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Temperature control on wastewater and downstream nitrous oxide emissions in an urbanized river system
Abstract
Although eutrophic urban rivers receiving loads of wastewater represent an important anthropogenic source of N2O, little is known as to how temperature and other environmental factors affect temporal variations in N2O emissions from wastewater treatment plants (WWTPs) and downstream rivers. Two-year monitoring at a WWTP and five river sites was complemented with available water quality data, laboratory incubations, and stable isotopes in N2O and NO3- to explore how wastewater effluents interact with seasonal changes in environmental conditions to affect downstream metabolic processes and N2O emissions from the lower Han River traversing the megacity Seoul. Water quality data from four WWTPs revealed significant inverse relationships between water temperature and the concentrations or fluxes of total N (TN) in effluents. Increased TN fluxes at low temperatures concurred with N2O surges in WWTP effluents and downstream rivers, counteracting the long-term decline in TN fluxes resulting from enhanced wastewater treatments. Incubation experiments with river water and sediment, in isolation or combined, implied the hypoxic winter sediment as a large source of N2O, whereas the anoxic summer sediment produced a smaller amount of N2O only when it was added with oxic water. For both WWTP effluents and downstream rivers, bulk isotope ratios and intramolecular distribution of 15N in N2O distinctly differed between summer and winter, indicating incomplete denitrification in the hypoxic sediment at low temperatures as a primary downstream source adding to WWTP-derived N2O. Winter surges in wastewater TN and sediment N2O release highlight temperature variability as an underappreciated control over anthropogenic N2O emissions from increasingly urbanized river systems worldwide.
... Generally, riverine N 2 O is produced as a byproduct of microbially mediated denitrification and nitrification of terrestrial nitrogen (Beaulieu et al., 2011;Marzadri et al., 2021;Yang and Lei, 2018), while CH 4 derives from the degradation of organic matter in the anaerobic sediment via methanogenesis Wang et al., 2018). It is generally assumed that organic carbon and inorganic nitrogen load within rivers are closely related to riverine CH 4 and N 2 O yield (Chun et al., 2020;Lin et al., 2022). However, river ecosystem is a dynamic bioreactor with uncontrollable reaction conditions. ...
... seasonal variability of riverine CH 4 and N 2 O emissions since it can strongly regulate in-stream microbial proliferation and metabolism (Beaulieu et al., 2010;Chun et al., 2020;Hinshaw and Dahlgren, 2013). However, temperature showed no relationship with CH 4 and N 2 O concentrations in our study. ...
... In many rivers disturbed by cities, denitrification was found to be the main pathway of N 2 O production Stow et al., 2005;Yang and Lei, 2018). While, autotrophic nitrification and anaerobic ammonification-nitrification have been widely found in polluted waters (Chun et al., 2020;Ho et al., 2022;Nirmal Rajkumar et al., 2008). When we only analyzed data of urban reaches, NO 3 − and NH 4 + still presented positive correlations with N 2 O concentration. ...
Urban rivers have been proved to be the hot spots of atmospheric methane (CH 4) and nitrous oxide (N 2 O) emissions. However, the rivers across rural-urban interface, which are undergoing significant environmental changes in the process of urbanization, received little attention. In this study, we conducted seasonal investigations in two small suburban rivers in mountainous southwest China, to clarify the spatial-temporal characteristics of CH 4 and N 2 O concentrations and fluxes in relation to watershed land use change and their potential controls. The results showed that, the small suburban rivers acted as strong CH 4 and N 2 O emitters to atmosphere with averaged fluxes of 960.2±966.6 and 205.1±240.8 µmol⋅m − 2 ⋅d − 1 , respectively. The concentrations and fluxes of CH 4 and N 2 O presented a sharp increase and spatial variability when the rivers cross the rural-urban interface. Such discontinuous spatial pattern of CH 4 and N 2 O emission in river continuum were closely related to watershed land use, and could be modeled as functions of the urban land use proportion. The urban land proportion in the basins could explain 70% and 59% of the total spatial variations of CH 4 and N 2 O emissions, respectively. Stepwise multiple regression models (SMRM) based on water environment parameters were established in our study and indicate that DTC, NH 4 + , NO 3 − , TP and conductivity play key controls on CH 4 and N 2 O emissions in the two studied mountainous suburban rivers. Moreover, SMRM and urban land use based functions were tested to have good predictions based on constrained mountainous samples, could provide a possible path to extrapolate CH 4 and N 2 O emissions in mountainous urbanized-rivers at the whole regional network scale. We also found evidences for the sewage-dominated tributary with abnormally high CH 4 and N 2 O concentrations, which was partially responsible for the sharp increase of CH 4 and N 2 O fluxes in urban reaches. The results of structural equation model provided strong evidence for direct and indirect controls of urban expansion on longitudinal differentiation of riverine CH 4 and N 2 O emissions crossing rural-urban interface. We argued that CH 4 and N 2 O emissions from rivers in the mountainous rural-urban area are sensitive to the watershed urbanization and water pollution, and suggested that land use management, nutrient and organic matter control can be promising avenues to mitigate riverine CH 4 and N 2 O emissions.
... In recent years, more and more research have emphasized that severely polluted urban rivers and streams are hotspots of CH 4 and N 2 O emissions to the atmosphere (Brigham et al., 2019;Chun et al., 2020;Wang et al., 2018b). For instance, CH 4 fluxes from urban sewage draining rivers in Tianjin, China, were reported 10-fold higher than these from natural rivers (Hu et al., 2018). ...
... Even higher CH 4 fluxes were pointed out by Martinez-Cruz et al. (2017) in tropical urban rivers in Mexico City, where the peak flux reached up to 1094 mmol m − 2 d − 1 (259 times the average of global rivers). Meanwhile, strikingly high N 2 O fluxes have been also observed in several heavily polluted urban rivers, including Han River, Adyar River, Xiaoyue River, Nanfei River (Chun et al., 2020;Nirmal Rajkumar et al., 2008;Wang et al., 2020;Yang et al., 2011). Yu et al. (2017) and Yu et al. (2013) pointed out that urban rivers had generally higher CH 4 and N 2 O fluxes than rural ones via a systematic investigation of river network in the Shanghai metropolitan area. ...
... There were generally predictable correlations between organic carbon (OC), nitrate (NO 3 − ), ammonia (NH 4 + ) and dissolved oxygen (DO), with CH 4 and N 2 O concentrations in urbanized rivers (Borges et al., 2018;Burgos et al., 2015;He et al., 2018), suggesting that large nutrient supply and hypoxic environments are two main drivers of higher CH 4 and N 2 O fluxes. Meanwhile, previous studies on rivers in Tianjin Hu et al., 2018), Shanghai Yu et al., 2013;Yu et al., 2017), Chongqing (He et al., 2017;Tang et al., 2021a;Wang et al., 2018b), Seoul (Chun et al., 2020;Jin et al., 2018) have all emphasized that exogenous sewage was important artificial source of CH 4 and N 2 O in urban waters. CH 4 and N 2 O concentrations in Ohio River increased 38.2 times when flowing through a series of municipal sewage treatment plants (Beaulieu et al., 2010). ...
Streams draining urban areas are usually regarded as hotspots of methane (CH4) and nitrous oxide (N2O) emissions. However, little is known about the coupling effects of watershed pollution and restoration on CH4 and N2O emission dynamics in heavily polluted urban streams. This study investigated the CH4 and N2O concentrations and fluxes in six streams that used to be heavily polluted but have undergone different watershed restorations in Southwest China, to explore the comprehensive influences of pollution and restoration. CH4 and N2O concentrations in the six urban streams ranged from 0.12 to 21.32 μmol L⁻¹ and from 0.03 to 2.27 μmol L⁻¹, respectively. The calculated diffusive fluxes of CH4 and N2O were averaged of 7.65 ± 9.20 mmol m⁻² d⁻¹ and 0.73 ± 0.83 mmol m⁻² d⁻¹, much higher than those in most previous reports. The heavily polluted streams with non-restoration had 7.2 and 7.8 times CH4 and N2O concentrations higher than those in the fully restored streams, respectively. Particularly, CH4 and N2O fluxes in the fully restored streams were 90% less likely than those found in the unrestored ones. This result highlighted that heavily polluted urban streams with high pollution loadings were indeed hotspots of CH4 and N2O emissions throughout the year, while comprehensive restoration can effectively weaken their emission intensity. Sewage interception and nutrient removal, especially N loadings reduction, were effective measures for regulating the dynamics of CH4 and N2O emissions from the heavily polluted streams. Based on global and regional integration, it further elucidated that increasing environment investments could significantly improve water quality and mitigate CH4 and N2O emissions in polluted urban streams. Overall, our study emphasized that although urbanization could inevitably strengthen riverine CH4 and N2O emissions, effective eco-restoration can mitigate the crisis of riverine greenhouse gas emissions.
... This balance is regulated by microbial communities, consisting of N 2 O producers and consumers (Zhao et al., 2018). Exploiting N 2 O-consuming bacteria has gathered attention for mitigating N 2 O emissions (Duan et al., 2021;Shan et al., 2021), and their activities are influenced by environmental conditions, e.g., temperature, oxygen, and nitrogen loading (Chun et al., 2020;Rosamond et al., 2012;Wang et al., 2015). Primarily, nitrogen loading stimulates N 2 O emissions and the nitrogen removal capacity in rivers (Weisener et al., 2017;Zhou et al., 2019). ...
... Significant variation in nitrogen loading from WWTP discharge and urban runoff may greatly alter the riverine microbial processes associated with nitrogen conversion (Lofton et al., 2007). Treated water discharge from WWTPs affected the balance of nitrogen transformation and eventually enhanced N 2 O emissions from downstream waterways (Chun et al., 2020). Despite the significance of N 2 O emission, little is known about the relative contribution of WWTP effluent to microbial activities associated with the N 2 O production and consumption in urban rivers. ...
... The sampling campaign and laboratory analysis determined that the peak N 2 O concentration (1.25 µg-N L − 1 ) in the river was far below the levels previously reported in urban rivers, but was comparable with agricultural river systems (Table 2). This result contradicts previous studies observing the substantially higher N 2 O emissions in urban rivers receiving higher nitrogen loadings (Chun et al., 2020;Wang et al., 2020;Zhang et al., 2021). Further, the EF 5r in the Tama River (0.015 ± 0.004% in summer; 0.018 ± 0.003% in winter) was lower than the EF 5r in other urban river systems with comparable nitrogen loadings (Table 2), and was one order of magnitude lower than the IPCC default value (0.25%). ...
Urban rivers receive used water derived from anthropogenic activities and are a crucial source of the potent greenhouse gas nitrous oxide (N2O). However, considerable uncertainties still exist regarding the variation and mechanisms of N2O production in response to the discharge of treated sewage from municipal wastewater treatment plants (WWTPs). This study investigated N2O concentrations and microbial processes responsible for nitrogen conversion upstream and downstream of WWTPs along the Tama River flowing through Tokyo, Japan. We evaluated the effect of treated sewage on dissolved N2O concentrations and inherent N2O consumption activities in the river sediments. In summer and winter, the mean dissolved N2O concentrations were 0.67 µg-N L⁻¹ and 0.82 µg-N L⁻¹, respectively. Although the dissolved N2O was supersaturated (mean 288.7% in summer, mean 240.7% in winter) in the river, the N2O emission factors (EF5r, 0.013%–0.025%) were significantly lower than those in other urban rivers and the Intergovernmental Panel on Climate Change default value (0.25%). The nitrate (NO3⁻) concentration in the Tama River increased downstream of the WWTPs discharge sites, and it was the main nitrogen constituent. An increasing trend of NO3⁻ concentration was observed from upstream to downstream, along with an increase in the N2O consumption potential of the river sediment. A multiple regression model showed that NO3⁻ is the crucial factor influencing N2O saturation. The diversity in the upstream microbial communities was greater than that in the downstream ones, indicating the involvement of treated sewage discharge in shaping the microbial communities. Functional gene quantification for N2O production and consumption suggested that nirK-type denitrifiers likely contributed to N2O production. Structural equation models (SEMs) revealed that treated sewage discharged from WWTPs increased the NO3⁻ loading from upstream to downstream in the river, inducing changes in the microbial communities and enhancing the N2O consumption activities. Collectively, aerobic conditions limited denitrification and in turn facilitated nitrification, leading to low N2O emissions even despite high NO3⁻ loadings in the Tama River. Our findings unravel an overestimation of the N2O emission potential in an urban oxygen-rich river affected by treated sewage discharge.
