Coupled cycles of dissolved oxygen and nitrous oxide in rivers along a trophic gradient in southern Ontario, Canada

River metabolic fingerprints and regimes reveal ecosystem responses to enhanced wastewater treatment

Although many studies have examined how improvements in wastewater treatment impact river nutrient concentrations and loads, there has been much less focus on measuring river metabolism to evaluate the wider aquatic ecosystem benefits of reducing nutrient inputs to rivers. The objectives of this study were to evaluate the effects of enhanced wastewater treatment (nitrification) on river metabolism in the Grand River, Canada's largest river draining into Lake Erie. Metabolic fingerprints and regimes (calculated from high-frequency dissolved oxygen [DO] measurements) were used to visualize whole-river ecosystem functional responses to these wastewater treatment upgrades. There was a 60% reduction in ecosystem respiration during summer, in response to reductions in effluent total ammonia inputs, causing a shift from net heterotrophy to net autotrophy, and contraction of river metabolic fingerprints. This resulted in major improvements in summer DO concentrations, with reductions in the percentage of days during summer that DO minima fell below water-quality guidelines for protection of aquatic early life stages, from 88% to ≤16%. The results also point to potential cascading impacts on coupled phosphorus and nitrogen cycles, which may generate further improvements in river water quality. During the summer, high rates of river metabolism and nutrient retention may result in measured water-column nutrient concentrations potentially underestimating nutrient pressures. This study also demonstrates the value of combining river metabolism with nutrient monitoring for a more holistic understanding of the role of nutrients in river ecosystem health and function.

Varying water column stability controls the denitrification process in a subtropical reservoir, Southwest China

Reservoirs are regarded as hotspots of nitrogen transformation and potential sources of nitrous oxide (N₂O). However, it remains unclear how the hydrological conditions due to dam construction control the processes of nitrogen transformation in reservoir waters. To address this issue, we examined the spatial-temporal characteristics of nitrate concentrations, δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, δ¹⁸O-H₂O, relative water column stability (RWCS), and related environmental factors in a subtropical eutrophic reservoir (Hongfeng Reservoir, HFR), Southwest China. We found that denitrification was the most important nitrogen transformation process in the HFR and that higher denitrification intensity was associated with increased RWCS in summer, which suggested hydrological control of the denitrification process. In contrast, low RWCS conditions favored the nitrification process in the HFR in winter. Additionally, dissolved oxygen (DO; p < 0.05) and nitrate concentrations (p < 0.01) had significant impacts on the denitrification rate. We also found that the spatiotemporal RWCS variations were a prerequisite for regulating DO/nitrate stratification and the coupling/decoupling of nitrification-denitrification at the local and global scales. This study would advances our knowledge of the impacts of RWCS and thermal stratification on nitrogen transformation processes in reservoirs.

Impact of nitrogen compound variability of sewage treated water on N2O production in riverbeds

Nitrous oxide (N₂O), a strong greenhouse and ozone depleting gas, is known to be generated in the river environment. However, the impact of sewage treated water on the production mechanism has not been clarified. In this study, N₂O production in the upper reach of a river was evaluated by field survey and activity test. The results demonstrated that the N₂O production activity of the river pebbles increased with the inflow of the sewage treated water, which was supported by field survey results, such as the dissolved N₂O concentrations and water quality. The emission factors of N₂O were determined to be 0.02-0.05% in nitrification and 0.01-0.025% in denitrification. Our study shows that combining a field survey and an activity test improves the reliability of the results and leads to the appropriate quantitative evaluation. From a perspective of controlling the N₂O emissions from the sewage treatment plant, N₂O generation inside the plant is critical. However, appropriate nitrogen removal in the treatment plant is connected to the reduction of N₂O generation in the river environment.

