evaluated the characteristics and effects of water-filled pore space (WFPS) and N input on the emissions of nitrous oxide (N2O) and carbon dioxide (CO2) from silt loam soil receiving treated wastewater. Irrigation with treated wastewater (vs. distilled water) significantly increased cumulative N2O emission in soil (117.97 µg N kg−1). Cumulative N2O emissions showed an exponentially increase with the increasing WFPS in unamended soil, but the maximum occurred in the added urea soil incubated at 60% WFPS. N2O emissions caused by irrigation with treated wastewater combined with urea-N fertilization did not simply add linearly, but significant interaction (P<0.05) caused lower emissions than the production of N2O from the cumulative effects of treated wastewater and fertilizer N. Moreover, a significant impact on cumulative CO2 emission was measured in soil irrigated with treated wastewater. When treated wastewater was applied, there was significant interaction between WFPS and N input on N2O emission. Hence, our results indicated that irrigation with treated wastewater should cause great concern for increasing global warming potential due to enhanced emission of N2O and CO2.
a b s t r a c t Evaluation of carbon dynamics is of great concern worldwide in terms of climate change and soil fertility. However, the annual CO 2 flux and the effect of land management on the carbon budget are poorly understood in Sub-Saharan Africa, owing to the relative dearth of data for in situ CO 2 fluxes. Here, we evaluated seasonal variations in CO 2 efflux rate with hourly climate data in two dry tropical croplands in Tanzania at two sites with contrasting soil textures, viz. clayey or sandy, over four consecutive crop-cultivation periods of 40 months. We then: (1) estimated the annual CO 2 flux, and (2) evaluated the effect of land management (control plot, plant residue treatment plot, fertilizer treatment plot, and plant residue and fertilizer treatment plot) on the CO 2 flux and soil carbon stock at both sites. Estimated annual CO 2 fluxes were 1.0e2.2 and 0.9e1.9 Mg C ha À1 yr À1 for the clayey and sandy sites, respectively. At the end of the experiment, crop cultivation had decreased the surface soil carbon stocks by 2.4 and 3.0 Mg C ha À1 (soil depth 0e15 cm) at the clayey and sandy sites, respectively. On the other hand, plant residue application (7.5 Mg C ha À1 yr À1) significantly increased the surface soil carbon stocks, i.e., 3.5e3.8 and 1.7e2.1 Mg C ha À1 (soil depth 0e15 cm) at the clayey and sandy sites, respectively, while it also increased the annual CO 2 fluxes substantially, i.e., 2.5e4.0 and 2.4e3.4 Mg C ha À1 yr À1 for the clayey and sandy soils, respectively. Our results indicate that these dry tropical croplands at least may act as a carbon sink, though the efficiency of carbon accumulation was substantially lower in sandy soil (6.8e8.4%) compared to clayey soil (14.0e15.2%), possibly owing to higher carbon loss by leaching and macro-faunal activity.
Common agricultural practices such as excessive use of agro-chemicals, deep tillage and luxury irrigation have degraded soils,
polluted water resources and contaminated the atmosphere. There is increasing concern about interrelated environmental problems
such as soil degradation, desertification, erosion, and accelerated greenhouse effects and climate change. The decline in
organic matter content of many soils is becoming a major process of soil degradation, particularly in European semi-arid Mediterranean
regions. Degraded soils are not fertile and thus cannot maintain sustainable production. At the same time, the production
of urban and industrial organic waste materials is widespread. Therefore, strategies for recycling such organic waste in agriculture
must be developed. Here, we review long-term experiments (3–60 years) on the effects of organic amendments used both for organic
matter replenishment and to avoid the application of high levels of chemical fertilizers. The major points of our analysis
are: (1) many effects, e.g. carbon sequestration in the soil and possible build-up of toxic elements, evolve slowly, so it
is necessary to refer to long-term trials. (2) Repeated application of exogenous organic matter to cropland led to an improvement
in soil biological functions. For instance, microbial biomass carbon increased by up to 100% using high-rate compost treatments,
and enzymatic activity increased by 30% with sludge addition. (3) Long-lasting application of organic amendments increased
organic carbon by up to 90% versus unfertilized soil, and up to 100% versus chemical fertilizer treatments. (4) Regular addition
of organic residues, particularly the composted ones, increased soil physical fertility, mainly by improving aggregate stability
and decreasing soil bulk density. (5) The best agronomic performance of compost is often obtained with the highest rates and
frequency of applications. Furthermore, applying these strategies, there were additional beneficial effects such as the slow
release of nitrogen fertilizer. (6) Crop yield increased by up to 250% by long-term applications of high rates of municipal
solid waste compost. Stabilized organic amendments do not reduce the crop yield quality, but improve it. (7) Organic amendments
play a positive role in climate change mitigation by soil carbon sequestration, the size of which is dependent on their type,
the rates and the frequency of application. (8) There is no tangible evidence demonstrating negative impacts of heavy metals
applied to soil, particularly when high-quality compost was used for long periods. (9) Repeated application of composted materials
enhances soil organic nitrogen content by up to 90%, storing it for mineralization in future cropping seasons, often without
inducing nitrate leaching to groundwater.
Soil respiration is known to be highly variable with time. Less is known, however, about the spatial variability of heterotrophic soil respiration at the plot scale. We simultaneously measured soil heterotrophic respiration, soil temperature, and soil water content at 48 locations with a nested sampling design and at 76 locations with a regular grid plus refinement within a 13- by 14-m bare soil plot for 15 measurement dates. Soil respiration was measured with a closed chamber covering a surface area of 0.032 m(2). A geostatistical data analyses indicated a mean range of 2.7 m for heterotrophic soil respiration. We detected rather high coefficients of variation of CO(2) respiration between 0.13 and 0.80, with an average of 0.33. The number of observations required to estimate average respiration fluxes at a 5% error level ranged between 5 and 123. The analysis of the temporal persistence revealed that a subset of 17 sampling locations is sufficient to estimate average respiration fluxes at a tolerable root mean square error of 0.15 g C m(-2) d(-1). Statistical analysis revealed that the spatiotemporal variability of heterotrophic soil respiration could be explained by the state variables soil temperature and water content. The spatial variability of respiration was mainly driven by variability in soil water content; the variability in the soil water content was almost an order of magnitude higher than the variability in soil temperature.
