Spatial Variation in Denitrification and N2O Emission in Relation to Nitrate Removal Efficiency in a N-stressed Riparian Buffer Zone

Ammonium-derived nitrous oxide is a global source in streams

Global riverine nitrous oxide (N2O) emissions have increased more than 4-fold in the last century. It has been estimated that the hyporheic zones in small streams alone may contribute approximately 85% of these N2O emissions. However, the mechanisms and pathways controlling hyporheic N2O production in stream ecosystems remain unknown. Here, we report that ammonia-derived pathways, rather than the nitrate-derived pathways, are the dominant hyporheic N2O sources (69.6 ± 2.1%) in agricultural streams around the world. The N2O fluxes are mainly in positive correlation with ammonia. The potential N2O metabolic pathways of metagenome-assembled genomes (MAGs) provides evidence that nitrifying bacteria contain greater abundances of N2O production-related genes than denitrifying bacteria. Taken together, this study highlights the importance of mitigating agriculturally derived ammonium in low-order agricultural streams in controlling N2O emissions. Global models of riverine ecosystems need to better represent ammonia-derived pathways for accurately estimating and predicting riverine N2O emissions.

Spatial and Temporal Variability and Driving Factors of Carbon Dioxide and Nitrous Oxide Fluxes in Alpine Wetland Ecosystems

Plants regulate greenhouse gas (GHG) fluxes in wetland ecosystems, but the mechanisms of plant removal and plant species that contribute to GHG emissions remain unclear. In this study, the fluxes of carbon dioxide (CO2) and nitrous oxide (N2O) were measured using the static chamber method from an island forest dominated by two different species, namely Betula platyphylla (BP) and Larix gmelinii (LG), in a marsh wetland in the Great Xing’an Mountains. Four sub-plots were established in this study: (1) bare soil after removing vegetation under BP (SBP); (2) bare soil after removing vegetation under LG (SLG); (3) soil with vegetation under BP (VSBP); and (4) soil with vegetation under LG (VSLG). Additionally, the contributions of the dark respiration from plant aerial parts under BP (VBP) and LG (VLG) to GHG fluxes were calculated. We found that the substantial spatial variability of CO2 fluxes ranged from −25.32 ± 15.45 to 187.20 ± 74.76 mg m−2 h−1 during the study period. The CO2 fluxes decreased in the order of SBP > VSLG > VSBP > SLG > VLG > VBP, indicating that vegetation species had a great impact on CO2 emissions. Particularly, the absence of vegetation promoted CO2 emission in both BP and LG. Additionally, CO2 fluxes showed dramatically seasonal variations, with high CO2 fluxes in late spring (May) and summer (June, July, and August), but low fluxes in late summer (August) and early autumn (September). Soil temperatures at 0−20 cm depth were better predictors of CO2 fluxes than deeper soil temperatures. N2O fluxes were varied in different treatments with the highest N2O fluxes in SLG and the lowest N2O fluxes in VBP. Meanwhile, no significant correlation was found between N2O fluxes and air or soil temperatures. Temporally, negative N2O fluxes were observed from June to October, indicating that soil N2O fluxes were reduced and emitted as N2, which was the terminal step of the microbial denitrification process. Most of the study sites were CO2 sources during the warm season and CO2 sinks in the cold season. Thus, soil temperature plays an important role in CO2 fluxes. We also found that the CO2 flux was positively related to pH in a 10 cm soil layer and positively related to moisture content (MC) in a 50 cm soil layer in VSBP and VSLG. However, the CO2 flux was negatively related to pH in a 30 cm soil layer in SBP and SLG. Our findings highlight the effects of vegetation removal on GHG fluxes, and aid in the scientific management of wetland plants.

Spatiotemporal analysis of land use pattern and stream water quality in southern Alberta, Canada

Alberta has over 70% of total irrigated land in Canada and 13 irrigation districts are mainly located along the rivers in southern Alberta. The runoff and irrigation return flow can carry excess nutrients, pesticides, and sediments, which adversely affect the river water quality. In the present study, the comprehensive spatiotemporal analysis of land use pattern and stream water quality in southern Alberta was conducted. The water quality monitoring on the South Saskatchewan River watershed within Alberta showed that most water quality indicators did not significantly change between 2003 and 2017. Land use maps demonstrated significant urban expansion and cropland decline in the study area from 2005 to 2015, while the irrigation area increased. Correlation analysis and redundancy analysis revealed that cropland, built-up land, and native grassland were three land use types that were positively correlated with water quality indicators. Stronger correlations between nitrogen concentration and built-up land were found in dry seasons compared with wet seasons. According to the results of cluster analysis, higher water quality indicator concentrations could be observed in groups with a higher proportion of cropland and built-up land. Significant relationships between land use patterns and water quality were found in this study, which demonstrated that further investigation is needed to identify the sources of water pollutants at a smaller scale.