... Excessive leaching of N from agricultural watersheds can cause intense eutrophication (Graeber et al., 2015;Smith et al., 1999). Dissolved inorganic N (DIN) is often the main target in watershed management, but DON fractions are also involved in critical environmental problems such as eutrophication, hypoxia, DOM bioavailability, and greenhouse gas (i.e., N 2 O) emissions (Chun et al., 2020;Li et al., 2018;Osburn et al., 2016). Organic N accounts for 12-94% of the riverine dissolved N pools (Li et al., 2016;Pisani et al., 2017;Sipler and Bronk, 2015). ...
... Management of DIN has been reported in agricultural and urban watersheds to improve water quality by controlling eutrophication, harmful algal blooms, and greenhouse gas emissions (Chun et al., 2020;Smith et al., 1999). However, many studies have reported a significant contribution of DON to agricultural runoff affected by inorganic fertilizer or manure (Graeber et al., 2015;Hussain et al., 2020;Li et al., 2018;Luek et al., 2020;Manninen et al., 2018). ...
Accelerated export of nitrogen-containing dissolved organic matter (DOM) or dissolved organic nitrogen (DON) to streams and rivers from agricultural watersheds has been reported worldwide. However, few studies have examined the dynamics of DOM molecular composition with the attention paid to the relative contributions of DON from various sources altered with flow conditions. In this study, end-member mixing analysis (EMMA) was conducted with the optical properties of DOM to quantify the relative contributions of several major organic matter sources (litter, reed, field soil, and manure) in two rivers of a small agricultural watershed. DOC and DON concentration increased during the storm events with an input of allochthonous DOM as indicated by an increase in specific ultraviolet absorbance at 254 nm (SUVA254) and a decrease in biological index (BIX), fluorescence index (FI), and protein-like component (%C3) at high discharge. EMMA results based on a Bayesian mixing model using stable isotope analysis in R (SIAR) were more accurate in source tracking than those using the traditional IsoSource program. Manure (>30%) and field soil (also termed as “manure-impacted field soil”) (>23%) end-members revealed their predominant contributions to the riverine DOM in SIAR model, which was enhanced during the storm event (up to 56% and 38%, respectively). The molecular composition of the riverine DOM exhibited a distinct footprint from the manure and manure-impacted field soil, with a larger number of CHON formulas and abundant polyphenols and condensed aromatics in peak flow samples in the studied rivers. The riverine DOM during peak flow contained many unique molecular formulas in both rivers (4980 and 2082) of which >60% originated from manure and manure-impacted field soil. Combining the EMMA with DOM molecular composition clearly demonstrated the effect of manure fertilizer on the riverine DOM of the watershed with intensive agriculture. This study provides insights into the source tracking and regulation of DON leaching from anthropogenically altered river systems worldwide.
... Seasonal floodings can deplete O 2 in saturated sediments, creating favorable conditions for anaerobic methanogens producing CH 4 in wetlands that are naturally formed or artificially constructed along floodplains (Sha et al. 2011). Nitrification and denitrification in river sediments are also sensitive to changes in the availability of O 2 and moisture in sediments, generating N 2 O as byproducts and hence increasing N 2 O emissions from rivers (Chun et al. 2020) and surrounding wetlands (Ruser et al. 2006). Although numerous individual studies have investigated climatic effects on the production of GHGs in river and wetland sediments, there has been limited research comparing the differential temperature responses of the three GHGs. ...
Despite the rising interest in understanding how climate change could affect the emissions of greenhouse gases (GHGs) from river systems, including floodplains, we still lack a mechanistic understanding of how changing environmental conditions, such as moisture and nutrient availability, limit the temperature responses of GHG production in floodplain sediments. To examine the environmental co-limitations on the temperature responses of three major GHGs (CO2, CH4, and N2O) produced in floodplain sediments, sediments from a constructed wetland on the floodplain of the lower Han River were incubated for 24 d at four temperatures spanning 4–28 ℃, under three conditions (closed, open/wetting, and open/drying). The net production of all three GHGs exhibited nonlinear temperature responses with gas-specific patterns and magnitudes of response varying over the incubation period. During the later incubation phase, positive temperature responses were weakened for the net production of CO2 and CH4 in the dried treatments, whereas a similar weakening occurred for N2O production in the wet treatments. This, combined with incubation-induced changes in dissolved organic carbon and its fluorescence components, indicated the lack or excess of moisture and associated changes in O2 and organic carbon availability as critical co-limiting factors for the temperature responses of GHG production. Warming decreased δ¹³C in the CH4 emitted from wet and hypoxic sediments, implying a stronger warming effect on CH4 production over oxidation. Unlike many studies assuming a consistent relationship between temperature and GHG production in sediments irrespective of other environmental conditions, our results suggest that warming effects on the GHG emissions from floodplain sediments would depend on the balance between gas production and consumption under the prevailing constraints of moisture, O2, and labile carbon availability.
... Dong et al. (2023) evaluated the potential impact of wastewater nitrogen discharge on estuarine N 2 O emissions globally. Here we compiled data from previous studies with direct N 2 O measurements in aquatic systems associated with WWTPs (not included in Dong et al., 2023) to assess the global impact of WWTPs on aquatic N 2 O concentrations or emissions (McElroy et al., 1978;Hemond and Duran, 1989;Toyoda et al., 2009;Beaulieu et al., 2010;Rosamond et al., 2012;Chun et al., 2020;Masuda et al., 2021Masuda et al., , 2018Dylla, 2019). WWTP effluents and water downstream of the WWTPs contain some of the highest N 2 O concentrations and fluxes observed in the aquatic system (Table 1 and Fig. S8). ...
Nitrous oxide (N2O), a potent greenhouse gas and ozone-destroying agent, is produced during nitrogen transformations in both natural and human-constructed environments. Wastewater treatment plants (WWTPs) produce and emit N2O into the atmosphere during the nitrogen removal process. However, the impact of WWTPs on N2O emissions in downstream aquatic systems remains poorly constrained. By measuring N2O concentrations at a monthly resolution over a year in the Potomac River estuary, a tributary of the Chesapeake Bay in the eastern United States, we found a strong seasonal variation in N2O concentrations and fluxes: N2O concentrations were larger in fall and winter, but the flux was larger in summer and fall. Observations at multiple stations across the Potomac River estuary revealed hotspots of N2O emissions downstream of WWTPs. N2O concentrations were higher at stations downstream of WWTPs compared to other stations (median: 21.2 nM vs. 16.2 nM) despite the similar concentration of dissolved inorganic nitrogen, suggesting the direct discharge of N2O from WWTPs into the aquatic system or a higher N2O production yield in waters influenced by WWTPs. Meta-analysis of N2O measurements associated with WWTPs globally revealed variable influence of WWTPs on downstream N2O concentrations and emissions. Since wastewater production has increased substantially with the growing population and is projected to continue to rise, accurately accounting for N2O emissions downstream of WWTPs is important for constraining and predicting future global N2O emissions. Efficient N2O removal, in addition to dissolved nitrogen removal, should be an essential part of water quality control in WWTPs.
... Thus, point sources seemed not to be the cause for the elevated N 2 O concentrations. However, discharge of WWTPs can potentially be important sources of N 2 O (Beaulieu et al., 2010;Chun et al., 2020;Brown et al., 2022), and the effect of wastewater input on N 2 O concentrations and emissions may change with altered river discharge, water temperature, and riverine nitrogen loads in the future. ...
Nitrous oxide (N2O) is a greenhouse gas, with a global warming potential 298 times that of carbon dioxide. Estuaries can be sources of N2O, but their emission estimates have significant uncertainties due to limited data availability and high spatiotemporal variability. We investigated the spatial and seasonal variability of dissolved N2O and its emissions along the Elbe Estuary (Germany), a well-mixed temperate estuary with high nutrient loading from agriculture. During nine research cruises performed between 2017 and 2022, we measured dissolved N2O concentrations, as well as dissolved nutrient and oxygen concentrations along the estuary, and calculated N2O saturations, flux densities, and emissions. We found that the estuary was a year-round source of N2O, with the highest emissions in winter when dissolved inorganic nitrogen (DIN) loads and wind speeds are high. However, in spring and summer, N2O saturations and emissions did not decrease alongside lower riverine nitrogen loads, suggesting that estuarine in situ N2O production is an important source of N2O. We identified two hotspot areas of N2O production: the Port of Hamburg, a major port region, and the mesohaline estuary near the maximum turbidity zone (MTZ). N2O production was fueled by the decomposition of riverine organic matter in the Hamburg Port and by marine organic matter in the MTZ. A comparison with previous measurements in the Elbe Estuary revealed that N2O saturation did not decrease alongside the decrease in DIN concentrations after a significant improvement of water quality in the 1990s that allowed for phytoplankton growth to re-establish in the river and estuary. The overarching control of phytoplankton growth on organic matter and, subsequently, on N2O production highlights the fact that eutrophication and elevated agricultural nutrient input can increase N2O emissions in estuaries.
... For example, Mander et al. (2014) reported that δ 18 O was a good indicator of denitrification in the riparian zone of an Alder Forest with high nitrogen loading due to upstream nitrogen chemical fertilizer application. Recently, there has been an increase in the number of studies of source partitioning based on the isotopic ratio of dissolved N 2 O in the aqueous environment, such as those applied to the groundwater (Well et al., 2005a) and to the urban rivers (Toyoda et al., 2009;Thuan et al., 2018;Ma et al., 2019;Chun et al., 2020) and to the deep sea (Toyoda et al., 2019). For example, Thuan et al. (2018) showed from isotope ratios that nitrate reduction is the dominant source of dissolved N 2 O in the Tama River and that it is highly correlated with the expression level of the nitrate reductase (NirK) gene. ...
Oil palm plantations in Southeast Asia are the largest supplier of palm oil products and have been rapidly expanding in the last three decades even in peat-swamp areas. Oil palm plantations on peat ecosystems have a unique water management system that lowers the water table and, thus, may yield indirect N2O emissions from the peat drainage system. We conducted two seasons of spatial monitoring for the dissolved N2O concentrations in the drainage and adjacent rivers of palm oil plantations on peat swamps in Sarawak, Malaysia, to evaluate the magnitude of indirect N2O emissions from this ecosystem. In both the dry and wet seasons, the mean and median dissolved N2O concentrations exhibited over-saturation in the drainage water, i.e., the oil palm plantation drainage may be a source of N2O to the atmosphere. In the wet season, the spatial distribution of dissolved N2O showed bimodal peaks in both the unsaturated and over-saturated concentrations. The bulk δ15N of dissolved N2O was higher than the source of inorganic N in the oil palm plantation (i.e., N fertilizer and soil organic nitrogen) during both seasons. An isotopocule analysis of the dissolved N2O suggested that denitrification was a major source of N2O, followed by N2O reduction processes that occurred in the drainage water. The δ15N and site preference mapping analysis in dissolved N2O revealed that a significant proportion of the N2O produced in peat and drainage is reduced to N2 before being released into the atmosphere.