Although soil respiration represents an important C transfer from terrestrial ecosystems to the atmosphere, the effects of environmental and biological factors on soil respiration rates are not adequately understood. This is due primarily to the variety of processes that produce CO, within the soil. Thus, separating the main CO2-producing processes is needed to improve our understanding of soil C cycling and dynamics. Here, we describe and test a model that estimates Soil CO2 emissions derived from anabolic and catabolic processes, representing organic matter decomposition and root + rhizosphere respiration, respectively. Our model is based on the exponential response of organic matter decomposition with respect to temperature, and it requires only measurements of total soil CO2 emissions and soil temperature as inputs. To test the model, we relied on published measurements of soil respiration rates and soil temperatures in a maize (Zea mays L.) field in Ottawa, Canada, and on independent estimations of soil and root contributions for this field made on the basis of stable-C isotope measurements of soil-derived CO2- Modelbased and isotope estimations correlated significantly (r(2) = 0.91, P < 10-9) on a daily basis. Model-based estimations for root + rhizosphere respiration rates for the entire growing season totaled 145 g C m(-2) or 27% of CO2 emissions, and those based on C isotopes totaled 158 g C m(-2) or 30% of the total emissions. The excellent correspondence between model-based and isotope-based estimations suggests that this relatively simple model can be used to distinguish root from soil contributions to soil CO2 emissions in temperate-zone, annual croplands free of significant water stress.
Changes in agricultural management can potentially increase the accumulation rate of soil organic C (SOC), thereby sequestering CO 2 from the atmosphere. This study was conducted to quantify potential soil C sequestration rates for different crops in response to decreasing tillage intensity or enhancing rotation complexity, and to estimate the duration of time over which sequestration may occur. Analyses of C sequestration rates were completed using a global database of 67 long-term agricultural experiments, consisting of 276 paired treatments. Results indicate, on average, that a change from conventional tillage (CT) to no-till (NT) can sequester 57 ± 14 g C m -2 yr -1 , excluding wheat (Triticum aestivum L.)-fallow systems which may not result in SOC accumulation with a change from CT to NT. Enhancing rotation complexity can sequester an average 20 ± 12 g C m -2 yr -1 , excluding a change from continuous corn (Zea mays L.) to corn-soybean (Glycine max L.) which may not result in a significant accumulation of SOC. Carbon sequestration rates, with a change from CT to NT, can be expected to peak in 5 to 10 yr with SOC reaching a new equilibrium in 15 to 20 yr. Following initiation of an enhancement in rotation complexity, SOC may reach a new equilibrium in approximately 40 to 60 yr. Carbon sequestration rates, estimated for a number of individual crops and crop rotations in this study, can be used in spatial modeling analyses to more accurately predict regional, national, and global C sequestration potentials.
Soil-surface CO2 efflux and its spatial and temporal variations were examined in an 8-y-old ponderosa pine plantation in the Sierra Nevada Mountains in California from June 1998 to August 1999. Continuous measurements of soil CO2 efflux, soil temperatures and moisture were conducted on two 20 × 20 m sampling plots. Microbial biomass, fine root biomass, and the physical and chemical properties of the soil were also measured at each of the 18 sampling locations on the plots. It was found that the mean soil CO2 efflux in the plantation was 4.43 µmol m−2 s−1 in the growing season and 3.12 µmol m−2 s−1 in the nongrowing season. These values are in the upper part of the range of published soil-surface CO2 efflux data. The annual maximum and minimum CO2 efflux were 5.87 and 1.67 µmol m−2 s−1, respectively, with the maximum occurring between the end of May and early June and the minimum in December. The diurnal fluctuation of CO2 efflux was relatively small (
In terrestrial ecosystems, soil respiration is a key pathway of carbon to the atmosphere. It is highly variable in time and space. Its temporal variability at a single point can be reasonably described by changes in soil temperature and moisture. However, it is much more difficult to determine the drivers of its spatial variability. The aim of this study was to elucidate the interrelationship between the spatial variability of soil respiration and the spatial variability of soil redistribution as well as other soil and crop properties. The study was carried out in a small agricultural watershed (4.2ha) subjected to water and tillage erosion processes. During three crop cycles (one of sugar beet, two of winter wheat) soil respiration, soil temperature, and soil moisture were measured in situ at least bi-weekly at 20–22 locations. The first stage was to analyse the interrelation of soil temperature as an important control of soil respiration and soil redistribution. In the second stage, measured CO2 effluxes were standardised to 15°C and mean fluxes at each location were calculated for the sugar beet year, as well as two growing phases under winter wheat. The mean CO2 effluxes for the five resulting phases at each measuring location were correlated to soil and crop properties and modelled soil redistribution. Moreover, the intercorrelation of all explanatory variables was analysed using principal component analyses, and these principal components were correlated to standardised CO2 effluxes. Except for the second phase in 2009 with combined autotrophic and heterotrophic respiration, which was dominated by root respiration, in all phases there was a tendency that CO2 effluxes at erosional sites were smaller than at depositional sites. The combined analysis of CO2 effluxes, erosion, and other explanatory variables indicates that for heterotrophic respiration (between rows in the case of sugar beet and before significant plant growth in the case of winter wheat) the spatial variability of median grain size and bulk density had the most consistent effect on the spatial variability of soil respiration. In contrast, soil moisture was less important and topsoil SOC had more or less no effect on CO2 effluxes. Only in two measuring phases (Phase 2 2008 and Phase 1 2009) did the combined analysis show that total erosion was one of the dominant variables for the spatial variability in CO2 effluxes. In general, the relatively inconsistent effect of soil erosion status on spatial variability of CO2 effluxes is somewhat surprising, as there are a number of different reasons which support the assumption that CO2 effluxes at erosional sites should be smaller than at depositional sites. A major reason for the observed behaviour might be the compensating effect of tillage and water erosion and the counteracting effects of both processes on soil respiration. This underlines the importance of field-scale studies to gain further insight into the interrelation of soil redistribution and CO2 effluxes.