Intra- and Inter-Annual Variability in the Dissolved Inorganic Nitrogen in an Urbanized River before and after Wastewater Treatment Plant Upgrades: Case Study in the Grand River (Southwestern Ontario)

External nitrogen (N) inputs originating from human activities act as essential nutrients accumulation in aquatic ecosystems or it is exported elsewhere, where the assimilation capacity is surpassed. This research presents a multi-annual case study of the dissolved inorganic nitrogen (DIN) in an urban river in Ontario (Canada), assessed changes in N downstream of the largest wastewater treatment plant (WTP) in the watershed. Changes in the DIN effluent discharge, in-river concentrations and loads were observed comparing the intra- and inter-annual variability (2010–2013) before, during and after WTP upgrades. These upgrades reduced the ammonium concentration in the river from 0.44 to 0.11 mg N-NH4+/L (year average), but the N load in the effluent increased. In the river, nitrate and ammonium concentrations responded to seasonal variability, being higher during the low temperature (>10 °C) and high flow seasons (spring and spring melt). Among years, changes in the DIN concentration are likely controlled by the effluent to river dilution ratio, which variability resides on the differences in river discharge between years. This suggest that the increasing trend in the DIN concentration and loads are the result of agricultural and urban additions, together with reduced N assimilation, in addition to N loads responding to variable river discharge. Finally, we propose monitoring both concentrations and loads, as they provide answers to different questions for regulatory agencies and water managers, allowing tailored strategies for different purposes, objectives and users.

Assessment of nitrous oxide production in eutrophicated rivers with inflow of treated wastewater based on investigation and statistical analysis

Accurate estimation and control of greenhouse gas emissions have been recognized as imperative in recent years. Therefore, frequent investigations under uniform environmental conditions are required to better understand this concept. Thus, six sampling sites with characteristic concentrations of nitrogen and other water quality parameters were selected to investigate the behavior of water quality parameters throughout the year and to statistically examine the correlations among the parameters. Dissolved nitrous oxide (D-N₂O) showed the highest positive correlation coefficient with NO₂-N among nitrogen species. The results of the principal component analysis suggested that river water quality could be broadly classified based on photosynthesis and contamination from treated wastewater. Photosynthesis caused an increase in pH, with concomitant decrease in D-N₂O concentration. Using the results of multiple regression analysis, D-N₂O was accurately estimated based on nitrogen concentration, pH, and concentration of organic matter in various situations. The results of a detailed survey suggested that D-N₂O was produced in the river from nitrogen sources released from the wastewater treatment plant. The main roles of the wastewater treatment plant for D-N₂O behavior in the river were the supply of the nitrogen source that was the precursor of D-N₂O, the supply of the nutrients that induced the photosynthesis, and the direct supply of D-N₂O at a low water temperature. The models based on multiple regression analysis could efficiently predict the D-N₂O concentration produced in rivers at sites downstream of the wastewater treatment plant, except for the direct supply of D-N₂O as an effluent at low water temperature.

Control of the Hydraulic Load on Nitrous Oxide Emissions from Cascade Reservoirs

Nitrous oxide (N₂O) emissions show large variability among dam reservoirs, which makes it difficult to estimate N₂O contributions to global greenhouse gases (GHGs). Because river damming alters hydraulic residence time and water depth, the hydraulic load (i.e., the ratio of the mean water depth to the residence time) was hypothesized to control N₂O emissions from dam reservoirs. To test this hypothesis, we investigated N₂O fluxes and related parameters in the cascade reservoirs along the Wujiang River in Southwest China. The N₂O fluxes showed obvious temporal and spatial variations, ranging from -7.86 to 337.22 μmol m^-2 d^-1, with an average of 12.76 μmol m^-2 d^-1. Nitrification was the main pathway of N₂O production in these reservoirs, and seasonal dissolved oxygen (DO) stratification played an important role in regulating the N₂O production. The reservoir N₂O flux had a significant negative logarithmic relationship with the hydraulic load, suggesting its control of the N₂O emission. This was because the hydraulic load was a prerequisite for regulating the nitrification-denitrification and the DO stratification in the dam reservoirs. This empirical relationship will help to estimate the contribution of reservoir N₂O emissions to global GHGs.