... Aside from nonlinearity, the WWTP parameters may also be characterized by dynamic behaviors. These may result from seasonal changes, such as higher oxidation rates owing to higher temperatures in some months of the year [61]. Increased human activity during waking hours (e.g., 08:00 to 22:00) also results in more water consumption, and thus rising pollutant concentrations in wastewater during those hours [44,62]. ...
The on-line monitoring of wastewater treatment plant (WWTP) operations is a challenge due to interference and breakdown from the harsh conditions endured by sensors in wastewater. To lessen the dependence on hardware sensors, mathematical models have been developed for estimating wastewater parameters. These so-called software (soft) sensors have advanced significantly, from mechanistic modelling to the latest machine learning models. The current review aimed to characterize these advancements in WWTP soft sensors by (1) identifying the current status of WWTP soft sensors; (2) analyzing the advancements in soft sensor development methods over time; and (3) evaluating WWTP soft sensors in relation to hardware technology. It is difficult to define an all-encompassing ‘state-of-the-art’ owing to significant variations in the physical and statistical properties of different WWTPs. However, the study was able to evaluate the effectiveness of these methods in specific contexts, based on the statistical properties of the dataset used for soft sensor development. It found that, although neural networks have remained the dominant methodology for soft sensor development since the early 2000s, some decision tree-based approaches have shown promising performance and enhanced robustness. It also highlights the importance of adjunct statistical methods for handling multicollinearity and noise, which are common problems in WWTP datasets. Opportunities to use soft sensor modelling approaches to enhance hardware sensor performance have also been identified. Continuous improvements in the reliability and range of measurement of hardware sensors, are expected to enhance the performance and scope of application of WWTP soft sensors.
The IPCC’s estimates of N2O emission focus on soils in forests, grasslands and agricultural lands, but often neglect the contributions from aquatic systems, especially small coastal water bodies in build-up areas. In this study, we conducted seasonal sampling of N2O concentrations, N2O fluxes and the relevant functional gene abundances in various water bodies (farmland ditches, aquaculture sewage ditches, tidal creeks, aquaculture ponds, town sewage ditches, and restored aquaculture ponds-to-wetlands) in the Min River Estuary (MRE) in Southeast China. The results showed that all the water bodies were consistently oversaturated in N2O relative to the overlying atmosphere. The town sewage ditches were hotspots for N2O production, with a mean dissolved N2O concentration of ~42.9 nmol L-1 (range 24.7–62.5 nmol L-1), which is 2.9–13.7 times greater than those in the other water bodies. The estimated N2O emission from town sewage ditches was ~1097.6 nmol m-2 h-1, which was ~28.2, 10.7, 4.4 and 3.4 times that from farmland ditches, aquaculture sewage ditches, tidal creeks and aquaculture ponds, respectively. Nitrogen substrate availability and abundance of AOB amoA and nirS genes were the key factors driving the variations in N2O concentration and emission among the various water bodies. Our results highlighted that coastal small water bodies were strong N2O emission source per unit area, but they tend to be poorly surveyed and need to be considered in the national greenhouse gas inventory.
The riverine N2O fluxes are assumed to linearly increase with nitrate loading. However, this linear relationship with a uniform EF5r is poorly constrained, which impedes the N2O estimation and mitigation. Our meta-analysis discovered a universal N2O emission baseline (EF5r = k/[NO3−], k = 0.02) for natural rivers. Anthropogenic impacts caused an overall increase in baselines and the emergence of hotspots, which constitute two typical patterns of anthropogenic sources. The k values of agricultural and urban rivers increased to 0.09 and 0.05, respectively, with 11% and 14% of points becoming N2O hotspots. Priority control of organic and NH4+ pollution could eliminate hotspots and reduce emissions by 51.6% and 63.7%, respectively. Further restoration of baseline emissions on nitrate removal is a long-term challenge considering population growth and declining unit benefits (ΔN-N2O/N-NO3−). The discovery of EF-lines emphasized the importance of targeting hotspots and managing baseline emissions sustainably to balance social and environmental benefits.
Estuaries are strong sources of N2O to the atmosphere; yet we still lack insights into the impact of their biogeochemical dynamics on the emissions of this powerful greenhouse gas. Here, we investigated the spatiotemporal dynamics of the N cycle in an estuary with a focus on the emission mechanisms and pathways of N2O. By coupling N2O isotopocule analysis and substrate NO3- isotope analysis, we found that nutrient availability, oxygen level, salinity gradient and temperature variation were major drivers of the N2O emissions from the Scheldt Estuary. In winter, lower temperature and higher O2 concentration diminished denitrification rates and reduction of N2O to N2, while both were enhanced in warmer summer, causing higher fraction of reduced N2O. As a result, we found comparable N2O fluxes and dissolved concentrations between the two seasons. Decrease in salinity level and increase in NO3- concentration accelerated N2O production when moving upstream of the estuary where more urbanization and higher NO3- from wastewater discharges were found. However, these drivers had no significant effect on the fraction of N2O derived by either denitrification or nitrification and/or fungal denitrification since the fractional proportion of these pathways showed no spatiotemporal variations, remaining around 89% and 11%, respectively. These findings challenge the conventional notion that N2O fluxes are generally higher in summer because of higher denitrification rates while confirming that denitrification is the most important pathway of N2O production in the estuaries. Furthermore, our study highlight the importance of combining various isotope analyses to gain in-depth understanding about N2O emission pathways and N cycling in dynamic systems like estuaries.
River water quality plays a crucial role in numerous industries, necessitating the precise delineation of riparian buffer zones and the implementation of comprehensive management measures to preserve water quality. Determining the optimal riparian buffer zone through the impact of landscape metrics on water quality has been widely applied to river management. However, failure to differentiate the same indicators in areas with different anthropogenic activities could lead to inaccuracies in identifying the optimal riparian buffer zone, particularly in regions with notable gradient in anthropogenic activities. Here, we incorporated a new landscape intensity (NLI) indicator into the landscape metrics to better understand controls of water quality and optimal width of riparian buffer zone, considering the differences between areas with different anthropogenic activities. Based on water quality monitoring data from June 2010 to April 2013 in a typical agricultural-urban gradient with different anthropogenic activities, we adopted redundancy analysis (RDA) to quantify the spatial scale effects and seasonal dependence of various landscape metrics impact on river water quality, and then to reveal the importance of landscape metrics in explaining water quality by variance partitioning analysis (VPA). Results showed that landscape metrics were more effective at explaining the water quality variations during the dry season than the wet season (56.48% ∼ 68.99% vs 53.28% ∼ 58.91%). The 1000 m riparian buffer zone was found to be optimal for explaining water quality changes during the dry season, while the 200 m riparian buffer zone was optimal during the wet season. In addition, we found that NLI was the most important landscape metric in our study and could explain 26% to 52% of the variation in water quality. This study provides a new insight for developing a landscape metric that considers differences in anthropogenic activities, which can help us to better understand water quality changes and preserve aquatic ecosystems.
Ostreobium, the major algal symbiont of the coral skeleton, remains understudied despite extensive research on the coral holobiont. The enclosed nature of the coral skeleton might reduce the dispersal and exposure of residing bacteria to the outside environment, allowing stronger associations with the algae. Here, we describe the bacterial communities associated with cultured strains of 5 Ostreobium clades using 16S rRNA sequencing. We shed light on their likely physical associations by comparative analysis of three datasets generated to capture (1) all algae associated bacteria, (2) enriched tightly attached and potential intracellular bacteria, and (3) bacteria in spent media. Our data showed that while some bacteria may be loosely attached, some tend to be tightly attached or potentially intracellular. Although colonised with diverse bacteria, Ostreobium preferentially associated with 34 bacterial taxa revealing a core microbiome. These bacteria include known nitrogen cyclers, polysaccharide degraders, sulphate reducers, antimicrobial compound producers, methylotrophs, and vitamin B12 producers. By analysing co-occurrence networks of 16S rRNA datasets from Porites lutea and Paragoniastrea australensis skeleton samples, we show that the Ostreobium-bacterial associations present in the cultures are likely to also occur in their natural environment. Finally, our data show significant congruence between the Ostreobium phylogeny and the community composition of its tightly associated microbiome, largely due to the phylosymbiotic signal originating from the core bacterial taxa. This study offers insight into the Ostreobium microbiome and reveals preferential associations that warrant further testing from functional and evolutionary perspectives.
To investigate the major triggers of nitrous oxide (N2O) production in a full-scale wastewater treatment plant, N2O emissions and wastewater characteristics (ammonia, nitrite, nitrate, total nitrogen, dissolved inorganic carbon, dissolved organic carbon, pH, temperature, dissolved oxygen and specific oxygen uptake rate), the results of variations in the cycling of a sequential batch reactor (SBR, where only full nitrification was performed), were monitored seasonally for 16 months. Major triggers of N2O production were investigated based on a seasonal measured database using a random forest (RF) model and sensitivity analysis, which was applied to identify important input variables. As the result of seasonal monitoring in the full-scale SBR, the N2O emission factor relative to daily total nitrogen removal ranged from 0.05 to 2.68%, corresponding to a range of N2O production rate from 0.02 to 0.70 kg-N/day. Results from the RF model and sensitivity analysis revealed that emissions during nitrification were directly or indirectly related to nitrite accumulation, temperature, ammonia loading rate and the specific oxygen uptake rate ratio between ammonia oxidizing bacteria and nitrite oxidizing bacteria (sOUR-ratio). However, changes in the microbial community did not significantly impact N2O emissions. Based on these results, the sOUR-ratio could represent the major trigger for N2O emission in a full-scale BNR system: a higher sOUR-ratio value with an average of 3.13 ± 0.23 was linked to a higher N2O production rate with an average value of 1.27 ± 0.12 kg-N/day (corresponding to 3.96 ± 1.20% of N2O emission factor relative to daily TN removal), while a lower sOUR-ratio with an average value of 2.39 ± 0.27 was correlated with a lower N2O production average rate of 0.17 ± 0.11 kg-N/day (corresponding to 0.74 ± 0.69% of N2O emission factor) (p-value = 0.00001, Mann-Whitney test).
Surrounded by intense anthropogenic activities, urban polluted rivers have increasingly been reported as a significant source of greenhouse gases (GHGs). However, unlike pollution and climate change, no integrated urban water models have investigated the GHG production in urban rivers due to system complexity. In this study, we proposed a novel integrated framework of mechanistic and data-driven models to qualitatively assess the risks of GHG accumulation in an urban river system in different water management interventions. Particularly, the mechanistic model delivered elaborated insights into river states in four intervention scenarios in which the installation of a new wastewater treatment plant using two different technologies, together with new sewage systems and additional retention tanks, were assessed during dry and rainy seasons. From the insights, we applied fuzzy rule-based models as a decision support tool to predict the GHG accumulation risks and identify their driving factors in the scenarios. The obtained results indicated the important role of new discharge connection and additional storage capacity in decreasing pollutant concentrations, consequently, reducing the risks. Moreover, among the major variables explaining the GHG accumulation in the rivers, DO level was considerably affected by the reaeration capacity of the rivers that was strongly dependent on river slope and flow. Furthermore, river water quality emerged as the most critical variable explaining the pCO2 and N2O accumulation that implied that the more polluted and anaerobic the sites were, the higher were their GHG accumulation. Given its simplicity and transparency, the proposed modeling framework can be applied to other river basins as a decision support tool in setting up integrated urban water management plans.