Policies that encourage greenhouse-gas emitters to mitigate emissions through terrestrial carbon (C) offsets -C sequestration in soils or biomass -will promote practices that reduce erosion and build soil fertility, while fostering adaptation to climate change, agricultural development, and rehabilitation of degraded soils. However, none of these benefits will be possible until changes in C stocks can be documented accurately and cost-effectively. This is particularly challenging when dealing with changes in soil organic C (SOC) stocks. Precise methods for measuring C in soil samples are well established, but spatial variability in the factors that determine SOC stocks makes it difficult to document change. Widespread interest in the benefits of SOC sequestration has brought this issue to the fore in the development of US and international climate policy. Here, we review the challenges to documenting changes in SOC stocks, how policy decisions influence offset documentation requirements, and the benefits and drawbacks of different sampling strategies and extrapolation methods.
Minimum (MT) or no tillage (NT) and increased cropping intensity can enhance soil structure and raise carbon sequestration in agricultural soils. The effectiveness of these procedures depends on soil type, crops, and tillage management systems. Increases in the organic carbon content may be affected by crop type, crop rotation and the quality and quantity of crop residues left on the soil surface. Soil organic carbon (SOC) is a good indicator of soil quality and conservation. The present study was conducted from 1994 to 2004 at Torrepadierne, Burgos, a cereal farming area in Spain, on Typic Calcixerolls soil with a 1.8% soil organic matter (SOM) content. The average annual rainfall in the area is 448mm. A split-plot experimental design was used, in which the main factor was the tillage system – conventional (CT), minimum (MT) or no-till (NT) – and the sub-factor crop rotation – cereal/cereal (C–C), cereal/fallow (C–F) and cereal/legume (C–L). Fallow/cereal and legume/cereal were added to these sequences to have the same crops every year. The present study was conducted to determine the effect of tillage systems and cropping sequences on SOC patterns after 10 years of soil management. At a depth of 0–10cm, the SOC content was significantly higher with NT than CT or MT, by 58% and 11%, respectively. SOC values were 41% higher with MT, in turn, than with CT. At a depth of 10–20cm, the SOC content was 30% higher with NT than with CT and 7% higher than with MT. And at 20–30cm, it was 7% higher with MT than with CT, 12% higher with NT than CT and 9% higher with no-till than minimum-till. In 2004, at the end of the 10 years period, SOC was 25% greater with NT than CT, 16% greater with NT than MT, and 17% higher with MT than CT. Crop rotation was not observed to have any significant effect on the SOC content in 2004, however. These findings suggest that carbon sequestration in the 30cm layer can be improved if NT or MT are used in lieu of conventional practice. The total crop residue returning to the soil was significantly greater in plots sown with legume after cereal harvest than in plots left fallow. It also enhanced SOC sequestration in non- or minimally tilled soils.
A field experiment was conducted to examine the influences of long-term applications of maize straw and organic manure on carbon dioxide (CO2) emissions from a cultivated Mollisol in northeast China and to evaluate the responses of soil CO2 fluxes to temperature and moisture. Soil CO2 flux was measured using closed chamber and gas chromatograph techniques. Our results indicated that the application of organic amendments combined with fertilizer nitrogen, phosphorus and potassium (NPK) accelerated soil CO2 emissions during the maize growing season, whereas NPK fertilization alone did not impact cumulative CO2 emissions. Cumulative CO2 emissions were higher from soils amended with pig manure relative to those with maize residue. Cumulative CO2 emissions during the growing season were 988 and 1130 g CO2 m(-2) under applications of 7500 and 22,500 kg ha(-1) pig manure combined with NPK, respectively, which were 42 and 63% higher than the emissions from the control (694 g CO2 m(-2)). The applications of 2250 and 4500 kg ha(-1) maize straw combined with NPK marginally increased soil CO2 emissions by 23 and 28% compared with the control, respectively. A log-transformed multiple regression model including both soil temperature and moisture explained 50-88% of the seasonal variation in soil CO2 fluxes. Cumulative soil CO2 emissions were affected more by applied treatments than by soil temperature and moisture. Our results suggest that the magnitude of the impact of soil amendments on CO2 emissions from Mollisols primarily depends on the type of organic amendments applied, whereas the application rate has limited impacts. (C) 2013 Elsevier Masson SAS. All rights reserved.
Impacts of organic manure and inorganic fertilizer on total organic carbon (CT), water-soluble organic C (CWS), microbial biomass C (CMB), particulate organic C (CP), labile organic C (CL), C storage and sequestration, and C management index (CMI) in surface soil (0–20 cm) were investigated in a 20-yr field experiment under a greenhouse vegetable system in northeast China. The treatments included unfertilized control (CK), N fertilizer (N), balanced NPK fertilizer (NPK), organic manure alone (M) and the NPK fertilizer combined with the manure (MNPK). Under the treatments of N and NPK, CT content and C storage were not significantly changed over the experimental period, while CWS, CMB, CP, CL concentrations and CMI were significantly increased compared with the unfertilized control. In comparison with the control, the manure treatments, M and MNPK, significantly increased CT content and C storage, sequestrating organic C of 8.9 and 9.2 Mg/ha, respectively. The M and MNPK treatments showed higher CWS, CMB, CP and CL concentrations and CMI than the other three treatments. Pearson’s correlation coefficients were used to show that CWS, CMB, CP, CL and CMI could be useful indicators for assessing soil quality and total C changes. The M treatment is effective in sequestrating soil C, but resulted in lower crop yield compared with the NPK treatment. The MNPK treatment showed the greatest increases in crop yield and C sequestration in the greenhouse vegetable system.