Methane and nitrous oxide temporal and spatial variability in two midwestern USA streams containing high nitrate concentrations

Concentrations and emissions of greenhouse gases CO₂, CH₄, and N₂O commonly are examined individually in aquatic environments in which each is expected to be relatively important; however, their co-occurrence and dynamic interactions in fluvial settings could provide important information about their controlling biogeochemical processes and potential contributions to global climate change. Spatial and temporal variability of CH₄, N₂O, and CO₂ concentrations were measured from June 1999 to September 2003 in two nitrate-rich (40-1200 μM) streams draining agricultural land in the midwestern USA that differed ~13-fold in flow. Seasonal (biweekly), diel (hourly), and transport-oriented (reach-scale) sampling approaches were compared. Dissolved gas concentrations exceeded atmospheric equilibrium values up to 700- and 16-fold, for CH₄ and N₂O, respectively. Mean concentrations were higher in the larger stream than in the smaller stream. In both streams, CH₄ emissions were generally higher in summer-fall and negatively correlated with flow and NO₃^- concentration while N₂O emissions were generally higher in winter/spring and positively correlated with flow and NO₃^-. In the small stream, diel variations in the concentrations, emissions, and isotopic compositions of CH₄, N₂O, and NO₂^- resulted from diel variations in sources, sinks, and air-water gas exchange velocities. Seasonal mean total (CH₄ + N₂O) area-normalized emission rates, expressed as CO₂ warming potential equivalents, were similar for the two streams, but the total reach-scale emission rate for the larger stream, including CO₂, was about 2.9 times that of the smaller stream (131.6 vs 46.0 kg CO₂ equivalents km^-1 day^-1, respectively). The CH₄ contribution to this flux was 9-28%, despite the relatively high NO₃^- and O₂ concentrations in the streams, indicating contributions from upwelling groundwater or reactions in streambed sediment.

Quantifying the fate of wastewater nitrogen discharged to a Canadian river

Addition of nutrients, such as nitrogen, can degrade water quality in lakes, rivers, and estuaries. To predict the fate of nutrient inputs, an understanding of the biogeochemical cycling of nutrients is needed. We develop and employ a novel, parsimonious, process-based model of nitrogen concentrations and stable isotopes that quantifies the competing processes of volatilization, biological assimilation, nitrification, and denitrification in nutrient-impacted rivers. Calibration of the model to nitrogen discharges from two wastewater treatment plants in the Grand River, Ontario, Canada, show that ammonia volatilization was negligible relative to biological assimilation, nitrification, and denitrification within 5 km of the discharge points.

Distribution and emission of N2O in the largest river-reservoir system along the Yellow River

Rivers and reservoirs are affected by human activities and are sources of the greenhouse gas nitrous oxide (N₂O). Concentrations of N₂O in the middle and lower reaches of the Yellow River and Xiaolangdi Reservoir, China, were measured in June and December 2017. Fluxes were estimated by boundary layer method to explore their controlling factors, especially the impact of damming and reservoir operation. In the middle and lower reaches of the Yellow River, N₂O concentrations in surface waters were 26.65 ± 14.67 nmol L^-1 in summer and 21.16 ± 5.35 nmol L^-1 in winter. In comparison, the concentrations of N₂O in the reservoir were 32.94 ± 17.32 nmol L^-1 in summer and 23.73 ± 5.60 nmol L^-1 in winter. The longitudinal distribution of N₂O along the river exhibited different patterns with surface N₂O decreasing downstream towards the dam in summer but increasing in winter. Vertical profiles of N₂O concentrations in the reservoir showed an increase with depth in summer but were almost vertically uniform in winter. In winter, N₂O that had accumulated in the bottom water in summer was transported to the surface by vertical mixing and released into the atmosphere. Dissolved oxygen (DO), water temperature, and in situ biological production were the main factors affecting the distribution of N₂O. The mean emissions rates of N₂O from the surface waters were 13.7 ± 8.8 μmol m^-2 d^-1 in summer and 13.2 ± 7.6 μmol m^-2 d^-1 in winter. Approximately 1.31 × 10⁶ mol N₂O was released from the reservoir surface in 2017, which represents 0.12% of the annual N₂O emissions from global reservoirs. The construction of dams increased N₂O emission from the lower reaches of the river by 4.53 × 10⁵ mol and 1.22 × 10⁵ mol due to the discharge of the bottom water and the water and sediment regulation, respectively. This study demonstrates that the construction of dams and reservoir operation practices have made the Xiaolangdi Reservoir a key area for N₂O emissions.