Nitrous oxide (N2O) emissions from inland waters remain a major source of uncertainty in global greenhouse gas budgets. N2O emissions are typically estimated using emission factors (EFs), defined as the proportion of the terrestrial nitrogen (N) load to a water body that is emitted as N2O to the atmosphere. The Intergovernmental Panel on Climate Change (IPCC) has proposed EFs of 0.25% and 0.75%, though studies have suggested that both these values are either too high or too low. In this work, we develop a mechanistic modeling approach to explicitly predict N2O production and emissions via nitrification and denitrification in rivers, reservoirs, and estuaries. In particular, we introduce a water residence time dependence, which kinetically limits the extent of denitrification and nitrification in water bodies. We revise existing spatially‐explicit estimates of N loads to inland waters to predict both lumped watershed and half‐degree grid cell emissions and EFs worldwide, as well as the proportions of these emissions that originate from denitrification and nitrification. We estimate global inland water N2O emissions of 10.6‐19.8 Gmol N yr⁻¹ (148‐277 Gg N yr⁻¹), with reservoirs producing most N2O per unit area. Our results indicate that IPCC EFs are likely overestimated by up to an order of magnitude, and that achieving the magnitude of the IPCC's EFs is kinetically improbable in most river systems. Denitrification represents the major pathway of N2O production in river systems, whereas nitrification dominates production in reservoirs and estuaries.
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Surface water concentrations of CO2, CH4, and N2O have rarely been measured simultaneously in river systems modified by human activities, contributing to large uncertainties in estimating global riverine emissions of greenhouse gases (GHGs). Basin-wide surveys of the three GHGs were combined with a small number of measurements of C isotope ratios in dissolved organic matter (DOM), CO2, and CH4 in the Han River basin, South Korea, to examine how longitudinal patterns of the three gases and DOM are affected by four cascade dams along a middle section of the North Han River (hereafter termed “middle reach”) and treated wastewater discharged to the lower Han River (“lower reach”) traversing the Seoul metropolitan area. Monthly monitoring and two-season comparison were conducted at 6 and 15 sites, respectively, to measure surface water gas concentrations and ancillary water quality parameters including concentrations of dissolved organic carbon (DOC) and optical properties of DOM. The basin-wide surveys were complemented with a sampling cruise along the lower reach and synoptic samplings along an urban tributary delivering effluents from a large wastewater treatment plant (WWTP) to the lower reach. The levels of pCO2 were relatively low in the middle reach (51–2465 µatm), particularly at the four dam sites (51–761 µatm), compared with those found in the largely forested upper basin with scattered patches of croplands (163–2539 µatm), the lower reach (78–11298 µatm), and three urban tributaries (2120–11970 µatm). The upper and middle reaches displayed generally low concentration ranges of CH4 and N2O, with some local peaks influenced by agricultural runoff and impoundments. By comparison, the lower reach exhibited exceptionally high concentrations of CH4 (1.2–15766 nmol L-1) and N2O (7.5–1396 nmol L-1), which were significantly correlated with different sets of variables such as DO and PO43- for CH4 and NH4+ and NO3- for N2O. Downriver increases in the levels of DOC and optical properties such as fluorescence index (FI) and protein-like fluorescence indicated an increasing DOM fraction of anthropogenic and microbial origin. The concentrations of the three GHGs and DOC were similar in magnitude and temporal variation at a WWTP discharge and the receiving tributary, indicating a disproportionate contribution of the WWTP effluents to the tributary gas and DOC exports to the lower reach. The values of δ13C in surface water CO2 and CH4 measured during the sampling cruise along the lower reach, combined with δ13C and Δ14C in DOM sampled across the basin, implied a strong influence of the wastewater-derived gases and aged DOM delivered by the urban tributaries. The downstream enrichment of 13C in CO2 and CH4 suggested that the spatial distribution of these gases across the eutrophic lower reach may also be constrained by multiple concomitant processes including outgassing, photosynthesis, and CH4 oxidation. The overall results suggest that dams and urban wastewater may create longitudinal discontinuities in riverine metabolic processes leading to large spatial variations in the three GHGs correlating with different combinations of DOM properties and nutrients. Further research is required to evaluate the relative contributions of anthropogenic and in-stream sources of the three gases and DOM in eutrophic urbanized river systems and constrain key factors for the contrasting impoundment effects such as autotrophy-driven decreases in pCO2 and in-lake production of CH4 and N2O.
Human activities are drastically altering water and material flows in river systems across Asia. These anthropogenic perturbations have rarely been linked to the carbon (C) fluxes of Asian rivers that may account for up to 40–50 % of the global fluxes. This review aims to provide a conceptual framework for assessing the human impacts on Asian river C fluxes, along with an update on anthropogenic alterations of riverine C fluxes. Drawing on case studies conducted in three selected rivers (the Ganges, Mekong, and Yellow River) and other major Asian rivers, the review focuses on the impacts of river impoundment and pollution on CO2 outgassing from the rivers draining South, Southeast, and East Asian regions that account for the largest fraction of river discharge and C exports from Asia and Oceania. A critical examination of major conceptual models of riverine processes against observed trends suggests that to better understand altered metabolisms and C fluxes in “anthropogenic land-water-scapes”, or riverine landscapes modified by human activities, the traditional view of the river continuum should be complemented with concepts addressing spatial and temporal discontinuities created by human activities, such as river impoundment and pollution. Recent booms in dam construction on many large Asian rivers pose a host of environmental problems, including increased retention of sediment and associated C. A small number of studies that measured greenhouse gas (GHG) emissions in dammed Asian rivers have reported contrasting impoundment effects: decreased GHG emissions from eutrophic reservoirs with enhanced primary production vs. increased emissions from the flooded vegetation and soils in the early years following dam construction or from the impounded reaches and downstream estuaries during the monsoon period. These contrasting results suggest that the rates of metabolic processes in the impounded and downstream reaches can vary greatly longitudinally over time as a combined result of diel shifts in the balance between autotrophy and heterotrophy, seasonal fluctuations between dry and monsoon periods, and a long-term change from a leaky post-construction phase to a gradual C sink. The rapid pace of urbanization across southern and eastern Asian regions has dramatically increased municipal water withdrawal, generating annually 120 km3 of wastewater in 24 countries, which comprises 39 % of the global municipal wastewater production. Although municipal wastewater constitutes only 1 % of the renewable surface water, it can disproportionately affect the receiving river water, particularly downstream of rapidly expanding metropolitan areas, resulting in eutrophication, increases in the amount and lability of organic C, and pulse emissions of CO2 and other GHGs. In rivers draining highly populated metropolitan areas, lower reaches and tributaries, which are often plagued by frequent algal blooms and pulsatile CO2 emissions from urban tributaries delivering high loads of wastewater, tended to exhibit higher levels of organic C and the partial pressure of CO2 (pCO2) than less impacted upstream reaches and eutrophic impounded reaches. More field measurements of pCO2, together with accurate flux calculations based on river-specific model parameters, are required to provide more accurate estimates of GHG emissions from the Asian rivers that are now underrepresented in the global C budgets. The new conceptual framework incorporating discontinuities created by impoundment and pollution into the river continuum needs to be tested with more field measurements of riverine metabolisms and CO2 dynamics across variously affected reaches to better constrain altered fluxes of organic C and CO2 resulting from changes in the balance between autotrophy and heterotrophy in increasingly human-modified river systems across Asia and other continents.
Nitrous oxide (N2O) is a potent greenhouse gas and produced in
denitrification and nitrification by various microorganisms. Site preference
(SP) of 15N in N2O, which is defined as the difference in the
natural abundance of isotopomers 14N15NO and 15N14NO
relative to 14N14NO, has been reported to be a useful tool to
quantitatively distinguish N2O production pathways. To determine
representative SP values for each microbial process, we firstly measured SP
of N2O produced in the enzyme reaction of hydroxylamine oxidoreductase
(HAO) purified from two species of ammonia oxidizing bacteria (AOB),
Nitrosomonas europaea and Nitrosococcus oceani, and that of
nitric oxide reductase (NOR) from Paracoccus denitrificans. The SP
value for NOR reaction (−5.9 ± 2.1‰) showed nearly the same
value as that reported for N2O produced by P. denitrificans in
pure culture. In contrast, SP value for HAO reaction
(36.3 ± 2.3‰) was a little higher than the values reported
for N2O produced by AOB in aerobic pure culture. Using the SP values
obtained by HAO and NOR reactions, we calculated relative contribution of the
nitrite (NO2–) reduction (which is followed by NO reduction) to
N2O production by N. oceani incubated under different O2
availability. Our calculations revealed that previous in vivo studies might
have underestimated the SP value for the NH2OH oxidation pathway
possibly due to a small contribution of NO2– reduction pathway.
Further evaluation of isotopomer signatures of N2O using common enzymes
of other processes related to N2O would improve the isotopomer analysis
of N2O in various environments.
Estimates of global riverine nitrous oxide (N2 O) emissions contain great uncertainity. We conducted a meta-analysis incorporating 169 observations from published literature to estimate global riverine N2 O emission rates and emission factors. Riverine N2 O flux was significantly correlated with NH4 , NO3 and DIN (NH4 +NO3 ) concentrations, loads and yields. The emission factors EF(a) (i.e., the ratio of N2 O emission rate and DIN load) and EF(b) (i.e., the ratio of N2 O and DIN concentrations) values were comparable and showed negative correlations with nitrogen concentration, load and yield and water discharge, but positive correlations with the dissolved organic carbon:DIN ratio. After individually evaluating 82 potential regression models based on EF(a) or EF(b) for global, temperate zone, and sub-tropical zone datasets, a power function of DIN yield multiplied by watershed area was determined to provide the best fit between modeled and observed riverine N2 O emission rates (EF(a): R(2) =0.92 for both global and climatic zone models, n=70; EF(b): R(2) =0.91 for global model and R(2) =0.90 for climatic zone models, n=70). Using recent estimates of DIN loads for 6400 rivers, models estimated global riverine N2 O emission rates of 29.6-35.3 (mean=32.2) Gg N2 O-N yr(-1) and emission factors of 0.16-0.19% (mean=0.17%). Global riverine N2 O emission rates are forecasted to increase by 35%, 25%, 18% and 3% in 2050 compared to the 2000s under the Millennium Ecosystem Assessment's Global Orchestration, Order from Strength, Technogarden, and Adapting Mosaic scenarios, respectively. Previous studies may overestimate global riverine N2 O emission rates (300-2100 Gg N2 O-N yr(-1) ) since they ignore declining emission factor values with increasing nitrogen levels and channel size, as well as neglect differences in emission factors corresponding to different nitrogen forms. Riverine N2 O emission estimates will be further enhanced through refining emission factor estimates, extending measurements longitudinally along entire river networks, and improving estimates of global riverine nitrogen loads. This article is protected by copyright. All rights reserved.
Isotopocule ratios of N2O (δ15N, δ18O and SP = 15N site preference within the linear N2O molecule) are useful parameters to identify sources of this greenhouse gas and provide an insight into production and consumption mechanisms in a complex bacterial system. We measured isotopocule ratios of dissolved N2O in simulated wastewater with activated sludge under variable conditions of key factors including dissolved oxygen (DO), carbon-to-nitrogen ratio (C/N ratio), mixed liquor suspended solid (MLSS), and water temperature in oxic and anoxic conditions. Under oxic condition, lower DO concentration causes greater N2O accumulation. Observed low SP (–2.6 to +7.8‰ at 25°C and –7.2 to +9.2‰ at 18°C), which is unique to N2O production pathway, and the relation of nitrogen isotope ratios between N2O and its substrate (NH4+) suggests that N2O is produced mainly by NO2– reduction by autotrophic nitrifiers (nitrifier-denitrification). The N2O production mechanism in this condition was not altered by changes in DO of 0.5–3.0 mg L–1. Under anoxic conditions, NO2– reduction by denitrifying bacteria (heterotrophic denitrification) is the dominant contributor to N2O production. Also, N2O reduction to N2 occurred simultaneously, as implied by isotopocule analysis. The C/N ratio had a negligible effect on the N2O production mechanism. During anoxic N2O decomposition experiment, a positive correlation between δ18O and δ15Nbulk (slope = 2.2) and between SP and 15Nbulk (slope = 0.9) of N2O, which indicates the occurrence of N2O reduction, were found. The N2O reduction rate was increased by the high MLSS concentration. Moreover, isotopic enrichment factors (ε), which are specific to biological reaction, during N2O reduction were estimated as –9.5 ± 1.0‰ for δ15Nbulk, –28.7 ± 3.7‰ for δ18O and –10.0 ± 2.2‰ for SP of N2O.