Dissimilatory nitrate reduction to ammonium (DNRA) in traditional municipal wastewater treatment plants in China: Widespread but low contribution

Recent reports on the occurrence and contribution of dissimilatory nitrate reduction to ammonium (DNRA) in marine, inland water, and soil systems have greatly improved our understanding of the global nitrogen (N) cycle. This also promoted the investigation of the role and ecological features of DNRA in anthropogenic ecosystems. However, so far, the use of DNRA in municipal wastewater treatment plants (WWTPs), which are one of the most common and largest biotechnologically artificial water ecosystems, has not been investigated. Accordingly, this study focused on the abundance, activity, community structure, and diversity of DNRA bacteria in full-scale WWTPs. DNRA bacteria were detected in all treatment units in six tested municipal WWTPs, even in aerobic zones (dissolved oxygen > 2 mg L^-1). Although the relative abundance of DNRA bacteria (0.2-4.0%) was less than that of denitrifying bacteria (0.7-10.1%) among all investigated samples, the abundance of DNRA bacteria still reaches 10⁹ gene copies g^-1. However, ¹⁵N-isotope tracing indicated that the potential DNRA rates were significantly lower (0.4-2.1 nmol N g^-1 h^-1) than those of denitrification (9.5-15.7 nmol N g^-1 h^-1), but higher than anammox rate (0.3-1.3 nmol N g^-1 h^-1). The DNRA bacterial community structure was primarily affected by temperature gradient despite the treatment process. High-throughput sequencing analysis targeting the DNRA nrfA gene showed that Nitrospira accounted for the largest proportion of nrfA genes among all samples (6.2-36.3%), followed by Brocadia (5.9-22.1%). Network analysis further indicated that Nitrospira played an important role in both the DNRA bacterial community and entire bacterial community in municipal WWTPs. These results suggest that the ecological habitats of DNRA bacteria in anthropogenic ecosystems were far more abundant than previously assumed. However, the contribution to N transformation by the widespread DNRA was not significant in traditional municipal WWTPs.

Riparian land-use systems impact soil microbial communities and nitrous oxide emissions in an agro-ecosystem

Riparian buffer systems (RBS) are considered a best management practice (BMP) in agricultural landscapes to intercept soil nitrogen (N) and phosphorus (P) leaching and surface runoff into aquatic ecosystems. However, these environmental benefits could be offset by increased greenhouse gas (GHG) emissions, including nitrous oxide (N₂O). The main sources of N₂O in soil are linked to processes which are mediated by soil microbial communities. These microorganisms play crucial roles in N-cycling and in the reduction of nitrate to N₂, and N₂O gases. This study was conducted to determine the abundance and diversity of microbial communities and functional genes associated with N-cycling and their influence on N₂O emissions in different riparian land-use: undisturbed natural forest (UNF), rehabilitated site (RH), grass buffer (GRB), and an adjacent agricultural land (AGR). Soil was sampled concurrently with N₂O emissions on July 13, 2017. DNA was extracted and used to target key N-cycling genes for N-fixation (nifH), nitrification: (amoA), and denitrification (nirS, nirK, and nosZ) via quantitative PCR, and for high throughput sequencing of total bacterial and fungal communities. Non-metric multidimensional scaling (NMDS) was used to examine microbial community composition and indicated significant differences in bacterial (p < 0.001) and fungal (p < 0.0085) communities between sites. Bacterial abundance differed significantly (p = 0.0005) between RBS and AGR sites with the highest populations occurring in the UNF (2.1 × 10¹⁰ copies g^-1 dry soil), and lowest in AGR (5.3 × 10⁹ copies g^-1 dry soil). However, the AGR site had the highest ammonia-oxidizing bacteria (AOB) abundance, indicating that nitrification is highest at this site. The abundance of the nosZ gene was highest in RH and GRB demonstrating the capacity for complete denitrification at these sites, lowering measured N₂O. These results suggest N-cycling microbial community dynamics differ among RBS and are influencing N₂O emissions in the sites investigated.