Diel and seasonal nitrous oxide fluxes determined by floating chamber and gas transfer equation methods in agricultural irrigation watersheds in southeast China

Agricultural nitrate leaching and runoff incurs high nitrogen loads in agricultural irrigation watersheds, constituting one of important sources of atmospheric nitrous oxide (N₂O). Two independent sampling campaigns of N₂O flux measurement over diel cycles and N₂O flux measurements once a week over annual cycles were carried out in an agricultural irrigation watershed in southeast China using floating chamber (chamber-based) and gas transfer equation (model-based) methods. The diel and seasonal patterns of N₂O fluxes did not differ between the two measurement methods. The diel variation in N₂O fluxes was characterized by the pattern that N₂O fluxes were greater during nighttime than daytime periods with a single flux peak at midnight. The diel variation in N₂O fluxes was closely associated with water environment and chemistry. The time interval of 9:00-11:00 a.m. was identified to be the sampling time best representing daily N₂O flux measurements in agricultural irrigation watersheds. Seasonal N₂O fluxes showed large variation, with some flux peaks corresponding to agricultural irrigation and drainage episodes and heavy rainfall during the crop-growing period of May to November. On average, N₂O fluxes calculated by model-based methods were 27% lower than those determined by the chamber-based techniques over diel or annual cycles. Overall, more measurement campaigns are highly needed to assess regional agricultural N₂O budget with low uncertainties.

Municipal wastewater treatment plant effluent-induced effects on freshwater mussel populations and the role of mussel refugia in recolonizing an extirpated reach

Global human population and urbanization continually increase the volume of wastewater entering aquatic environments. Despite efforts to treat these effluents, they contribute a diverse suite of substances that enter watersheds at concentrations that have the potential to elicit adverse effects on aquatic organisms. The relationship between wastewater treatment plant (WWTP) effluent exposure and biological responses within aquatic ecosystems remains poorly understood, especially at the population level. To examine the effect of WWTP effluents on sentinel invertebrates, freshwater mussels were assessed in the Grand River, Ontario, in populations associated with the outfall of a major WWTP. This watershed, within the Laurentian Great Lakes basin, has a diverse community of twenty-five species of mussels, including nine Species at Risk, and is representative of many habitats that receive WWTP effluents regionally as well as globally. Surveys were conducted to assess the presence and species richness of freshwater mussels. In total, 55 sites downstream of the WWTP were examined using timed visual searches with one or 2 h of effort spent searching 100 m segments. Although seven species of mussels were found in moderate abundance (mean of 8 mussels per hour of searching across 2 sites) upstream of the WWTP outfall, no live mussels were observed for 7.0 km downstream of the WWTP. Long-term water quality monitoring data indicate that ammonia and nitrite concentrations along with large seasonal declines in diel dissolved oxygen were associated with the extirpation of mussels downstream of the WWTP. The first live mussels found downstream were below the confluence with a major tributary indicating that in addition to an improvement in water quality to a state that enables mussels (and/or their fish hosts) to survive, a nearby mussel refuge may have facilitated the recolonization of the depauperate WWTP-impacted zone.

Factors Related with CH4 and N2O Emissions from a Paddy Field: Clues for Management implications

Paddy fields are major sources of global atmospheric greenhouse gases, including methane (CH₄) and nitrous oxide (N₂O). The different phases previous to emission (production, transport, diffusion, dissolution in pore water and ebullition) despite well-established have rarely been measured in field conditions. We examined them and their relationships with temperature, soil traits and plant biomass in a paddy field in Fujian, southeastern China. CH₄ emission was positively correlated with CH₄ production, plant-mediated transport, ebullition, diffusion, and concentration of dissolved CH₄ in porewater and negatively correlated with sulfate concentration, suggesting the potential use of sulfate fertilizers to mitigate CH₄ release. Air temperature and humidity, plant stem biomass, and concentrations of soil sulfate, available N, and DOC together accounted for 92% of the variance in CH₄ emission, and Eh, pH, and the concentrations of available N and Fe³⁺, leaf biomass, and air temperature 95% of the N₂O emission. Given the positive correlations between CH₄ emission and DOC content and plant biomass, reduce the addition of a carbon substrate such as straw and the development of smaller but higher yielding rice genotypes could be viable options for reducing the release of greenhouse gases from paddy fields to the atmosphere.