Inland waters are important sites of nitrogen processing1,2, and represent a significant component of the global budget of nitrous oxide, a powerful greenhouse gas. Measurements have focused on nitrogen-rich temperate rivers, with low-nitrogen freshwater systems at high latitudes receiving less attention. Here we measured surface water nitrous oxide partial pressures and calculated fluxes across 321 rivers, lakes and ponds in three boreal regions of Québec, Canada. Fluxes to the atmosphere ranged from-23.1 to 115.7 μmolm-2 d-1, with high variability among ecosystem types, regions and seasons. Surprisingly, over 40% of the systems sampled were under-saturated in nitrous oxide during the summer, and one region's aquatic network was a net atmospheric sink. Fluxes could not be predicted from the relatively narrow range in nitrogen concentrations, but the aquatic systems acting as sinks tended to have lower pH, higher dissolved organic carbon and lower oxygen concentrations. Given the large variability in observed fluxes, we estimate that high-latitude aquatic networks may emit from-0.07 to 0.20 TgN2O-N yr-1. The potential of boreal aquatic networks to act as net atmospheric nitrous oxide sinks highlights the extensive uncertainty in our understanding of global freshwater nitrous oxide budgets.
Effective mitigation for N2O emissions, now the third most important anthropogenic greenhouse gas and the largest remaining anthropogenic source of stratospheric ozone depleting substances, requires understanding of the sources and how they may increase this century. Here we update estimates and their uncertainties for current anthropogenic and natural N2O emissions and for emissions scenarios to 2050. Although major uncertainties remain, 'bottom-up' inventories and 'top-down' atmospheric modeling yield estimates that are in broad agreement. Global natural N2O emissions are most likely between 10 and 12 Tg N2O-N yr−1. Net anthropogenic N2O emissions are now about 5.3 Tg N2O-N yr−1. Gross anthropogenic emissions by sector are 66% from agriculture, 15% from energy and transport sectors, 11% from biomass burning, and 8% from other sources. A decrease in natural emissions from tropical soils due to deforestation reduces gross anthropogenic emissions by about 14%. Business-as-usual emission scenarios project almost a doubling of anthropogenic N2O emissions by 2050. In contrast, concerted mitigation scenarios project an average decline of 22% relative to 2005, which would lead to a near stabilization of atmospheric concentration of N2O at about 350 ppb. The impact of growing demand for biofuels on future projections of N2O emissions is highly uncertain; N2O emissions from second and third generation biofuels could remain trivial or could become the most significant source to date. It will not be possible to completely eliminate anthropogenic N2O emissions from agriculture, but better matching of crop N needs and N supply offers significant opportunities for emission reductions.
Nitrous oxide is a potent greenhouse gas, and it destroys stratospheric ozone. Seventeen per cent of agricultural nitrous oxide emissions come from the production of nitrous oxide in streams, rivers and estuaries, in turn a result of inorganic nitrogen input through leaching, runoff and sewage. The Intergovernmental Panel on Climate Change and global nitrous oxide budgets assume that riverine nitrous oxide emissions increase linearly with dissolved inorganic nitrogen loads, but data are sparse and conflicting. Here we report measurements over two years of nitrous oxide emissions in the Grand River, Canada, a seventh-order temperate river that is affected by agricultural runoff and outflow from a waste-water treatment plant. Emissions were disproportionately high in urban areas and during nocturnal summer periods. Moreover, annual emission estimates that are based on dissolved inorganic nitrogen loads overestimated the measured emissions in a wet year and underestimated them in a dry year. We found no correlations of nitrous oxide emissions with nitrate or dissolved inorganic nitrogen, but detected negative correlations with dissolved oxygen, suggesting that nitrate concentrations did not limit emissions. We conclude that future increases in nitrate export to rivers will not necessarily lead to higher nitrous oxide emissions, but more widespread hypoxia most likely will.
Carbon dioxide (CO2) transfer from inland waters to the atmosphere, known as CO2 evasion, is a component of the global carbon cycle. Global estimates of CO2 evasion have been hampered, however, by the lack of a framework for estimating the inland water surface area and gas transfer velocity and by the absence of a global CO2 database. Here we report regional variations in global inland water surface area, dissolved CO2 and gas transfer velocity. We obtain global CO2 evasion rates of 1.8 petagrams of carbon (Pg C) per year from streams and rivers and 0.32 Pg C yr(-1) from lakes and reservoirs, where the upper and lower limits are respectively the 5th and 95th confidence interval percentiles. The resulting global evasion rate of 2.1 Pg C yr(-1) is higher than previous estimates owing to a larger stream and river evasion rate. Our analysis predicts global hotspots in stream and river evasion, with about 70 per cent of the flux occurring over just 20 per cent of the land surface. The source of inland water CO2 is still not known with certainty and new studies are needed to research the mechanisms controlling CO2 evasion globally.
Urban ecosystems are constantly evolving, and they are expected to change in both space and time with active management or degradation. An urban watershed continuum framework recognizes a continuum of engineered and natural hydrologic flowpaths that expands hydrologic networks in ways that are seldom considered. It recognizes that the nature of hydrologic connectivity influences downstream fluxes and transformations of carbon, contaminants, energy, and nutrients across 4 space and time dimensions. Specifically, it proposes that (1) first order streams are largely replaced by urban infrastructure (e.g. storm drains, ditches, gutters, pipes) longitudinally and laterally within watersheds, (2) there is extensive longitudinal and lateral modification of organic carbon and nutrient retention in engineered headwaters (3) there are longitudinal downstream pulses in material and energy exports that are amplified by interactive land-use and hydrologic variability, (4) there are vertical interactions between leaky pipes and ground water that influence stream solute transport, (5) the urban watershed continuum is a transformer and transporter of materials and energy based on hydrologic residence times, and (6) temporally, there is an evolution of biogeochemical cycles and ecosystem functions as land use and urban infrastructure change over time. We provide examples from the Baltimore Ecosystem Study Long-Term Ecological (LTER) site along 4 spatiotemporal dimensions. Long-term monitoring indicates that engineered headwaters increase downstream subsidies of nitrate, phosphate, sulfate, carbon, and metals compared with undeveloped headwaters. There are increased longitudinal transformations of carbon and nitrogen from suburban headwaters to more urbanized receiving waters. Hydrologic connectivity along the vertical dimension between ground water and leaky pipes from Baltimore’s aging infrastructure elevates stream solute concentrations. Across time, there has been increased headwater stream burial, evolving stormwater management, and long-term salinization of Baltimore’s drinking water supply. Overall, an urban watershed continuum framework proposes testable hypotheses of how transport/transformation of materials and energy vary along a continuum of engineered and natural hydrologic flowpaths in space and time. Given interest in transitioning from sanitary to sustainable cities, it is necessary to recognize the evolving relationship between infrastructure and ecosystem function along the urban watershed continuum.
Nitrous oxide (N2O) is produced in the lower Seine river at 40 to 60 kg N-N2O d(-1), in a section greatly affected by effluents from the Acheres Wastewater treatment plant (WWTP). Because water column denitrification is weak in this sector, we hypothesized that N2O could be produced by the nitrifier-denitrification process. To understand the controls of N2O emission, nitrifying bacterial cultures were grown from Seine river water (in batches and continuous flow experiments). The population diversity of ammonia oxidizing bacteria (AOB) in these experiments was determined by denaturing gradient gel electrophoresis (DGGE) and found to be similar to those naturally present in the Seine. We determined nitrification kinetics of the 2 functional bacteria populations (AOB and nitrite oxidizing bacteria [NOB]). During nitrifying batch and continuous flow experiments, the N2O production kinetics were examined under contrasted conditions. We tested the effect of dissolved oxygen, ammonia and nitrite concentrations on the N2O Production rate. To our knowledge, this is the first study determining the optimal concentrations of O-2, NH4+ and NO2- that will lead to maximum N2O production by nitrifier-denitrification of mixed nitrifying bacteria populations from natural freshwater. We tested a range of oxygen concentrations and observed a peak in N2O emission within the narrow range from 1.1 to 1.5 Mg O-2 l(-1). We plotted the N2O production as a function of ammonium and nitrite concentrations under optimal dissolved oxygen conditions. The results followed the hyperbolic Michaelis-Menten type curves, and kinetics parameters (V-max and K-s) were determined. The maximum N2O production rate (V-max) was estimated at 8 to 9 mu g N-N2O Mg C biomass(-1) h(-1). The half-saturation constants of nitrifier-denitrification were K-s = 1.5 to 3 mg N-NH4+ l(-1) for ammonium, and K-s = 1 to 4 mg N-NO2- l(-1) for nitrite.
The continuous increase of nitrous oxide (N2O) abundance in the atmosphere is a global concern. Multiple pathways of N2O production occur in soil, but their significance and dependence on oxygen (O2) availability and nitrogen (N) fertilizer source are poorly understood. We examined N2O and nitric oxide (NO) production under 21%, 3%, 1%, 0.5%, and 0% (vol/vol) O2 concentrations following urea or ammonium sulfate [(NH4)2SO4] additions in loam, clay loam, and sandy loam soils that also contained ample nitrate. The contribution of the ammonia (NH3) oxidation pathways (nitrifier nitrification, nitrifier denitrification, and nitrification-coupled denitrification) and heterotrophic denitrification (HD) to N2O production was determined in 36-h incubations in microcosms by (15)N-(18)O isotope and NH3 oxidation inhibition (by 0.01% acetylene) methods. Nitrous oxide and NO production via NH3 oxidation pathways increased as O2 concentrations decreased from 21% to 0.5%. At low (0.5% and 3%) O2 concentrations, nitrifier denitrification contributed between 34% and 66%, and HD between 34% and 50% of total N2O production. Heterotrophic denitrification was responsible for all N2O production at 0% O2. Nitrifier denitrification was the main source of N2O production from ammonical fertilizer under low O2 concentrations with urea producing more N2O than (NH4)2SO4 additions. These findings challenge established thought attributing N2O emissions from soils with high water content to HD due to presumably low O2 availability. Our results imply that management practices that increase soil aeration, e.g., reducing compaction and enhancing soil structure, together with careful selection of fertilizer sources and/or nitrification inhibitors, could decrease N2O production in agricultural soils.