Hotspot of dissimilatory nitrate reduction to ammonium (DNRA) process in freshwater sediments of riparian zones

Dissimilatory nitrate reduction to ammonium (DNRA), an important intermediate process in the N-cycle, links N-compound oxidation and reduction processes. Hence, the oxic-anoxic interface would be the hotspot of the DNRA process. In freshwater ecosystems, the riparian zone is the most typical carrier of the oxic-anoxic interface. Here we report spatio-temporal evidence of a higher abundance and rate of DNRA in the riparian zone than in the open water sediments based on molecular and ¹⁵N isotopic-tracing technologies, hence signifying a hotspot for the DNRA process. These abudance and rates were significantly higher than those in open water sediments. ¹⁵N isotopic paring technology revealed that the DNRA hotspot promoted higher rates of N-compound oxidation (NO₂^-), reduction (NO₃^- and DNRA), and N₂ production (anammox and denitrification) in the riparian zone than those in open water sediment. However, high-through sequencing analysis showed that the DNRA bacteria in the riparian zone and openwater sediments were insignificantly different. Network and correlation analysis showed that the DNRA abundance and rates were significantly positively correlated with TOM, TC/NH₄⁺, and TC/NO₂^-, but not with the dominant genera (Anaeromyxobacter, Lacunisphaera, and Sorangium), which played different roles on the connection in the respective community networks. The DNRA process in the riparian zone could be driven mainly by the related environmental biogeochemical characteristics induced by anthropogenic changes, followed by microbial processes. This result provides valuable information for the management of riparian zones because anthropogenic changes in the riparian water table are expected to increase, inducing consequent changes in the reduction from NO₃^- to NH₄⁺.

Nitrate reduction in the reed rhizosphere of a riparian zone: From functional genes to activity and contribution

The increased nitrogen (N) fertilizer usage caused substantial nitrate (NO₃^-) leaching into groundwater and eutrophication in downstream aquatic systems. Riparian zones positioned as the link interfaces of terrestrial and aquatic environments are effective in NO₃^- removal. However, the microbial mechanisms regulating NO₃^- reduction in riparian zones are still unclear. In this study, four microbial NO₃^- reduction processes were explored in fine-scale riparian soil horizons by isotopic tracing technique, qPCR of functional gene, high-throughput amplicon sequencing, and phylogenetic molecular ecological network analysis. Interestingly, anaerobic ammonium oxidation (anammox) contributed to NO₃^- removal of up to 48.2% only in waterward sediments but not in landward soil. Denitrification was still the most significant contributor to NO₃^- reduction (32.0-91.8%) and N-losses (51.7-100%). Additionally, dissimilatory nitrate reduction to ammonium (DNRA) played a key role in NO₃^- reduction (4.4-67.5%) and was even comparable to denitrification. Community structure analysis of denitrifying, anammox, and DNRA bacterial communities targeting the related functional gene showed that spatial heterogeneity played a greater role than both temporal and soil type (rhizosphere and non-rhizosphere soil) variability in microbial community structuring. Denitrification and DNRA communities were diverse, and their activities did not depend on gene abundance but were significantly related to organic matter, suggesting that gene abundance alone was insufficient in assessing their activity in riparian zones. Based on networks, DNRA plays a keystone role among the microbial NO₃^- reducers. As the last line of defense in the interception of terrestrial NO₃^-, these findings contribute to our understanding of NO₃^- removal mechanisms in riparian zones, and could potentially be exploited to reduce the diffusion of NO₃^- pollution.

Anammox and denitrification separately dominate microbial N-loss in water saturated and unsaturated soils horizons of riparian zones