Freshwater mussels in an urban watershed: Impacts of anthropogenic inputs and habitat alterations on populations

The substantial increase in urbanization worldwide has resulted in higher emissions of wastewater to riverine systems near urban centers, which often impairs aquatic populations and communities. This study examined the effect of urbanization on freshwater mussel populations, including Species at Risk in two rivers receiving wastewater. The influence of anthropogenic activities was assessed in a watershed in the Laurentian Great Lakes basin, one that historically supported one of the most diverse mussel faunas in Canada. In the Grand River (ON), four sites along a 60km reach spanning from an upstream reference site to an urban-impacted downstream area were examined. In the Speed River, mussel populations at six sites along a 10km reach, selected to bracket specific anthropogenic inputs and structures were assessed. A semi-quantitative visual search method revealed that catch per unit effort in the Grand River declined by >60% from the upstream reference site to the area downstream of an urban center. The size (length) frequency distribution of the most abundant species, Lasmigona costata, was significantly (p≤0.008) different upstream of the majority of urban inputs (45-130mm) compared to downstream of the cities (85-115mm). In the Speed River, impoundments and wastewater treatment plants (WWTP) reduced both the diversity and catch per effort. Most striking were 84 and 95% changes in the number of mussels found on either side of two impoundments, and a 98% drop in mussels immediately downstream of a WWTP outfall. These population level effects of decreased abundance and underrepresentation of smaller mussels downstream of the urban area correspond to previously documented impacts at the biochemical and whole organism level of biological organization in wild mussels at this location. Our results demonstrate that poor water quality and physical barriers in urban environments continue to impair susceptible populations and communities of aquatic animals.

Indirect nitrous oxide emissions from streams within the US Corn Belt scale with stream order

N2O is an important greenhouse gas and the primary stratospheric ozone depleting substance. Its deleterious effects on the environment have prompted appeals to regulate emissions from agriculture, which represents the primary anthropogenic source in the global N2O budget. Successful implementation of mitigation strategies requires robust bottom-up inventories that are based on emission factors (EFs), simulation models, or a combination of the two. Top-down emission estimates, based on tall-tower and aircraft observations, indicate that bottom-up inventories severely underestimate regional and continental scale N2O emissions, implying that EFs may be biased low. Here, we measured N2O emissions from streams within the US Corn Belt using a chamber-based approach and analyzed the data as a function of Strahler stream order (S). N2O fluxes from headwater streams often exceeded 29 nmol N2O-N m(-2) ⋅ s(-1) and decreased exponentially as a function of S. This relation was used to scale up riverine emissions and to assess the differences between bottom-up and top-down emission inventories at the local to regional scale. We found that the Intergovernmental Panel on Climate Change (IPCC) indirect EF for rivers (EF5r) is underestimated up to ninefold in southern Minnesota, which translates to a total tier 1 agricultural underestimation of N2O emissions by 40%. We show that accounting for zero-order streams as potential N2O hotspots can more than double the agricultural budget. Applying the same analysis to the US Corn Belt demonstrates that the IPCC EF5r underestimation explains the large differences observed between top-down and bottom-up emission estimates.