1] Site preference (SP), the difference in d 15 N between the central and outer nitrogen atoms in N 2 O, is a powerful approach for apportioning fluxes of N 2 O from soils to nitrification and denitrification (Sutka et al., 2006). A critical aspect of the use of SP data to apportion sources of N 2 O to nitrification and denitrification is the need to evaluate data for isotope shifts that may have occurred during N 2 O reduction in soils prior to its escape to the atmosphere. We present data on the isotopologue effects during reduction of N 2 O during anaerobic incubation of soils and pure cultures of denitrifying bacteria. Isotopic enrichment factors for N 2 O reduction in soil mesocosms experiments varied between À9.2 and À1.8% for nitrogen and between À25.1 and À5.1% for oxygen. In pure cultures of Psuedomonas stutzeri and Psuedomonas denitrificans we observed isotopic enrichment factors for SP of À5.0 and À6.8%, respectively. We further find that N 2 O consumption produces consistent relationships between d 18 O and d 15 N and d 18 O and the d 15 N of the central N atom in N 2 O of 2.5 and 1.6, respectively, which are clearly diagnostic of this process. Our results indicate that SP may be altered during reduction of N 2 O and thus bias evaluations of its origins. To understand the impacts of N 2 O reduction in soil flux studies on source isotope signals we modeled the isotope effects of N 2 O production occurring simultaneous with reduction and find increasingly curvilinear relationships between d 18 O and d 15 N and d 18 O and d 15 N a with increased reduction. Consequently, a deviation from the linear mixing relationship between soil-derived and atmospheric N 2 O is an indication of extensive reduction. On the basis of our characterization of isotopic fractionation during N 2 O reduction, we show that the rate of reduction would have to be substantially greater than 10% of that of production to impact SP estimates of N 2 O from denitrification by more than a few percent. Nonetheless, reduction results in a small, but potentially important, increase in SP away from values proposed for bacterial denitrification (0%) toward those associated with production from nitrification (33%) (Sutka et al., 2006). On this basis, estimates of the proportion of N 2 O derived from denitrification obtained from SP values are underestimates and therefore conservative. (2007), Isotopologue effects during N 2 O reduction in soils and in pure cultures of denitrifiers, J. Geophys. Res., 112, G02005, doi:10.1029/2006JG000287.
Nitrous oxide (N₂O) is a long-lived and potent greenhouse gas produced during microbial nitrification and denitrification. In developed countries, centralized water reclamation plants often use these processes for N removal before effluent is used for irrigation or discharged to surface water, thus making this treatment a potentially large source of N₂O in urban areas. In the arid but densely populated southwestern United States, water reclamation for irrigation is an important alternative to long-distance water importation. We measured N₂O concentrations and fluxes from several wastewater treatment processes in urban southern California. We found that N removal during water reclamation may lead to in situ N₂O emission rates that are three or more times greater than traditional treatment processes (C oxidation only). In the water reclamation plants tested, N₂O production was a greater percentage of total N removed (1.2%) than traditional treatment processes (C oxidation only) (0.4%). We also measured stable isotope ratios (δN and δO) of emitted N₂O and found distinct δN signatures of N₂O from denitrification (0.0 ± 4.0 ‰) and nitrification reactors (-24.5 ± 2.2 ‰), respectively. These isotope data confirm that both nitrification and denitrification contribute to N₂O emissions within the same treatment plant. Our estimates indicate that N₂O emissions from biological N removal for water reclamation may be several orders of magnitude greater than N₂O emissions from agricultural activities in highly urbanized southern California. Our results suggest that wastewater treatment that includes biological nitrogen removal can significantly increase urban N₂O emissions.
Inland waters (lakes, reservoirs, streams, and rivers) are often substantial methane (CH4) sources in the terrestrial landscape. They are, however, not yet well integrated in global greenhouse gas (GHG) budgets.
Data from 474 freshwater ecosystems and the most recent global water area estimates indicate that freshwaters emit at least
103 teragrams of CH4 year−1, corresponding to 0.65 petagrams of C as carbon dioxide (CO2) equivalents year−1, offsetting 25% of the estimated land carbon sink. Thus, the continental GHG sink may be considerably overestimated, and
freshwaters need to be recognized as important in the global carbon cycle.
Nitrous oxide (N(2)O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Anthropogenic nitrogen (N) loading to river networks is a potentially important source of N(2)O via microbial denitrification that converts N to N(2)O and dinitrogen (N(2)). The fraction of denitrified N that escapes as N(2)O rather than N(2) (i.e., the N(2)O yield) is an important determinant of how much N(2)O is produced by river networks, but little is known about the N(2)O yield in flowing waters. Here, we present the results of whole-stream (15)N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N(2)O at rates that increase with stream water nitrate (NO(3)(-)) concentrations, but that <1% of denitrified N is converted to N(2)O. Unlike some previous studies, we found no relationship between the N(2)O yield and stream water NO(3)(-). We suggest that increased stream NO(3)(-) loading stimulates denitrification and concomitant N(2)O production, but does not increase the N(2)O yield. In our study, most streams were sources of N(2)O to the atmosphere and the highest emission rates were observed in streams draining urban basins. Using a global river network model, we estimate that microbial N transformations (e.g., denitrification and nitrification) convert at least 0.68 Tg·y(-1) of anthropogenic N inputs to N(2)O in river networks, equivalent to 10% of the global anthropogenic N(2)O emission rate. This estimate of stream and river N(2)O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change.
Phosphorus removal from freshwater systems has wide-ranging ecological consequences
Nitrogen is a requisite and highly demanded element for living organisms on the earth. However, increasing human activities have greatly altered the global nitrogen cycle, especially for rivers and streams, resulting in eutrophication, formation of hypoxic zones, and increased production of N2O, a powerful greenhouse gas. This review focuses on three aspects of the nitrogen cycle in streams and rivers. We firstly introduce the distributions and concentrations of nitrogen compounds in streams and rivers as well as the techniques for sources tracing of nitrogen pollution. Secondly, the overall picture of nitrogen transformations in rivers and streams conducted by organisms is described, especially focusing on the roles of suspended particle-water surfaces in the overlying water, sediment-water interfaces, and riparian zone in the nitrogen cycle of streams and rivers. The coupling of nitrogen and other element (C, S, and Fe) cycles in streams and rivers is also briefly covered. Finally, we analyze nitrogen budget of river systems as well as nitrogen loss as N2O and N2 through the fluvial network, and give a summary of the effects and consequences of human activities and climate change on the riverine nitrogen cycle. In addition, future directions about the research on nitrogen cycle in river systems are outlined.
Continuous underway measurements were combined with a basin-scale survey to examine human impacts on CO2 outgassing in a highly urbanized river system in Korea. While the partial pressure of CO2 (pCO2) was measured at 15 sites using syringe equilibration, three cruises employing an equilibrator were done along a 30-km transect in the Seoul metropolitan area. The basin-scale survey revealed longitudinal increases in surface water pCO2 and dissolved organic carbon (DOC) in the downstream reach. Downstream increases in pCO2, DOC, fluorescence index, and inorganic N and P reflected disproportionately large contributions from wastewater treatment plant (WWTP) effluents carried by major urban tributaries. Cruise transects exhibited strong localized peaks of pCO2 up to 13,000 μatm and 13CO2 enrichment along the confluences of tributaries at average flow, whereas CO2 pulses were dampened by increased flow during the monsoon period. Fluctuations in pCO2 along the eutrophic reach downstream of the confluences reflected environmental controls on the balance between photosynthesis, biodegradation, and outgassing. The results underscore WWTP effluents as an anthropogenic source of nutrients, DOC, and CO2 and their influences on algal blooms and associated C dynamics in eutrophic urbanized river systems, warranting further research on urbanization-induced perturbations to riverine metabolic processes and carbon fluxes.
Rivers are important sources of N2O emissions into the atmosphere. Nevertheless, N2O production processes in rivers are not well identified. We measured concentrations and isotopic ratios of N2O, NH4⁺, NO2⁻, and NO3⁻ in surface water to identify the microbial processes of N2O production along the Tama River in Japan. We also measured the functional gene abundance of nitrifiers and denitrifiers (amoA-bacteria, nirK, nirS, nosZ clade I, nosZ clade II) together with concentrations of dissolved organic carbon (DOC) and fluorescence intensities of protein and humic components of dissolved organic matter (DOM) to support the elucidation of N2O production processes. The observed nitrogen (δ¹⁵N) and oxygen (δ¹⁸O) of N2O were within the expected isotopic range of N2O produced by nitrate reduction, indicating that N2O was dominantly produced by denitrification. The positive significant correlation between N2ONet concentration and nirK gene abundance implied that nitrifiers and denitrifiers are contributors to N2O production. Fluorescence intensities of protein and humic components of DOM and concentrations of DOC did not show significant correlations with N2O concentrations, which suggests that DOC and abundance of DOM components do not control dissolved N2O. Measurement of isotope ratios of N2O and its substrates was found to be a useful tool to obtain evidence of denitrification as the main source of N2O production along the Tama River.
Nitrous oxide (N2O) is an important greenhouse gas and an ozone-depleting substance which can be emitted from wastewater treatment systems (WWTS) causing significant environmental impacts. Understanding the N2O production pathways and their contribution to total emissions is the key to effective mitigation. Isotope technology is a promising method that has been applied to WWTS for quantifying the N2O production pathways. Within the scope of WWTS, this article reviews the current status of different isotope approaches, including both natural abundance and labelled isotope approaches, to N2O production pathways quantification. It identifies the limitations and potential problems with these approaches, as well as improvement opportunities. We conclude that, while the capabilities of isotope technology have been largely recognized, the quantification of N2O production pathways with isotope technology in WWTS require further improvement, particularly in relation to its accuracy and reliability.
Nitrous oxide (N2O) is an important pollutant which is emitted during the biological nutrient removal (BNR) processes of wastewater treatment. Since it has a greenhouse effect which is 265 times higher than carbon dioxide, even relatively small amounts can result in a significant carbon footprint. Biological nitrogen (N) removal conventionally occurs with nitrification/denitrification, yet also through advanced processes such as nitritation/denitritation and completely autotrophic N-removal. The microbial pathways leading to the N2O emission include hydroxylamine oxidation and nitrifier denitrification, both activated by ammonia oxidizing bacteria, and heterotrophic denitrification. In this work, a critical review of the existing literature on N2O emissions during BNR is presented focusing on the most contributing parameters. Various factors increasing the N2O emissions either per se or combined are identified: low dissolved oxygen, high nitrite accumulation, low chemical oxygen demand to nitrogen ratio, slow growth of denitrifying bacteria, uncontrolled pH and temperature. However, there is no common pattern in reporting the N2O generation amongst the cited studies, a fact that complicates its evaluation. When simulating N2O emissions, all microbial pathways along with the potential contribution of abiotic N2O production during wastewater treatment at different dissolved oxygen/nitrite levels should be considered. The undeniable validation of the robustness of such models calls for reliable quantification techniques which simultaneously describe dissolved and gaseous N2O dynamics. Thus, the choice of the N-removal process, the optimal selection of operational parameters and the establishment of validated dynamic models combining multiple N2O pathways are essential for studying the emissions mitigation.
Rationale:
Triple oxygen and nitrogen isotope ratios in nitrate are powerful tools for assessing atmospheric nitrate formation pathways and their contribution to ecosystems. N2 O decomposition using microwave-induced plasma (MIP) has been used only for measurements of oxygen isotopes to date, but it is also possible to measure nitrogen isotopes during the same analytical run.
Methods:
The main improvements to a previous system are (i) an automated distribution system of nitrate to the bacterial medium, (ii) N2 O separation by gas chromatography before N2 O decomposition using the MIP, (iii) use of a corundum tube for microwave discharge, and (iv) development of an automated system for isotopic measurements. Three nitrate standards with sample sizes of 60, 80, 100, and 120 nmol were measured to investigate the sample size dependence of the isotope measurements.
Results:
The δ17 O, δ18 O, and Δ17 O values increased with increasing sample size, although the δ15 N value showed no significant size dependency. Different calibration slopes and intercepts were obtained with different sample amounts. The slopes and intercepts for the regression lines in different sample amounts were dependent on sample size, indicating that the extent of oxygen exchange is also dependent on sample size. The sample-size-dependent slopes and intercepts were fitted using natural log (ln) regression curves, and the slopes and intercepts can be estimated to apply to any sample size corrections. When using 100 nmol samples, the standard deviations of residuals from the regression lines for this system were 0.5‰, 0.3‰, and 0.1‰, respectively, for the δ18 O, Δ17 O, and δ15 N values, results that are not inferior to those from other systems using gold tube or gold wire.
Conclusions:
An automated system was developed to measure triple oxygen and nitrogen isotopes in nitrate using N2 O decomposition by MIP. This system enables us to measure both triple oxygen and nitrogen isotopes in nitrate with comparable precision and sample throughput (23 min per sample on average), and minimal manual treatment. Copyright © 2016 John Wiley & Sons, Ltd.