Fertilized agroecosystems may show considerable leaching of the mobile nitrogen (N) compound NO₃^-, which pollutes groundwater and causes eutrophication of downstream waterbodies. Riparian buffer zones, positioned between terrestrial and aquatic environments, effectively remove NO₃^- and serve as a hotspot for N₂O emissions. However, microbial processes governing NO₃^- reduction in riparian zones still remain largely unclear. This study explored the underlying mechanisms of various N-loss processes in riparian soil horizons using isotopic tracing techniques, molecular assays, and high-throughput sequencing. Both anaerobic ammonium oxidation (anammox) and denitrification activity were maximized in the riparian fringe rather than in the central zones. Denitrifying anaerobic methane oxidation (damo) process was not detected. Interestingly, both contrasting microbial habitats were separated by a groundwater table, which forms an important biogeochemical interface. Denitrification dominated cumulative N-losses in the upper unsaturated soil, while anammox dominated the lower oxic saturated soil horizons. Archaeal and bacterial ammonium oxidation that couple dissimilatory nitrate reduction to ammonium (DNRA) with a high cell-specific rate promoted anammox even further in oxic subsurface horizons. High-throughput sequencing and network analysis showed that the anammox rate positively correlated with Candidatus 'Kuenenia' (4%), rather than with the dominant Candidatus 'Brocadia'. The contribution to N-loss via anammox increased significantly with the water level, which was accompanied by a significant reduction of N₂O emission (∼39.3 ± 10.6%) since N-loss by anammox does not cause N₂O emissions. Hence, water table management in riparian ecotones can be optimized to reduce NO₃^- pollution by shifting from denitrification to the environmentally friendly anammox pathway to mitigate greenhouse gas emissions.

Evaluating the net effect of sulfadimidine on nitrogen removal in an aquatic microcosm environment

Antibiotics enter into aquatic pond sediments by wastewater and could make detrimental effects on microbial communities. In this study, we examined the effects of sulfadimidine on nitrogen removal when added to experimental pond sediments. We found that sulfadimidine increased the number of sulfadimidine resistant bacteria and significantly increased the abundance of sul2 at the end of the incubation time (ANOVA test at Tukey HSD, P < 0.05). In addition, sulfadimidine decreased the N₂O reduction rate as well as the amount of nitrate reduction. Pearson correlation analysis revealed that the N₂O reduction rate was significantly and negatively correlated with narG (r = -0.679, P < 0.05). In contrast, we found a significant positive correlation between the amount of nitrate reduction and the abundance of narG (r = 0.609, P < 0.05) and nirK (r = 0.611, P < 0.05). High-throughput sequencing demonstrated that Actinobacteria, Euryarchaeota, Gemmatimonadetes, Nitrospirae, Burkholderiaceae (a family of Proteobacteria), and Thermoanaerobaculaceae (a family of Firmicutes) decreased with sulfadimidine exposure. In sediments, Actinobacteria, Bacteroidetes, Cyanobacteria, Epsilonbacteraeota, Euryarchaeota, Firmicutes, Gemmatimonadetes, and Spirochaetesat may play key roles in nitrogen transformation. Overall, the study exhibited a net effect of antibiotic exposure regarding nitrogen removal in an aquatic microcosm environment through a combination of biochemical pathways and molecular pathways, and draws attention to controlling antibiotic pollution in aquatic ecosystems.

Spatial and Temporal Variation in Soil Nitrous Oxide Emissions from a Rehabilitated and Undisturbed Riparian Forest

Riparian zones enhance water quality and provide wildlife habitat, but high nutrient input in agricultural landscapes causes nitrous oxide (NO) emissions, potentially negating their benefits of C sequestration. The objectives of this study were to quantify spatiotemporal NO emissions in a rehabilitated and undisturbed natural riparian forest. We also determined soil and vegetation characteristics, and their role in driving spatiotemporal NO emissions. Mean NO-N emissions were not significantly ( < 0.05) different between rehabilitated (7.62 μg m h) and undisturbed (5.93 μg m h) riparian forests. The greatest ( < 0.05) NO-N emissions in both riparian forests were observed during the summer. Soil moisture, temperature, and N were significantly correlated to NO-N emissions. Our results show that soil and vegetation characteristics varied between the two riparian forests, but differences in NO-N emissions were negligible. We also found that NO emissions were influenced by soil characteristics and seasonality, rather than vegetation characteristics or spatial position.

Laboratory study on nitrate removal and nitrous oxide emission in intact soil columns collected from nitrogenous loaded riparian wetland, Northeast China

Nitrate (NO3−) pollution of surface and groundwater systems is a major problem globally. For some time now wetlands have been considered potential systems for improving water quality. Nitrate dissolved in water moving through wetlands can be removed through different processes, such as the denitrification process, where heterotrophic facultative anaerobic bacteria use NO3− for respiration, leading to the production of nitrogen (N₂) and nitrous oxide (N₂O) gases. Nitrate removal and emission of N₂O in wetlands can vary spatially, depending on factors such as vegetation, hydrology and soil structure. This study intended to provide a better understanding of the spatial variability and processes involved in NO3− removal and emission of N₂O in riparian wetland soils. We designed a laboratory experiment simulating surface water flow through soil columns collected from different sites dominated by different plant species within a wetland. Water and gas samples for NO3−,NH4+ and N₂O analyses were collected every 5 days for a period of 30 days. The results revealed significant removal of NO3− in all the soil columns, supporting the role of riparian wetland soils in removing nitrogen from surface runoff. Nitrate removal at 0 and 10cm depths in sites dominated by Phragmites australis and Carex schnimdtii was significantly higher than in the site dominated by Calamagrostis epigeio. Nitrous oxide emissions varied spatially and temporally with negative flux observed in sites dominated by P. australis and C. schnimdtii. These results reveal that in addition to the ability of wetlands to remove NO3−, some sites within wetlands are also capable of consuming N₂O, hence mitigating not only agricultural nitrate pollution but also climate change.