Depending upon how soil is managed, it can serve as a source or sink for atmospheric carbon dioxide (CO2). As the atmospheric CO2 concentration continues to increase, more attention is being focused on the soil as a possible sink for atmospheric CO2. This study was conducted to examine the short-term effects of crop rotation and N fertilization on soil CO2 emissions in Central Iowa. Soil CO2 emissions were measured during the growing seasons of 2003 and 2004 from plots fertilized with three N rates (0, 135, and 270kgNha−1) in continuous corn and a corn–soybean rotation in a split-plot design. Soil samples were collected in the spring of 2004 from the 0–15cm soil depth to determine soil organic C content. Crop residue input was estimated using a harvest index based on the measured crop yield. The results show that increasing N fertilization generally decreased soil CO2 emissions and the continuous corn cropping system had higher soil CO2 emissions than the corn–soybean rotation. Soil CO2 emission rate at the peak time during the growing season and cumulative CO2 under continuous corn increased by 24 and 18%, respectively compared to that from corn–soybean rotation. During this period, the soil fertilized with 270kgNha−1 emitted, on average, 23% less CO2 than the soil fertilized with the other two N rates. The greatest difference in CO2 emission rate was observed in 2004; where plots that received 0N rate had 31% greater CO2 emission rate than plots fertilized with 270kgNha−1. The findings of this research indicate that changes in cropping systems can have immediate impact on both rate and cumulative soil CO2 emissions, where continuous corn caused greater soil CO2 emissions than corn soybean rotation.
Variation in soil temperature can account for most of the seasonal and diel variation in soil CO2 efflux, but the temperature effect is not always consistent, and other factors such as soil water content are known to influence soil respiration. The objectives of this research were to study the spatial and temporal variation in soil respiration in a temperate forested landscape and to evaluate temperature and soil water functions as predictors of soil respiration. Soil CO2 fluxes were measured with chambers throughout an annual cycle in six study areas at the Harvard Forest in central Massachusetts that include soil drainage classes from well drained to very poorly drained. The mean annual estimate of soil CO2 efflux was 7.2 Mg ha–1, but ranged from 5.3 in the swamp site to 8.5 in a well-drained site, indicating that landscape heterogeneity is related to soil drainage class. An exponential function relating CO2 fluxes to soil temperature accounted for 80% of the seasonal variation in fluxes across all sites (Q10 = 3.9), but the Q10 ranged from 3.4 to 5.6 for the individual study sites. A significant drought in 1995 caused rapid declines in soil respiration rates in August and September in five of the six sites (a swamp site was the exception). This decline in CO2 fluxes correlated exponentially with decreasing soil matric potential, indicating a mechanistic effect of drought stress. At moderate to high water contents, however, soil water content was negatively correlated with soil temperature, which precluded distinguishing between the effects of these two confounded factors on CO2 flux. Occurrence of high Q10 values and variation in Q10 values among sites may be related to: (i) confounding effects of high soil water content; (ii) seasonal and diel patterns in root respiration and turnover of fine roots that are linked to above ground phenology and metabolism; and (iii) variation in the depth where CO2 is produced. The Q10 function can yield reasonably good predictions of annual fluxes of CO2, but it is a simplification that masks responses of root and microbial processes to variation in temperature and water content throughout the soil.
Partitioning the root-derived CO2 efflux from soil (frequently termed rhizosphere respiration) into actual root respiration (RR, respiration by autotrophs) and rhizomicrobial respiration (RMR, respiration by heterotrophs) is crucial in determining the carbon (C) and energy balance of plants and soils. It is also essential in quantifying C sources for rhizosphere microorganisms and in estimation of the C contributing to turnover of soil organic matter (SOM), as well as in linking net ecosystem production (NEP) and net ecosystem exchange (NEE). Artificial-environment studies such as hydroponics or sterile soils yield unrealistic C-partitioning values and are unsuitable for predicting C flows under natural conditions. To date, several methods have been suggested to separate RR and RMR in nonsterile soils: 1) component integration, 2) substrate-induced respiration, 3) respiration by excised roots, 4) comparison of root-derived ¹⁴CO2 with rhizomicrobial ¹⁴CO2 after continuous labeling, 5) isotope dilution, 6) model-rhizodeposition technique, 7) modeling of ¹⁴CO2 efflux dynamics, 8) exudate elution, and 9) δ¹³C of CO2 and microbial biomass. This review describes the basic principles and assumptions of these methods and compares the results obtained in the original papers and in studies designed to compare the methods.
We use the Terrestrial Ecosystem Model (TEM, Version 4.1) and the land cover data set of the international geosphere–biosphere program to investigate how increasing atmospheric CO2 concentration and climate variability during 1900–1994 affect the carbon storage of terrestrial ecosystems in the conterminous USA, and how carbon storage has been affected by land-use change. The estimates of TEM indicate that over the past 95 years a combination of increasing atmospheric CO2 with historical temperature and precipitation variability causes a 4.2% (4.3 Pg C) decrease in total carbon storage of potential vegetation in the conterminous US, with vegetation carbon decreasing by 7.2% (3.2 Pg C) and soil organic carbon decreasing by 1.9% (1.1 Pg C). Several dry periods including the 1930s and 1950s are responsible for the loss of carbon storage. Our factorial experiments indicate that precipitation variability alone decreases total carbon storage by 9.5%. Temperature variability alone does not significantly affect carbon storage. The effect of CO2 fertilization alone increases total carbon storage by 4.4%. The effects of increasing atmospheric CO2 and climate variability are not additive. Interactions among CO2, temperature and precipitation increase total carbon storage by 1.1%. Our study also shows substantial year-to-year variations in net carbon exchange between the atmosphere and terrestrial ecosystems due to climate variability. Since the 1960s, we estimate these terrestrial ecosystems have acted primarily as a sink of atmospheric CO2 as a result of wetter weather and higher atmospheric CO2 concentrations. For the 1980s, we estimate the natural terrestrial ecosystems, excluding cropland and urban areas, of the conterminous US have accumulated 78.2 Tg C yr−1 because of the combined effect of increasing atmospheric CO2 and climate variability. For the conterminous US, we estimate that the conversion of natural ecosystems to cropland and urban areas has caused a 18.2% (17.7 Pg C) reduction in total carbon storage from that estimated for potential vegetation. The carbon sink capacity of natural terrestrial ecosystems in the conterminous US is about 69% of that estimated for potential vegetation.