Suspended sediment (SPS) is ubiquitous in rivers, and SPS with different particle sizes and compositions may affect coupled nitrification-denitrification (CND) occurring on SPS significantly. However, there is no related research report. In this work, 15N isotope tracer technique was adopted to explore the CND in systems containing SPS (8 g L−1 and 1 g L−1) collected from the Yellow River with various particle sizes, including < 2, 2-20, 20-50, 50-100, and 100-200 μm. The results showed that the CND occurred on SPS and the CND rate was negatively related to particle size; both nitrification and denitrification rate constants increased with decreasing SPS particle size. For instance, SPS (8 g L−1) with a particle size below 2 μm had the highest 15N2 emission rate of 1.15 mg-N/(m3•d), which was 2.9 times that of 100-200 μm. This is because SPS with a smaller particle size had a larger specific surface area and a higher organic carbon content, which is beneficial for bacteria growth. Both the nitrifying and denitrifying bacteria population were positively correlated with CND rate (p<0.05). Different from the 15N2 production, 15N2O emission rate did not decrease with increasing SPS particle size. For the system containing 8 g L−1 SPS, 15N2O emission rate reached the highest of 1.05 μg-N/(m3•d) in the 50-100 μm SPS system, which was 17.5 times that of 100-200 μm. Similar results could be found from the system with 1 g L−1 SPS. This is due to the fact that the oxygen concentration at the SPS-water interface increased with SPS particle size, and the oxygen conditions might be most suitable for the production of N2O in the 50-100 μm system. This study suggests that SPS size and composition play an important role in nitrogen cycle of river systems, especially for the production of N2O.
High concentrations of ammonium (NH4⁺) in soil have been shown to inhibit nitrification, and fertilizer injection as conducted during CULTAN (controlled uptake long-term ammonium nutrition) management might thus have the potential to reduce N2O emission from arable soil. We conducted an incubation experiment with different NH4⁺ concentrations in soil that resembled concentrations as expected at and around injection spots (5000, 2250, 1000, 450, 0 mg NH4⁺-N kg⁻¹ soil) directly after fertilization and after dilution due to plant uptake or precipitation. N2O emission was measured in dynamic soil mesocosms over a period of 21 days. Acetylene inhibition and ¹⁵N tracer approaches were used to calculate the relative contribution of nitrification and denitrification to N2O emission. An isotopomer approach was applied to gain further insight into N2O producing processes. We expected lower contribution of nitrification-derived N2O to total N2O emission and a higher N2O/NO3⁻ ratio from nitrification with increasing NH4⁺ levels. Nitrification indeed declined with increasing NH4⁺ level, and no nitrification occurred in the 5000 mg NH4⁺-N kg⁻¹ soil treatment. A pool dilution approach showed that gross nitrification in 450 mg NH4⁺-N kg⁻¹ soil (nitrification rate: 4.96 mg NO3⁻-N kg soil d⁻¹) was by a factor of 2.6 and 6 higher than in 1000 and 2250 mg NH4⁺-N kg⁻¹ soil treatments. In the 5000 mg NH4⁺-N kg⁻¹ soil treatment, gross nitrification occurred at very small rates (0.1 mg NO3⁻-N kg soil d⁻¹). Similarly, N2O emission declined with increasing NH4⁺ level. The N2O yield of nitrification was between 0.07 and 0.15% of NO3⁻ production, but was not affected by increasing NH4⁺ level. Nitrification was the dominant source of N2O throughout the incubation at all NH4⁺ levels, and there was no significant change in the relative contribution of nitrification and denitrification with NH4⁺ level or time. This finding indicates that denitrification derived N2O emissions were similarly reduced at high NH4⁺ levels. Applying the non-equilibrium technique to our ¹⁵N tracer data revealed heterogeneous distribution of denitrification in soil, with at least two distinct NOx⁻ (NO3⁻ + NO2⁻) pools and spatial separation of NOx⁻ formation and consumption. The isotopomer approach provided reasonable results in comparison with the acetylene inhibition and ¹⁵N tracer approaches and indicated substantial contribution of nitrifier denitrification and/or coupled nitrification-denitrification (10–40%) to total N2O production.
CO2 evasion from rivers (FCO2) is an important component of the global carbon budget. Here, we present the first global maps of CO2 partial pressures (pCO2) in rivers of stream order 3 and higher and the resulting FCO2 at 0.5° resolution constructed with a statistical model. Our statistical model based upon a GIS based approach is used to derive a pCO2 prediction function trained on data from 1182 sampling locations. While data from Asia and Africa are scarce and the training data set is dominated by sampling locations from the Americas, Europe, and Australia, the sampling locations cover the full spectrum from high to low latitudes. The predictors of pCO2 are net primary production, population density and slope gradient within the river catchment as well as mean air temperature at the sampling location (r2=0.47). The predicted pCO2 map was then combined with spatially explicit estimates of stream surface area Ariver and gas exchange velocity k calculated from published empirical equations and data sets to derive the FCO2 map. Using Monte Carlo simulations, we assessed the uncertainties of our estimates. At the global scale, we estimate an average river pCO2 of 2400 (2019–2826) µatm and a FCO2 of 650 (483–846) Tg C yr−1 (5th and 95th percentile of confidence interval). Our global CO2 evasion is substantially lower than the recent estimate of 1800 Tg C yr−1 [Raymond et al., 2013] although the training set of pCO2 is very similar in both studies, mainly due to lower tropical pCO2 estimates in the present study. Our maps reveal strong latitudinal gradients in pCO2, Ariver and FCO2. The zone between 10°N and 10°S contributes about half of the global CO2 evasion. Collection of pCO2 data in this zone, in particular for African and South East Asian rivers is a high priority to reduce uncertainty on FCO2.
Denitrifying aquifers are sources of the greenhouse gas N2O. Isotopic signatures reflect processes of production and reduction of N2O, but it is not clear to which extent these can be used to quantify those processes. We investigated the spatial distribution of isotopologue values of N2O (δ18O, average δ15N, and 15N site preference, SP) in two denitrifying sandy aquifers to study N2O production and reduction and associated isotope effects in groundwater. For the first time, we combined this approach with direct estimation of N2O reduction from excess-N2 analysis. Groundwater samples were collected from 15 monitoring wells and four multilevel sampling wells and analysed for , dissolved N2O, dissolved O2, excess N2 from denitrification and isotopic signatures of and N2O. Both aquifers exhibited high concentrations with average concentrations of 22 and 15 mg N L−1, respectively. Evidence of intense denitrification with associated N2O formation was obtained from mean excess-N2 of 3.5 and 4.3 mg N L−1, respectively. Isotopic signatures of N2O were highly variable with ranges of 17.6–113.2‰ (δ18O), −55.4 to 89.4‰ (δ15Nbulk) and 1.8–97.9‰ (SP). δ15N and δ18O of ranged from −2.1‰ to 65.5‰ and from −5‰ to 33.5‰, respectively.
Global nitrogen (N) enrichment has resulted in increased nitrous oxide (N2 O) emission that greatly contributes to climate change and stratospheric ozone destruction, but little is known about the N2 O emissions from urban river networks receiving anthropogenic N inputs. We examined N2 O saturation and emission in the Shanghai city river network, covering 6300 km(2) , over 27 months. The overall mean saturation and emission from 87 locations was 770% and 1.91 mg N2 O-N•m(-2) •d(-1) , respectively. N2 O saturation did not exhibit a clear seasonality, but the temporal pattern was co-regulated by both water temperature and N loadings. Rivers draining through urban and suburban areas receiving more sewage N inputs had higher N2 O saturation and emission than those in rural areas. Regression analysis indicated that water ammonium (NH4 (+) ) and dissolved oxygen (DO) level had great control on N2 O production and were better predictors of N2 O emission in urban watershed. About 0.29 Gg N2 O-N•yr(-1) N2 O was emitted from the Shanghai river network annually, which was about 131% of IPCC's prediction using default emission values. Given the rapid progress of global urbanization, more study efforts, particularly on nitrification and its N2 O yielding, are needed to better quantify the role of urban rivers in global riverine N2 O emission. This article is protected by copyright. All rights reserved.
We present measurements of site preferences (SP) and bulk 15N/14N ratios (δ15NbulkN2O) of nitrous oxide (N2O) by quantum cascade laser absorption spectroscopy (QCLAS) as a powerful tool to investigate N2O production pathways in biological wastewater treatment. QCLAS enables high-precision N2O isotopomer analysis in real time. This allowed us to trace short-term fluctuations in SP and δ15NbulkN2O and, hence, microbial transformation pathways during individual batch experiments with activated sludge from a pilot-scale facility treating municipal wastewater. On the basis of previous work with microbial pure cultures, we demonstrate that N2O emitted during ammonia (NH4+) oxidation with a SP of -5.8 to 5.6 ‰ derives mostly from nitrite (NO2-) reduction (e.g. nitrifier denitrification), with a minor contribution from hydroxylamine (NH2OH) oxidation at the beginning of the experiments. SP of N2O produced under anoxic conditions was always positive (1.2 to 26.1 ‰), and SP values at the high end of this spectrum (24.9 to 26.1 ‰) are indicative of N2O reductase activity. The measured δ15NbulkN2O at the initiation of the NH4+ oxidation experiments ranged between -42.3 and -57.6 ‰ (corresponding to a nitrogen isotope effect Δδ15N = δ15Nsubstrate - δ15NbulkN2O of 43.5 to 58.8 ‰), which is considerably higher than under denitrifying conditions (δ15NbulkN2O 2.4 to -17 ‰; Δδ15N = 0.1 to 19.5 ‰). During the course of all NH4+ oxidation and nitrate (NO3-) reduction experiments, δ15NbulkN2O increased significantly, indicating net 15N enrichment in the dissolved inorganic nitrogen substrates (NH4+, NO3-) and transfer into the N2O pool. The decrease in 15N during NO2- and NH2OH oxidation experiments is best explained by inverse fractionation during the oxidation of NO2- to NO3-.
This chapter contains section titled:
A meta-analysis was conducted on 136 data sets of denitrification rates (DR) recorded both during the period of highest water temperature and monthly in five types of aquatic ecosystems: oceans, coastal environments, estuaries, lakes and rivers. There was a gradual increase of DR from the ocean to rivers and lakes at both scales, with the rivers showing the highest DR variability. Denitrification peaked during summertime and showed highest seasonal variability in lakes and rivers. High concentrations of nitrate and interstitially-dissolved organic carbon as well as low oxygen concentration in the overlying water enhanced DR both during summer and at a seasonal scale whereas total phosphorus did at the seasonal scale only. There was a positive linear relationship between overlying nitrate and DR over the range of 1–970µmol NO3 (r
2=0.86, P=0.001). DR in lakes and rivers might reach values doubling those in the more denitrifying terrestrial ecosystems (e.g. agrosystems). Discrepancies in DR and its controlling factors between site-specific studies and this meta-analysis may arise from environmental variability at two, often confounded, scales of observation: the habitat and the ecosystem level. Future studies on denitrification in aquatic environments should address the topic of spatial heterogeneity more thoroughly.