Proper interpretation of dissolved nitrous oxide isotopes, production pathways, and emissions requires a modelling approach

Stable isotopes ([Formula: see text]15N and [Formula: see text]18O) of the greenhouse gas N2O provide information about the sources and processes leading to N2O production and emission from aquatic ecosystems to the atmosphere. In turn, this describes the fate of nitrogen in the aquatic environment since N2O is an obligate intermediate of denitrification and can be a by-product of nitrification. However, due to exchange with the atmosphere, the [Formula: see text] values at typical concentrations in aquatic ecosystems differ significantly from both the source of N2O and the N2O emitted to the atmosphere. A dynamic model, SIDNO, was developed to explore the relationship between the isotopic ratios of N2O, N2O source, and the emitted N2O. If the N2O production rate or isotopic ratios vary, then the N2O concentration and isotopic ratios may vary or be constant, not necessarily concomitantly, depending on the synchronicity of production rate and source isotopic ratios. Thus prima facie interpretation of patterns in dissolved N2O concentrations and isotopic ratios is difficult. The dynamic model may be used to correctly interpret diel field data and allows for the estimation of the gas exchange coefficient, N2O production rate, and the production-weighted [Formula: see text] values of the N2O source in aquatic ecosystems. Combining field data with these modelling efforts allows this critical piece of nitrogen cycling and N2O flux to the atmosphere to be assessed.

Nonlinear response of riverine N2O fluxes to oxygen and temperature

One-quarter of anthropogenically produced nitrous oxide (N2O) comes from rivers and estuaries. Countries reporting N2O fluxes from aquatic surfaces under the United Nations Framework Convention on Climate Change typically estimate anthropogenic inorganic nitrogen loading and assume a fraction becomes N2O. However, several studies have not confirmed a linear relationship between dissolved nitrate (NO3-) and river N2O fluxes. We apply recursive partitioning analysis to examine the relationships between N2O flux and NO3-, dissolved oxygen (DO), temperature, land use and surficial geology in the Grand River, Canada, a seventh-order river in an agricultural catchment with substantial urban population. Results suggest that N2O flux is high when hypoxia exists. Temperature, not NO3-, was the primary correlate of N2O flux when hypoxia does not occur suggesting NO3- is not limiting N2O production and further increases in NO3- may not lead to comparable increases in N2O flux. This work indicates that a linear relationship between NO3- and N2O is unlikely to exist in most agricultural and urban impacted river systems. Most N2O is produced during hypoxia so quantifying the extent of hypoxia is a necessary first step to quantifying N2O fluxes in lotic systems. Predicted increases in riverine hypoxia via eutrophication and increased temperature due to climate change may drive nonlinear increases in N2O production.

Wastewater effluent impacts ammonia-oxidizing prokaryotes of the Grand River, Canada

The Grand River (Ontario, Canada) is impacted by wastewater treatment plants (WWTPs) that release ammonia (NH3 and NH4+) into the river. In-river microbial communities help transform this ammonia into more oxidized compounds (e.g., NO3- or N2), although the spatial distribution and relative abundance of freshwater autotrophic ammonia-oxidizing prokaryotes (AOP) are not well characterized. This study investigated freshwater N cycling within the Grand River, focusing on sediment and water columns, both inside and outside a WWTP effluent plume. The diversity, relative abundance, and nitrification activity of AOP were investigated by denaturing gradient gel electrophoresis (DGGE), quantitative real-time PCR (qPCR), and reverse transcriptase qPCR (RT-qPCR), targeting both 16S rRNA and functional genes, together with activity assays. The analysis of bacterial 16S rRNA gene fingerprints showed that the WWTP effluent strongly affected autochthonous bacterial patterns in the water column but not those associated with sediment nucleic acids. Molecular and activity data demonstrated that ammonia-oxidizing archaea (AOA) were numerically and metabolically dominant in samples taken from outside the WWTP plume, whereas ammonia-oxidizing bacteria (AOB) dominated numerically within the WWTP effluent plume. Potential nitrification rate measurements supported the dominance of AOB activity in downstream sediment. Anaerobic ammonia-oxidizing (anammox) bacteria were detected primarily in sediment nucleic acids. In-river AOA patterns were completely distinct from effluent AOA patterns. This study demonstrates the importance of combined molecular and activity-based studies for disentangling molecular signatures of wastewater effluent from autochthonous prokaryotic communities.