The experiment designed to quantify the effects of long-term tillage practices on soil organic carbon (SOC) storage and CO2 emissions, was conducted on long-term tillage and continuous corn (Zea mays L.). The experimental plots were established in 1962 on a Crosby silt loam (fine, mixed, mesic Aeric Ochraqualf) in Ohio. It consisted of moldboard plow till (MT) chisel till (CT), and no-till (NT) laid out in a randomized block design with four replications. After 43 yrs of continuous corn, the pool of SOC in the top 30 cm depth was significantly greater under NT (80.0 ± 3.7 Mg C ha−1) than under CT (45.3 ± 1.7 Mg C ha−1) and MT (44.8 ± 3.7 Mg C ha−1). A large proportion (68–74%) of SOC, in the 0–30 cm depth originated from corn residues (C4-C). On average, MT, CT and NT treatments sequestered C4-C in the top 30 cm at a rate of 0.73, 0.71 and 1.37 Mg ha−1 yr−1. The average daily CO2 fluxes (g CO2-C m−2 d−1) were greater under MT (2.14) and CT (2.07) than under NT (1.61). In addition, the daily CO2 fluxes were highest in summer (2.62–3.77 g CO2-C m−2 d−1), the lowest in winter (0.75–0.87 g CO2-C m−2 d−1), and were positively correlated with air (R2 = 0.78, P < 0.01) and soil temperatures in the top 20 cm (R2 = 0.76, P < 0.01) and negatively with soil water content (R2 = 0.57, P < 0.05). Tillage management had a significant influence on average daily CO2 fluxes during summer and autumn but not during winter and spring. Annual CO2 emissions calculated by extrapolating daily CO2 fluxes were significantly higher under MT (6.6 ± 0.3 Mg CO2-C ha−1 yr−1) and CT (6.2 ± 0.4 Mg CO2-C ha−1 yr−1) than under NT (5.5 ± 0.5 Mg CO2-C ha−1 yr−1; LSD = 0.25 Mg CO2-C ha−1 yr−1). These results indicated that, during the growing season, NT reduced CO2 emissions by an average of 0.7 and 0.6 Mg C ha−1 yr−1 compared to MT and CT, respectively.
It is crucial to advance the understanding of the soil carbon dioxide (CO2) flux and environmental factors for a better comprehension of carbon dynamics in subtropical ecosystems. Red soil, one of the typical agricultural soils in subtropical China, plays important roles in the global carbon budget due to their large potential to sequester C and replenish atmospheric C through soil CO2 flux. We examined the relationship between soil CO2 flux and environmental determinants in four different land use types of subtropical red soil-paddy (P), orchard (O), woodland (W) and upland (U) using static closed chamber method. Objectives were to evaluate the relationship of soil temperature, water-filled pore space (WFPS), and dissolved organic carbon (DOC) with the soil CO2 flux. Soil CO2 fluxes were measured on each site about every 14 days between 09:00 and 11:00 a.m. during 14-July 2004 to 25-April 2007 at the experimental station of Heshengqiao at Xianning, Hubei, China. Soil CO2 fluxes revealed seasonal fluctuations, with the tendency that maximum values occurred in summer, minimum in winter and intermediate values in spring and autumn except for paddy soil when it was submerged. Further, significant differences in soil CO2 fluxes were observed among the four soils, following the order of P > O > U ∼ W. Average soil CO2 fluxes were estimated as 901 ± 114, 727 ± 55, 554 ± 22 and 533 ± 27 (±S.D.) g CO2 m−2 year−1 in paddy, orchard, upland and woodland soils, respectively. Variations in soil CO2 flux were related to soil temperature, WFPS, and dissolved organic carbon with a combined R2 of 0.49–0.75. Soil temperature was an important variable controlling 26–59% of soil CO2 flux variability. The interaction of soil temperature and WFPS could explain 31–60% of soil CO2 flux variations for all the land use types. We conclude that soil CO2 flux from red soil is under environmental controls, soil temperature being the main variable, which interact with WFPS and DOC to control the supply of readily mineralizable substrates.
We assessed the impact of long-term manuring and fertilization on changes in different SOC fractions over ten years period (1994–2003) in a Typic Haplustept under intensive cropping with maize (Zea mays L.) — wheat (Triticum aestivum L.) — cowpea (Vigna unguiculata) in semi-arid, sub-tropical India. The application of graded doses of NPK from 50% (130 kg N, 35 kg P and 41.5 kg K ha− 1) to 150% (390 kg N, 105 kg P and 124 kg K ha− 1) in the cropping system significantly enhanced SOC, particulate organic C (POC) and KMnO4 oxidizable C (KMnO4–C) fractions in soil. The increase in these C fractions was greater when farmyard manure (FYM) was applied conjointly with 100% NPK (260 kg N, 70 kg P and 83 kg K ha− 1). This treatment showed highest amount of SOC (58.3 Mg C ha− 1 in 1994 and 72.1 Mg C ha− 1 in 2003), POC (5.30 Mg C ha− 1 in 1994 and 6.33 Mg C ha− 1 in 2003) and KMnO4-C (10.05 Mg C ha− 1 in 1994 and 11.2 Mg C ha− 1 in 2003) in 0–45 cm soil depth. The C sequestration rate in SOC calculated over ten year period (1994–2003) was highest with 100% NPK + FYM (997 kg C ha− 1 yr− 1) followed by the 150% NPK (553 kg C ha− 1 yr− 1). It was estimated that 17.1 to 34.0% of the gross C input over ten year period contributed towards the increase in SOC content, while C sequestration efficiency (CSE) in POC (varied between 1.28 and 2.58%) was lower than KMnO4-C (varied between 1.42 and 3.72%). The CSE was highest in 150% NPK treatment, while 100% NPK + FYM showed the lowest CSE. By applying the values of humification constant (h) and decay constant (k) in Jenkinson's equation, it is possible to predict SOC level in the year 2003 and the C inputs required to maintain the SOC level in the year 1994 (AE) were calculated from Jenkinson's equation. The low k value in native SOC was responsible for lower requirements of C input required to maintain SOC in equilibrium. Thus increase in SOC concentration under long-term maize–wheat–cowpea cropping was due to the fact that annual C input by the system was higher than AE. In semi-arid sub-tropical India, continuous adoption of 100% NPK + FYM treatment in maize–wheat–cowpea cropping system might sequester 1.83 Tg C yr− 1 which corresponds to about 1% of the fossil fuel emissions by India.