Dramatic source-sink transition of N2O in the water level fluctuation zone of the Three Gorges Reservoir during flooding-drying processes

Biogeochemical cycling of nitrous oxide (N₂O), a significant greenhouse gas (GHG), can influence global climate change. The production and emission of N₂O mediated by hydrological regimes is particularly active in water level fluctuation zones (WLFZs). However, the hydrological mechanisms affecting N₂O transformation and production across the water-sediment micro-interface remain unclear. In this study, intact sediment cores from the WLFZs of the Three Gorges Reservoir (TGR) were incubated for 24 days in a laboratory microcosm to identify the effects of the flooding-drying processes on the yield and emission of N₂O. Results showed a source-sink transition of N₂O in the first 1.5 days during the flooding period, with the water column subsequently acting as a sink relative to the atmosphere in the following experimental period. The source-sink transition was ascribed to changes in oxygen concentration in the water column and sediment regulation of NO₃^--N transformation, resulting in denitrification and N₂O production. Preliminary estimates on the mass budget of N₂O in a typical WLFZs of the TGR showed slight emission fluxes, ranging from 13.08 to 43.08 μmol m^-2 from flooding period to drying process. Although these N₂O emissions were relatively low, the emission peak detected during the initial period (first 1.5 days) of the flooding phase provides important knowledge on the mitigation of GHG emissions from hydropower sources, which should be incorporated into future reservoir operations.

Microbial Nitrogen Cycle Hotspots in the Plant-Bed/Ditch System of a Constructed Wetland with N2O Mitigation

Artificial microbial nitrogen (N) cycle hotspots in the plant-bed/ditch system were developed and investigated based on intact core and slurry assays measurement using isotopic tracing technology, quantitative PCR and high-throughput sequencing. By increasing hydraulic retention time and periodically fluctuating water level in heterogeneous riparian zones, hotspots of anammox, nitrification, denitrification, ammonium (NH₄⁺) oxidation, nitrite (NO₂^-) oxidation, nitrate (NO₃^-) reduction and DNRA were all stimulated at the interface sediments, with the abundance and activity being about 1-3 orders of magnitude higher than those in nonhotspots. Isotopic pairing experiments revealed that in microbial hotspots, nitrite sources were higher than the sinks, and both NH₄⁺ oxidation (55.8%) and NO₃^- reduction (44.2%) provided nitrite for anammox, which accounted for 43.0% of N-loss and 44.4% of NH₄⁺ removal in riparian zones but did not involve nitrous oxide (N₂O) emission risks. High-throughput analysis identified that bacterial quorum sensing mediated this anammox hotspot with B.fulgida dominating the anammox community, but it was B. anammoxidans and Jettenia sp. that contributed more to anammox activity. In the nonhotspot zones, the NO₂^- source (NO₃^- reduction dominated) was lower than the sink, limiting the effects on anammox. The in situ N₂O flux measurement showed that the microbial hotspot had a 27.1% reduced N₂O emission flux compared with the nonhotspot zones.

Stimulating ammonia oxidizing bacteria (AOB) activity drives the ammonium oxidation rate in a constructed wetland (CW)