Fish community responses to multiple municipal wastewater inputs in a watershed

Municipalities utilize aquatic environments to assimilate their domestic effluent resulting in eutrophication, anoxia, toxicity and endocrine disruption of aquatic biota. The objective of this study was to assess the potential cumulative impacts of municipal wastewater effluent (MWWE) discharges in the Grand River on the health status of a sentinel species and the fish community downstream of 2 MWWE discharges. The fish communities downstream of the MWWE outfalls demonstrated differences in the abundance and diversity, species and family richness, % tolerance and % vulnerability when compared to the fish community upstream or further downstream of these points of effluent discharge. In both years studied, the fish community exposed to MWWE in the riffle-run habitats demonstrated reductions in the proportion of the most prominent fish (Rainbow Darter, Ethoestoma caeruleum) downstream of the outfalls, and a significant increase in the proportion of large mobile, tolerant-omnivorous fish species such as suckers and sunfish. There was less variability in the responses of the fish community to MWWE in the same season between years than between seasons within the same year. An examination of how impaired health of a sentinel species exposed to MWWE discharges parallels changes in the fish community is also conducted. This study successfully demonstrates the cumulative impact of urban development, including multiple outfalls of treated wastewater effluents on fish populations and communities. Municipalities are the major source of nutrients and pharmaceuticals and personal care products to aquatic systems, and they need to consider their impacts carefully with increasing urban population growth and ageing demographics.

Diurnal pattern in nitrous oxide emissions from a sewage-enriched river

Estimates of N2O emission based on limit measurements could be highly inaccurate because of considerable diurnal variations in N2O flux due to rapid transformation of nutrients and diel change of dissolved oxygen (DO). In the present study, the N2O fluxes, dissolved N2O concentrations, and the controlling variables were measured hourly for 3d and night cycles at five sites on a typically sewage-enriched river in the Taihu region. There were no significant diurnal patterns in N2O emissions and dissolved N2O saturation, with respective mean value of 56.1μg N2O-Nm(-2)h(-1) (range=41.1μg N2O-Nm(-2)h(-1) to 87.7μg N2O-Nm(-2)h(-1)) and 813% (range=597-1372%), though distinct diurnal patterns were observed in DO concentration and river chemistry. However, the mean N2O emissions and the mean dissolved N2O saturation during the day (61.7μgNm(-2)h(-1) for N2O fluxes and 0.52μgNL(-1) for dissolved N2O saturation) were significantly higher than those during the night (50.1μgNm(-2)h(-1)for N2O fluxes and 0.44μgNL(-1) for dissolved N2O saturation). Factors controlling the N2O flux were pH, DO, NH4(+),SO4(2-), air temperature, and water temperature. Sampling at 19:00h could well represent the daily average N2O flux at the studied river.

Dissolved nitrous oxide concentrations and fluxes from the eutrophic San Joaquin River, California

Agriculturally impacted ecosystems can be a source of the greenhouse gas, nitrous oxide (N(2)O); yet in situ measurements of N(2)O fluxes are sparse, particularly in streams and rivers. Dissolved N(2)O was measured from 9 sites over a 13-month period and a gas exchange model was used to predict N(2)O fluxes. N(2)O fluxes were measured at 4 sites on 7 sampling dates using floating chambers. In addition, dissolved N(2)O in porewaters was measured at 4 sites at various depths from 2 to 30 cm. Dissolved N(2)O-N concentrations in surface waters (0.31-1.60 μg L(-1)) varied seasonally with highest concentrations in late fall and early summer and lowest in winter. Estimated N(2)O-N fluxes (26.2-207 μg m(-2) hr(-1)) were in relative agreement with measured N(2)O fluxes using floating chambers (9.5-372 μg m(-2) hr(-1)) and correlated strongly with temperature and nitrate concentrations (R(2) = 0.86). Maximum dissolved N(2)O-N:NO(3)(-)-N ratios were higher in sediment-porewaters at 0.16, compared to surface waters (0.010). The calculated EF5-r value (mean = 0.0028; range = 0.0012-0.0069) was up to 3 times greater than the current IPCC EF5-r emissions factor (0.0025 kg N(2)O-N emitted per kg of NO(3)(-)-N leached). The highest EF5-r values were found in the high-flow sampling events when dissolved N(2)O and NO(3)(-) concentrations were low, highlighting potential constraints in the IPCC methodology for large rivers.