Nitrous oxide (N2O) is an important greenhouse gas and a major sink for stratospheric ozone. In biological wastewater treatment, microbial processes such as autotrophic nitrification and heterotrophic denitrification have been identified as major sources; however, the underlying pathways remain unclear. In this study, the mechanisms of N2O production were investigated in a laboratory batch-scale system with activated sludge for treating municipal wastewater. This relatively complex mixed population system is well representative for full-scale activated sludge treatment under nitrifying and denitrifying conditions. Under aerobic conditions, the addition of nitrite resulted in strongly nitrite-dependent N2O production, mainly by nitrifier denitrification of ammonia-oxidizing bacteria (AOB). Furthermore, N2O is produced via hydroxylamine oxidation, as has been shown by the addition of hydroxylamine. In both sets of experiments, N2O production was highest at the beginning of the experiment, then decreased continuously and ceased when the substrate (nitrite, hydroxylamine) had been completely consumed. In ammonia oxidation experiments, N2O peaked at the beginning of the experiment when the nitrite concentration was lowest. This indicates that N2O production via hydroxylamine oxidation is favored at high ammonia and low nitrite concentrations, and in combination with a high metabolic activity of ammonia-oxidizing bacteria (at 2 to 3 mgO2/l); the contribution of nitrifier denitrification by AOB increased at higher nitrite and lower ammonia concentrations towards the end of the experiment. Under anoxic conditions, nitrate reducing experiments confirmed that N2O emission is low under optimal growth conditions for heterotrophic denitrifiers (e.g. no oxygen input and no limitation of readily biodegradable organic carbon). However, N2O and nitric oxide (NO) production rates increased significantly in the presence of nitrite or low dissolved oxygen concentrations.
Global models have indicated agriculturally impacted rivers and streams may be important sources of the greenhouse gas nitrous oxide (N(2)O). However, there is significant uncertainty in N(2)O budgets. Isotopic characterization can be used to help constrain N(2)O budgets. We present the first published measurements of the isotopic character of N(2)O emitted from low (2-4) order streams. Isotopic character of N(2)O varied seasonally, among streams, and over diel periods. On an annual basis, δ(18)O of emitted N(2)O (+47.4 to +51.4‰; relative to VSMOW) was higher than previously reported for larger rivers, but δ(15)N of emitted N(2)O (-16.2 to +2.4‰ among streams; relative to atmospheric N(2)) was similar to that of past studies. On an annual basis, all streams emitted N(2)O with lower δ(15)N than tropospheric N(2)O. Given these streams have elevated nitrate concentrations which are associated with enhanced N(2)O fluxes, this supports the hypothesis that streams are contributing to the accumulation of (15)N-depleted N(2)O in the troposphere.
Wastewater treatment processes are believed to be anthropogenic sources of nitrous oxide (N(2)O) and methane (CH(4)). However, few studies have examined the mechanisms and controlling factors in production of these greenhouse gases in complex bacterial systems. To elucidate production and consumption mechanisms of N(2)O and CH(4) in microbial consortia during wastewater treatment and to characterize human waste sources, we measured their concentrations and isotopomer ratios (elemental isotope ratios and site-specific N isotope ratios in asymmetric molecules of NNO) in water and gas samples collected by an advanced treatment system in Tokyo. Although the estimated emissions of N(2)O and CH(4) from the system were found to be lower than those from the typical treatment systems reported before, water in biological reaction tanks was supersaturated with both gases. The concentration of N(2)O, produced mainly by nitrifier-denitrification as indicated by isotopomer ratios, was highest in the oxic tank (ca. 4000% saturation). The dissolved CH(4) concentration was highest in in-flow water (ca. 3000% saturation). It decreased gradually during treatment. Its carbon isotope ratio indicated that the decrease resulted from bacterial CH(4) oxidation and that microbial CH(4) production can occur in anaerobic and settling tanks.
Emissions of nitrous oxide (N2O) from aquatic systems such as rivers and estuaries are enhanced as a result of human activities on land resulting in enhanced nitrogen availability in aquatic systems. These human activities include agricultural activities such as fertilizer use, as well as industrial activities resulting in nitrogen (N) losses to the environment. In this article, we analyze past and future trends in global emissions of N2O from rivers and estuaries. We calculate aquatic N2O emissions from trends in the export of nitrogen to coastal waters by world-wide rivers. These trends in riverine N exports are from the Global NEWS models, which are global, regionally explicit models developed in the NEWS (Nutrient Export from WaterShed) framework. The NEWS models calculate nutrient exports from land to coastal waters, taking into account different human activities on the land, as well as biological N2 fixation and different ways in which nitrogen is retained in watersheds, including the effect of dams. We present global total emissions of N2O for the years 1970, 2000, and for four scenarios for 2050, as well as regional patterns
This study examines N2O emissions from aquatic environments globally, particularly as they are affected by anthropogenic activity. The global distribution of N2O production in rivers and estuaries was modeled as a function of nitrification and denitrification rates, which were related to external nitrogen (N) inputs. N loading rates were estimated as a function of environmental parameters in the watersheds using two existing models that we adapted for global databases. Model estimated export of dissolved inorganic nitrogen (DIN) by world rivers to estuaries in 1990 is 20.8TgNyr-1 approximately 75% is estimated to be anthropogenic. DIN export to the Atlantic and Indian Oceans is similar (5.4TgNyr-1 and 4.6TgNyr-1, respectively); inputs to the Pacific are approximately 50% greater. China and southeast Asia account for over 50% of DIN export by world rivers. Globally, anthropogenic DIN export is predominately attributed to fertilizer N, followed by sewage and atmospheric deposition. About 8% of the total N inputs to the terrestrial environment can be accounted for as DIN export by rivers. Worldwide N2O emissions from rivers (55%), estuaries (11%), and continental shelves (33%) are calculated to be 1.9TgNyr-1. For rivers and estuaries, approximately 90% of N2O emissions are in the northern hemisphere in line with the regional distribution of DIN export by rivers. China and India account for about 50% of N2O emissions from rivers and estuaries. About 1% of the N input from fertilizers, atmospheric deposition, and sewage to watersheds is lost as N2O in rivers and estuaries. Globally, rivers and estuaries could account for approximately 20% of the current global anthropogenic N2O emissions and are similar in magnitude to a number of previously identified sources including direct emissions of N2O from soils induced by anthropogenic N inputs.
Context Abstract: Atmospheric concentrations of nitrous oxide, a greenhouse gas, are increasing due to human activities. Our analysis suggests that a third of global anthropogenic N2O emission is from aquatic sources (rivers, estuaries, continental shelves) and the terrestrial sources comprise the remainder. Over 80% of aquatic anthropogenic N2O emissions are from the Northern Hemisphere mid-latitudes consistent with the geographic distribution of N fertilizer use, human population and atmospheric N deposition. These N inputs to land have increased aquatic as well as terrestrial anthropogenic N2O emissions because a substantial portion enters aquatic systems and results in increased N2O production. Thus, wise management of N in the terrestrial environment could help reduce/control both aquatic and terrestrial N2O emissions.
River water has been suggested as a potential source of nitrous oxide (N2O), which is a greenhouse gas that is accumulating rapidly in the troposphere and which is a precursor to stratospheric NOx that depletes ozone. Previous studies on freshwater N2O sources have specifically examined estuaries where sedimentary N2O production might be important and a few points near anthropogenic nitrogen sources such as agricultural or municipal wastewater areas. Here we present the first observation of a temporal and horizontal distribution of N2O and its isotopomers between the midstream and estuary of an urban river. Surface water was supersaturated (100-6800%) with N2O at all stations during the study period. The average or maximum saturation value was greater than described in most previous reports. High N2O concentrations were observed near sewage plants and the unique signature of isotopomer ratios implied direct N2O addition from the plants. The isotopomer ratios also suggested N2O production/consumption at the sediment-water interface. Fluxes and isotopomer ratios of N2O, from the river to the atmosphere, estimated from our observations, indicated that the urban river is indeed a source of atmospheric N2O and that its production could be distinguished from other natural or anthropogenic sources using isotopomer ratios.
We report a new method for measurement of the isotopic composition of nitrate (NO3-) at the natural-abundance level in both seawater and freshwater. The method is based on the isotopic analysis of nitrous oxide (N20) generated from nitrate by denitrifying bacteria that lack N2O-reductase activity. The isotopic composition of both nitrogen and oxygen from nitrate are accessible in this way. In this first of two companion manuscripts, we describe the basic protocol and results for the nitrogen isotopes. The precision of the method is better than 0.2/1000 (1 SD) at concentrations of nitrate down to 1 microM, and the nitrogen isotopic differences among various standards and samples are accurately reproduced. For samples with 1 microM nitrate or more, the blank of the method is less than 10% of the signal size, and various approaches may reduce it further.
We report a novel method for measurement of the oxygen isotopic composition (18O/16O) of nitrate (NO3-) from both seawater and freshwater. The denitrifier method, based on the isotope ratio analysis of nitrous oxide generated from sample nitrate by cultured denitrifying bacteria, has been described elsewhere for its use in nitrogen isotope ratio (15N/14N) analysis of nitrate. (1) Here, we address the additional issues associated with 18O/16O analysis of nitrate by this approach, which include (1) the oxygen isotopic difference between the nitrate sample and the N20 analyte due to isotopic fractionation associated with the loss of oxygen atoms from nitrate and (2) the exchange of oxygen atoms with water during the conversion of nitrate to N2O. Experiments with 18O-labeled water indicate that water exchange contributes less than 10%, and frequently less than 3%, of the oxygen atoms in the N20 product for Pseudomonas aureofaciens. In addition, both oxygen isotope fractionation and oxygen atom exchange are consistent within a given batch of analyses. The analysis of appropriate isotopic reference materials can thus be used to correct the measured 18O/16O ratios of samples for both effects. This is the first method tested for 18O/16O analysis of nitrate in seawater. Benefits of this method, relative to published freshwater methods, include higher sensitivity (tested down to 10 nmol and 1 microM NO3-), lack of interference by other solutes, and ease of sample preparation.
Nitrogen inputs to the Gulf of Mexico have increased during recent decades and agricultural regions in the upper Midwest, such as those in Illinois, are a major source of N to the Mississippi River. How strongly denitrification affects the transport of nitrate (NO(3)-N) in Illinois streams has not been directly assessed. We used the nutrient spiraling model to assess the role of in-stream denitrification in affecting the concentration and downstream transport of NO(3)-N in five headwater streams in agricultural areas of east-central Illinois. Denitrification in stream sediments was measured approximately monthly from April 2001 through January 2002. Denitrification rates tended to be high (up to 15 mg N m(-2) h(-1)), but the concentration of NO(3)-N in the streams was also high (>7 mg N L(-1)). Uptake velocities for NO(3)-N (uptake rate/concentration) were lower than reported for undisturbed streams, indicating that denitrification was not an efficient N sink relative to the concentration of NO(3)-N in the water column. Denitrification uptake lengths (the average distance NO(3)-N travels before being denitrified) were long and indicated that denitrification in the streambed did not affect the transport of NO(3)-N. Loss rates for NO(3)-N in the streams were <5% d(-1) except during periods of low discharge and low NO(3)-N concentration, which occurred only in late summer and early autumn. Annually, most NO(3)-N in these headwater sites appeared to be exported to downstream water bodies rather than denitrified, suggesting previous estimates of N losses through in-stream denitrification may have been overestimated.
The dissolved nitrate concentrations and their nitrogen and oxygen isotopic ratios were analyzed in seasonal samples from Korea's Han River to ascertain the seasonal and spatial variations of dissolved nitrate and its possible sources. Nitrate concentrations in the South Han River (SHR) were much higher than those in the North Han River (NHR), probably because of the more extensive distribution of agricultural fields, residential areas and animal farms in the SHR drainage basin. The nitrogen isotopic composition of dissolved nitrate indicates that nitrate-nitrogen (NO(3)(-)-N) is derived mainly from atmospheric deposition and/or soil organic matter in the NHR but comes principally from manure or sewage, with only a minor contribution from atmospheric deposition or soil organic matter, in the SHR. The oxygen isotopic compositions of dissolved nitrate suggest that most atmospheric nitrate undergoes microbial nitrification before entering the river.
Seoul Wastewater Treatment Plants Statistics
- Seoul Metropolitan Government