The dynamics and the controlling factors of soil respiration measured with a closed static chamber method for continuous 2 years in grazed and ungrazed typical Leymus chinensis steppes, Inner Mongolia, PR China were analysed. There were similar diurnal and seasonal dynamics between the grazed and ungrazed plots. The diurnal patterns of soil respiration could be expressed as one-humped curves, reaching to the maximum at 11:00–14:00 and falling to the minimum at 1:00–3:00. During the growing season, the rates of soil respiration increased from the middle of June to the end of July and then gradually decreased. The seasonal changes of soil respiration were mainly influenced by moisture and temperature. When temperature was an independent controlling factor, it played a good role under the conditions of lower temperature (<15 °C) and lower moisture (<12%). However, the temperature models (e.g. linear, quadratic, power, exponential and Arrhenius models) did not reflect the stimulation effect of moisture on soil respiration with increasing temperature and moisture. Moisture was the single best predictor of hourly soil respiration rate in the arid and semi-arid grassland, but the mutual regulation by temperature and moisture did improve the predictive capacity of the models. Linear models could give better simulations than others did, and account for above 82% of the variation in soil respiration at the ungrazed plot. Although there was no further improvement in exponential, exponential-power and exponential-Arrhenius models for the simulation at the ungrazed plot, they did enhance the predictive capacity of soil respiration at the grazed plot (R2=0.87–0.88).
The response of soil respiration to varying environmental factors was studied in four Picea abies stands (47-, 87-, 111- and 146-year old) during the 1998 growing season. While within-site variations of soil CO2 efflux (up to 1.6 μmol CO2 m−2 s−1) were larger than their diurnal variability (<0.25 μmol CO2 m−2 s−1), spatial variations within a site were smaller than seasonal changes in soil respiration rates (up to 4.4 μmol CO2 m−2 s−1). Highest within-site variability of soil efflux was generally found during the summer months when maximum flux rates of 4–6 μmol CO2 m−2 s−1 were reached (coefficient of variation 40%). Soil temperatures (in the Of and Oh layers, and Ah horizon) showed a pronounced seasonal course, in contrast to soil moisture. An exponential equation best described the relationships between soil temperature in the Of layer and soil CO2 efflux (r2 between 0.75 and 0.81). However, an Arrhenius type equation always resulted in lower r2 values (0.52–0.71). The Q10 values ranged between 2.39 (146-year old stand) and 3.22 (87-year old stand), averaging 2.72 for the P. abies stands within the watershed. The removal of litter and organic layers generally affected soil CO2 efflux negatively. In three of the four P. abies stands (47-, 87-, 146-year old stands), soil respiration rates were reduced by 10–20% after removal of the L and Of layer, and by 30–40% after removal of the L and most of the Of and Oh layers. Thus, mineral soil respiration seemed to contribute a major fraction to the total soil CO2 flux (>60%). Trenching shallow fine roots during collar insertion and mechanical inhibition of root in-growth during the following months allowed fine root respiration to be separated from microbial respiration only in times of highest root growth. Microbial respiration seemed to dominate the respiratory CO2 loss from the forest floor (>70%). The comparison of the annual soil CO2 efflux in the 47-year old P. abies stand (about 710 g C m−2 yr−1) with annual litterfall and root net primary productivity estimates supported this conclusion.
It is well known that carbon storage capacity of forests will change in response to management practices such as fertilization. However, the influence of fertilization on belowground processes such as soil respiration, fine root production, and microbial biomass is still unclear. We measured soil respiration, fine root biomass production, and microbial biomass along a fertilization gradient (0, 56, 112, and 224 kg N ha−1 per year) in 7-year-old cottonwood and loblolly pine plantations, established on a well-drained, Redbay sandy loam (a fine-loamy, siliceous, thermic Rhodic Paleudlt), in northwest Florida. Soil respiration was measured monthly from June 2001 to May 2002 using the soda-lime technique. Fine root biomass production was quantified using the ingrowth core method during the same period. In addition, microbial biomass, soil temperature, moisture, soil pH, and organic matter were also measured along the same gradient for both species. Annual soil respiration rate was significantly greater (781 g C m−2 per year) in cottonwood than that (692 g C m−2 per year) in loblolly pine. Nitrogen fertilization had a significant negative effect on soil respiration in cottonwood, but no effect was observed in loblolly pine stands. Mean daily soil respiration rates exhibited significant exponential relationships with soil temperature both in cottonwood (R2=0.81) and loblolly pine (R2=0.51). Annual soil respiration rates in cottonwood stands were positively correlated with fine root production (r=0.64) and soil microbial biomass C (r=0.87) and negatively correlated with soil pH (r=−0.81). Annual soil respiration in loblolly pine stands was correlated positively with fine root production (r=0.54) and with organic matter content (r=0.74). Annual fine root production was significantly greater in cottonwood (221 g m−2 per year) than that in loblolly pine (144 g m−2 per year). Fertilization did not affect fine root production in both species. Microbial biomass, however, was significantly reduced by nitrogen fertilization in both species. We also observed an optimum range of soil pH (6.0±0.4), where highest microbial activity could be expected. Multiple regression analysis indicated that microbial biomass, soil organic matter, and soil pH were the major factors affecting soil respiration in cottonwood, while fine root production and soil organic matter were the major factors affecting soil respiration in loblolly pine. These results suggest that belowground responses to fertilization can vary widely between conifers and hardwoods.