An integrated approach to document high ammonium oxidation rate in Guanjinggang constructed wetland (GJG-CW) was performed and the results showed that the substantial ammonium oxidation rate could be obtained by enhancing Ammonia Oxidizing Bacteria (AOB) activity rather than Ammonia Oxidizing Archaea (AOA) activity. In the plant-bed/ditch system, ditch center and plant-bed fringe were two active zones for NH₄⁺-N removal with ammonium oxidation rate peaking at 2.98±0.04 and 2.15±0.02mgNkg^-1d^-1, respectively. The enhanced AOB activity were achieved by increasing water level fluctuations, extending hydraulic retention time (HRT) and stimulating substrate availability, which subsequently enhanced NH₄⁺-N removal by 34.06% in GJG-CW. However, the high AOB activity was not correlated with high AOB abundance, but was instead mostly determined by specific AOB taxa, particularly Nitrosomonas, which dominated in the active AOB. The increased cell-specific AOA activity and high AOA diversity were also achieved using those engineering measures. Although the AOA activity decreased overall with extended HRT and increased NH₄⁺-N contents in GJG-CW, AOA still played a major role on ammonium oxidation in plant-bed soil. The study illustrated that artificially enhancing AOB activity and certain species in anthropogenically polluted water ecosystems would be an effective strategy to improve NH₄⁺-N removal.

Increased Denitrification Rates Associated with Shifts in Prokaryotic Community Composition Caused by Varying Hydrologic Connectivity

While modern developments in agriculture have allowed for increases in crop yields and rapid human population growth, they have also drastically altered biogeochemical cycles, including the biotransformation of nitrogen. Denitrification is a critical process performed by bacteria and fungi that removes nitrate in surface waters, thereby serving as a potential natural remediation strategy. We previously reported that constant inundation resulted in a coupling of denitrification gene abundances with denitrification rates in sediments, but these relationships were not maintained in periodically-inundated or non-inundated environments. In this study, we utilized Illumina next-generation sequencing to further evaluate how the microbial community responds to these hydrologic regimes and how this community is related to denitrification rates at three sites along a creek in an agricultural watershed over 2 years. The hydrologic connectivity of the sampling location had a significantly greater influence on the denitrification rate (P = 0.010), denitrification gene abundances (P < 0.001), and the prokaryotic community (P < 0.001), than did other spatiotemporal factors (e.g., creek sample site or sample month) within the same year. However, annual variability among denitrification rates was also observed (P < 0.001). Furthermore, the denitrification rate was significantly positively correlated with water nitrate concentration (Spearman's ρ = 0.56, P < 0.0001), denitrification gene abundances (ρ = 0.23-0.47, P ≤ 0.006), and the abundances of members of the families Burkholderiaceae, Anaerolinaceae, Microbacteriaceae, Acidimicrobineae incertae sedis, Cytophagaceae, and Hyphomicrobiaceae (ρ = 0.17-0.25, P ≤ 0.041). Prokaryotic community composition accounted for the least amount of variation in denitrification rates (22%), while the collective influence of spatiotemporal factors and gene abundances accounted for 37%, with 40% of the variation related to interactions among all parameters. Results of this study suggest that the hydrologic connectivity at each location had a greater effect on the prokaryotic community than did spatiotemporal differences, where inundation is associated with shifts favoring increased denitrification potential. We further establish that while complex interactions among the prokaryotic community influence denitrification, the link between hydrologic connectivity, microbial community composition, and genetic potential for biogeochemical cycling is a promising avenue to explore hydrologic remediation strategies such as periodic flooding.

Edaphic Conditions Regulate Denitrification Directly and Indirectly by Altering Denitrifier Abundance in Wetlands along the Han River, China

Riparian wetlands play a critical role in retaining nitrogen (N) from upland runoff and improving river water quality, mainly through biological processes such as soil denitrification. However, the relative contribution of abiotic and biotic factors to riparian denitrification capacity remains elusive. Here we report the spatiotemporal dynamics of potential and unamended soil denitrification rates in 20 wetlands along the Han River, an important water source in central China. We also quantified the abundance of soil denitrifying microorganisms using nirK and nirS genes. Results showed that soil denitrification rates were significantly different between riparian and reservoir shoreline wetlands, but not between mountain and lowland wetlands. In addition, soil denitrification rates showed strong seasonality, with higher values in August (summer) and April (spring) but lower values in January (winter). The potential and unamended denitrification rates were positively correlated with edaphic conditions (moisture and carbon concentration), denitrifier abundance, and plant species richness. Path analysis further revealed that edaphic conditions could regulate denitrification rates both directly and indirectly through their effects on denitrifier abundance. Our findings highlight that not only environmental factors, but also biotic factors including denitrifying microorganisms and standing vegetation, play an important role in regulating denitrification rate and N removal capacity in riparian wetlands.