Five main biogenic sources of CO2 efflux from soils have been distinguished and described according to their turnover rates and the mean residence time of carbon. They are root respiration, rhizomicrobial respiration, decomposition of plant residues, the priming effect induced by root exudation or by addition of plant residues, and basal respiration by microbial decomposition of soil organic matter (SOM). These sources can be grouped in several combinations to summarize CO2 efflux from the soil including: root-derived CO2, plant-derived CO2, SOM-derived CO2, rhizosphere respiration, heterotrophic microbial respiration (respiration by heterotrophs), and respiration by autotrophs. These distinctions are important because without separation of SOM-derived CO2 from plant-derived CO2, measurements of total soil respiration have very limited value for evaluation of the soil as a source or sink of atmospheric CO2 and for interpreting the sources of CO2 and the fate of carbon within soils and ecosystems. Additionally, the processes linked to the five sources of CO2 efflux from soil have various responses to environmental variables and consequently to global warming. This review describes the basic principles and assumptions of the following methods which allow SOM-derived and root-derived CO2 efflux to be separated under laboratory and field conditions: root exclusion techniques, shading and clipping, tree girdling, regression, component integration, excised roots and insitu root respiration; continuous and pulse labeling, 13C natural abundance and FACE, and radiocarbon dating and bomb-14C. A short sections cover the separation of the respiration of autotrophs and that of heterotrophs, i.e. the separation of actual root respiration from microbial respiration, as well as methods allowing the amount of CO2 evolved by decomposition of plant residues and by priming effects to be estimated. All these methods have been evaluated according to their inherent disturbance of the ecosystem and C fluxes, and their versatility under various conditions. The shortfalls of existing approaches and the need for further development and standardization of methods are highlighted.
In this study, the influence of different soil temperature and moisture reduction functions for scaling decomposition rates of soil organic matter on the prediction of CO2 production and fluxes was analysed. For this purpose, soil temperature and moisture reduction functions of six soil carbon decomposition models (CANDY, CENTURY, DAISY, PATCIS, ROTHC, and SOILCO2) were implemented in the modified SOILCO2-ROTHC model. As a test scenario, a respiration experiment on a silt loam in Columbia (USA) was chosen, which consists of two periods both including soil respiration measurements in a wheat stand and a subsequent bare soil period. Additionally, the dataset contains measured soil temperature, soil moisture as well as CO2 concentrations within the soil profile. The cumulative CO2 fluxes simulated with different temperature reduction functions showed deviations up to 41% (1.77 t C ha− 1) for the six-month simulation period in 1981. The influence of moisture reduction was smaller with deviations up to 2% (0.10 t C ha− 1). A combination of corresponding temperature and moisture reduction functions resulted in the highest deviations up to 41% (1.80 t C ha− 1). Under field conditions the sensitivity towards soil temperature reduction was 6 to 7 times higher compared to soil moisture reduction. The findings of this study show that the choice of soil temperature and soil moisture reduction functions is a crucial factor for a reliable simulation of carbon turnover.
Soil is a potential C sink and could offset rising atmospheric CO2. The capacity of soils to store and sequester C will depend on the rate of C inputs from plant productivity relative to C exports controlled by microbial decomposition. Management practices, such as no-tillage and high intensity cropping sequences, have the potential to enhance C and N sequestration in agricultural soils. An investigation was carried out to study the influence of long-term applications of fertilizers and manures on different organic C fractions in a Typic Haplustept under intensive sequence of cropping with maize–wheat–cowpea in a semi-arid sub-tropic of India. In 0–15 cm, the bulk density was lowest (1.52 Mg m−3) in plots treated with 100% NPK + FYM, while the control treatment showed the highest value (1.67 Mg m−3). Balanced application of NPK (100% NPK) showed significantly lower bulk density (1.56 Mg m−3) over either 100% N (1.67 Mg m−3) or 100% NP (1.61 Mg m−3) in surface soils. The application of super-optimal dose of NPK (150% NPK) showed higher total organic C (TOC) (12.9 g C kg−1) over either 50% NPK (9.3 g C kg−1) or 100% NPK (10.0 g C kg−1) in 0–15 cm soil layer. There was an improvement in TOC in 100% NPK or 100% NP (9.3 g C kg−1) over 100% N (8.7 g C kg−1) in the same depth. The application of FYM with 100% NPK showed 15.2, 9.9 and 5.2 g C kg−1 in 0–15, 15–30 and 30–45 cm, respectively. Application of graded doses of NPK from 50 to 150% of recommendation NPK significantly enhanced other organic C fractions like, microbial biomass C (MBC), particulate organic C (POC) and KMnO4 oxidizable C (KMnO4–C) in all the three soil depths. The TOC in 0–45 cm soil depth in 150% NPK (63.5 Mg C ha−1) was increased by 39% over that in 50% NPK treatment (51.5 Mg C ha−1) and 29% over that in 100% NPK treatment (54.1 Mg C ha−1). Integrated use of farmyard manure with 100% NPK (100% NPK + FYM) emerged as the most efficient management system in accumulating largest amount of organic C (72.1 Mg C ha−1) in soil. Nevertheless, this treatment also sequestered highest amount of organic C (731 kg C ha−1 year−1). Particulate organic carbon, a physically protected carbon pool in soil, could well be protected in sub-surface soil layers than in surface soil layer as a means of carbon aggradations. Microbial metabolic quotient (qCO2) was significantly lower in 100% NPK + FYM over other treatments to indicate this to be the most efficient manuring practice to preserve organic carbon in soil where it facilitates aggradations of more recalcitrant organic C in soil. As compared to POC, total TOC proved to be a better predictor of MBC as it strongly correlated with total carbon mineralized from soil.
Extensive research has focused on the temperature sensitivity of soil respiration. However, in Mediterranean ecosystems, soil respiration may have a pulsed response to precipitation events, especially during prolonged dry periods. Here, we investigate temporal variations in soil respiration (Rs), soil temperature (T) and soil water content (SWC) under three different land uses (a forest area, an abandoned agricultural field and a rainfed olive grove) in a dry Mediterranean area of southeast Spain, and evaluate the relative importance of soil temperature and water content as predictors of Rs. We hypothesize that soil moisture content, rather than soil temperature, becomes the major factor controlling CO2 efflux rates in this Mediterranean ecosystem during the summer dry season. Soil CO2 efflux was measured monthly between January 2006 and December 2007 using a portable soil respiration instrument fitted with a soil respiration chamber (LI-6400-09). Mean annual soil respiration rates were 2.06 ± 0.07, 1.71 ± 0.09, and 1.12 ± 0.12 μmol m−2 s−1 in the forest, abandoned field and olive grove, respectively. Rs was largely controlled by soil temperature above a soil water content threshold value of 10% at 0–15 cm depth for forest and olive grove, and 15% for abandoned field. However, below those thresholds Rs was controlled by soil moisture. Exponential and linear models adequately described Rs responses to environmental variables during the growing and dry seasons. Models combining abiotic (soil temperature and soil rewetting index) and biotic factors (above-ground biomass index and/or distance from the nearest tree) explained between 39 and 73% of the temporal variability of Rs in the forest and olive grove. However, in the abandoned field, a single variable – either soil temperature (growing season) or rewetting index (dry season) – was sufficient to explain between 51 and 63% of the soil CO2 efflux. The fact that the rewetting index, rather than soil water content, became the major factor controlling soil CO2 efflux rates during the prolonged summer drought emphasizes the need to quantify the effects of rain pulses in estimates of net annual carbon fluxes from soil in Mediterranean ecosystems.