Efficiency and detrimental side effects of denitrifying bioreactors for nitrate reduction in drainage water

A laboratory column experiment was conducted to test the efficiency of denitrifying bioreactors for the nitrate (NO3-N) removal in drainage waters at different flow rates and after desiccation. In addition, we investigated detrimental side effects in terms of the release of nitrite (NO2-N), ammonium (NH4-N), phosphate (PO4-P), dissolved organic carbon (DOC), methane (CH4), and dinitrogen oxide (N2O). The NO3-N removal efficiency decreased with increasing NO3-N concentrations, increasing flow rates, and after desiccation. Bioreactors with purely organic fillings showed higher NO3-N removal rates (42.6-55.7 g NO3-N m(-3) day(-1)) than those with organic and inorganic fillings (6.5-21.4 g NO3-N m(-3) day(-1)). The release of NO2-N and DOC was considerable and resulted in concentrations of up to 800 μg NO2-N L(-1)and 25 mg DOC L(-1) in the effluent water. N2O concentrations increased by 4.0 to 15.3 μg N2O-N L(-1) between the influent and the effluent, while CH4 production rates were low. Our study confirms the high potential of denitrifying bioreactors to mitigate NO3-N pollution in drainage waters, but highlights also the potential risks for the environment.

The impact of a pulsing groundwater table on greenhouse gas emissions in riparian grey alder stands

Floods control greenhouse gas (GHG) emissions in floodplains; however, there is a lack of data on the impact of short-term events on emissions. We studied the short-term effect of changing groundwater (GW) depth on the emission of (GHG) carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in two riparian grey alder (Alnus incana) stands of different age in Kambja, southern Estonia, using the opaque static chamber (five replicates in each site) and gas chromatography methods. The average carbon and total nitrogen content in the soil of the old alder (OA) stand was significantly higher than in the young alder (YA) stand. In both stands, one part was chosen for water table manipulation (Manip) and another remained unchanged with a stable and deeper GW table. Groundwater table manipulation (flooding) significantly increases CH4 emission (average: YA-Dry 468, YA-Manip 8,374, OA-Dry 468, OA-Manip 4,187 μg C m(-2) h(-1)) and decreases both CO2 (average: OA-Dry 138, OA-Manip 80 mg C m(-2) h(-1)) and N2O emissions (average: OA-Dry 23.1, OA-Manip 11.8 μg N m(-2) h(-1)) in OA sites. There was no significant difference in CO2 and CH4 emissions between the OA and YA sites, whereas in OA sites with higher N concentration in the soil, the N2O emission was significantly higher than at the YA sites. The relative CO2 and CH4 emissions (the soil C stock-related share of gaseous losses) were higher in manipulated plots showing the highest values in the YA-Manip plot (0.03 and 0.0030 % C day(-1), respectively). The soil N stock-related N2O emission was very low achieving 0.000019 % N day(-1) in the OA-Dry plot. Methane emission shows a negative correlation with GW, whereas the 20 cm depth is a significant limit below which most of the produced CH4 is oxidized. In terms of CO2 and N2O, the deeper GW table significantly increases emission. In riparian zones of headwater streams, the short-term floods (e.g. those driven by extreme climate events) may significantly enhance methane emission whereas the long-term lowering of the groundwater table is a more important initiator of N2O fluxes from riparian gley soils than flood pulses.

Isotopologue ratios of N2O and N2 measurements underpin the importance of denitrification in differently N-loaded riparian alder forests

Known as biogeochemical hotspots in landscapes, riparian buffer zones exhibit considerable potential concerning mitigation of groundwater contaminants such as nitrate, but may in return enhance the risk for indirect N2O emission. Here we aim to assess and to compare two riparian gray alder forests in terms of gaseous N2O and N2 fluxes and dissolved N2O, N2, and NO3(-) in the near-surface groundwater. We further determine for the first time isotopologue ratios of N2O dissolved in the riparian groundwater in order to support our assumption that it mainly originated from denitrification. The study sites, both situated in Estonia, northeastern Europe, receive contrasting N loads from adjacent uphill arable land. Whereas N2O emissions were rather small at both sites, average gaseous N2-to-N2O ratios inferred from closed-chamber measurements and He-O laboratory incubations were almost four times smaller for the heavily loaded site. In contrast, groundwater parameters were less variable among sites and between landscape positions. Campaign-based average (15)N site preferences of N2O (SP) in riparian groundwater ranged between 11 and 44 ‰. Besides the strong prevalence of N2 emission over N2O fluxes and the correlation pattern between isotopologue and water quality data, this comparatively large range highlights the importance of denitrification and N2O reduction in both riparian gray alder stands.