To understand the effects of nitrogen fertilization on soil respiration in an intensively cultivated fluvo-aquic loamy soil, a field experiment was conducted in the Fengqiu State Key Agro-Ecological Experimental Station, Henan province, China. The experiment consisted of five treatments: unplanted and N-unfertilized soil (CK0), unplanted soil treated with 150 kg N ha− 1 (CKNL), maize (Zea mays L.) planted and N-unfertilized soil (N0), and planted soils fertilized with 150 kg N ha− 1 (NL) and 250 kg N ha− 1 (NH). Soil CO2 efflux during the maize growth season was significantly affected by soil temperature and also by soil moisture when the opposite effect of soil moisture below and above the optimum values was distinguished. There was a significant interdependence between soil temperature and soil moisture in the effect on soil CO2 efflux in the presence of maize plants. A logarithm transformed regression equation including soil temperature (T) and soil moisture (W) was developed as y = a + bT log(W). This equation accounted for 60–71% of the seasonal variation in soil CO2 efflux, which better depicted soil CO2 efflux than did a regression equation with soil temperature alone in the maize planted soils. Cumulative soil CO2 emissions in the CK0 and CKNL treatments were estimated as 229 ± 12 and 245 ± 17 g C m− 2, respectively during the experimental period and the application of N fertilizer slightly increased soil basal respiration by 6.5% through enhancing microbial biomass. In contrast, cumulative seasonal soil CO2 emissions were 7.4% lower in the NL (461 ± 33 g C m− 2) and NH (462 ± 13 g C m− 2) treatments than in the N0 treatment (498 ± 32 g C m− 2), indicating that N fertilization marginally significantly depressed soil respiration (p = 0.06). N application rates, however, did not exhibit any effects. Our results suggest that the effects of N fertilization on soil respiration mainly depended on the concentration of easily decomposed organic carbon in soil and N fertilization possibly reduced soil respiration in the planted soils when N released from the decomposition of native soil organic carbon roughly met the demand for maize growth. Yes Yes
Potential C and N mineralization and soil microbial biomass C (SMBC) are soil biological properties important in understanding nutrient and organic matter dynamics. Knowledge of soil water content at a matric potential near field capacity is needed to determine these biological properties. The objective of this study was to examine whether adjustment of soil water content to a common level of water-filled pore space (WFPS) may be an acceptable alternative that would require little prior analysis in comparison with adjustment based on matric potential. Potential C and N mineralization and SMBC were determined from 15 variably eroded soils of the Madison-Cecil-Pacolet association (clayey, kaolinitic, thermic Typic Kanhapludults) in response to WFPS. The levels of WFPS to achieve maximum activity and biomass under naturally settled conditions were unaffected by clay content and occurred at 0.42±0.03 m3 m-3 for net N mineralization during 24 days of incubation, 0.51±0.22 m3 m-3 for specific respiratory activity of SMBC, 0.60±0.07 m3 m-3 for cumulative C mineralization during 24 d of incubation, and 0.76±0.27 m3 m-3 for SMBC. Selecting a common WFPS level of 0.5 m3 m-3 resulted in 96±2%, 97±5%, 97±4%, and 88±10% of the maximum for these four properties, respectively, and was a reasonable compromise when attempting to estimate these properties during simultaneous incubations. Adjusting soil water content based on WFPS was simpler and nearly as reliable as based on matric potential, in which soil water content at -33 kPa varied from 0.16 to 0.30 g g-1.
Sequestration and storage of carbon (C) by agricultural soils has been cited as one potential part of the solution to soil degradation and global climate change. However, C sequestration in soils is a slow and dynamic process. The objective of this study was to evaluate the effects of crop rotation and N fertilizer management on soil organic C (SOC) levels at several points in time during 18 yr of a long-term study in the Western Corn Belt. Seven cropping systems (three monoculture, two 2-yr, and two 4-yr rotations) with three levels of N fertilizer were compared. Soil samples were taken in the spring in 1984, 1992, 1998, and 2002 to a depth of 30 cm in 0- to 7.5-, 7.5- to 15-, and 15- to 30-cm increments. No differences were obtained in SOC levels in 1984 at the beginning of the study. After 8 yr, rotation significantly increased SOC 449 kg ha(-1) across all cropping systems. From 1992 to 2002, SOC levels in the 0- to 7.5-cm depth decreased by 516 kg ha(-1) across all cropping systems. Soil organic C levels in the 7.5- to 15-cm depths in 1992 and 2002 demonstrated similar rotation effects to those in the surface 0- to 7.5-cm, being not significantly affected from 1984 to 1992 but being significantly decreased from 1992 to 2002 (568 kg SOC ha(-1) across all cropping systems). Many of the SOC gains in the surface 30 cm measured during the first 8 yr of the study were lost during the next 10 yr in all but the 4-yr cropping systems after 18 yr. The loss of SOC in this latter period occurred when depth of tillage was increased by using a tandem disk with larger-diameter disks. These results demonstrate that more than one point-in-time measurement from long-term experiments is necessary to monitor SOC changes when several management variables, such as cropping system and N fertilizer, are being used. They also indicate that apparent small changes in cultural practices, such as in depth of tillage in this experiment, can significantly change SOC dynamics in the soil. Subtle changes in cultural practices (e.g., tillage depth) can have significant long-term results, but long-term experiments are required to quantify their impact under variable climatic conditions.