Denitrification controls in urban riparian soils: implications for reducing urban nonpoint source nitrogen pollution

The purpose of this research was to thoroughly analyze the influences of environmental factors on denitrification processes in urban riparian soils. Besides, the study was also carried out to identify whether the denitrification processes in urban riparian soils could control nonpoint source nitrogen pollution in urban areas. The denitrification rates (DR) over 1 year were measured using an acetylene inhibition technique during the incubation of intact soil cores from six urban riparian sites, which could be divided into three types according to their vegetation. The soil samples were analyzed to determine the soil organic carbon (SOC), soil total nitrogen (STN), C/N ratio, extractable NO3 (-)-N and NH4 (+)-N, pH value, soil water content (SWC), and the soil nitrification potential to evaluate which of these factors determined the final outcome of denitrification. A nitrate amendment experiment further indicated that the riparian DR was responsive to added nitrate. Although the DRs were very low (0.099 ~ 33.23 ng N2O-N g(-1) h(-1)) due to the small amount of nitrogen moving into the urban riparian zone, the spatial and temporal patterns of denitrification differed significantly. The extractable NO3 (-)-N proved to be the dominant factor influencing the spatial distribution of denitrification, whereas the soil temperature was a determinant of the seasonal DR variation. The six riparian sites could also be divided into two types (a nitrate-abundant and a nitrate-stressed riparian system) according to the soil NO3 (-)-N concentration. The DR in nitrate-abundant riparian systems was significantly higher than that in the nitrate-stressed riparian systems. The DR in riparian zones that were covered with bushes and had adjacent cropland was higher than in grass-covered riparian sites. Furthermore, the riparian DR decreased with soil depth, which was mainly attributed to the concentrated nitrate in surface soils. The DR was not associated with the SOC, STN, C/N ratio, and pH. Nitrate supply and temperature finally decided the spatiotemporal distribution patterns of urban riparian denitrification. Considering both the low DR of existing riparian soils and the significance of nonpoint source nitrogen pollution, the substantial denitrification potential of urban riparian soils should be utilized to reduce nitrogen pollution using proper engineering measures that would collect the polluted urban rainfall runoff and make it flow through the riparian zones.

The phytoremediation potential of native plants on New Zealand dairy farms

Ecological restoration of marginal land and riparian zones in agricultural landscapes in New Zealand enhances the provision of above-ground ecosystem services. We investigated whether native endemic plant assemblages have remediation potential, through modifying soil nutrient and trace element mobility. Analysis of native plant foliage in situ indicated that selective uptake of a range of commonly deficient trace elements including Zn, B, Cu, Mn and Co could provide a browse crop to avoid deficiencies of these elements in livestock, although some native plants may enhance the risk of Mo and Cd toxicity. Native plant rhizospheres were found to modify soil physico-chemistry and are likely to influence lateral and vertical fluxes of chemical elements in drainage waters. Native plants on marginal land in agricultural landscapes could add value to dairy production systems whilst helping to resolve topical environmental issues.

Nitrous oxide production from soils amended with biogas residues and cattle slurry

The amount of residues generated from biogas production has increased dramatically due to the worldwide interest in renewable energy. A common way to handle the residues is to use them as fertilizers in crop production. Application of biogas residues to agricultural soils may be accompanied with environmental risks, such as increased NO emission. In 24-d laboratory experiments, NO dynamics and total production were studied in arable soils (sandy, clay, and organic) amended with one of two types of anaerobically digested biogas residues (BR-A and BR-B) generated from urban and agricultural waste and nondigested cattle slurry (CS) applied at rates corresponding to 70 kg NH-N ha. Total NO-N losses from the sandy soil were higher after amendment with BR-B (0.32 g NO-N m) than BR-A or CS (0.02 and 0.18 g NO-N m, respectively). In the clay soil, NO-N losses were very low for CS (0.02 g NO-N m) but higher for BR-A and BR-B (0.25 and 0.15 g NO-N m, respectively). In the organic soil, CS gave higher total NO-N losses (0.31 g NO-N m) than BR-A or BR-B (0.09 and 0.08 g NO-N m, respectively). Emission peaks differed considerably between soils, occurring on Day 1 in the organic soil and on Days 11 to 15 in the sand, whereas in the clay the peak varied markedly (Days 1, 6, and 13) depending on residue type. In all treatments, NH concentration decreased with time, and NO concentration increased. Potential ammonium oxidation and potential denitrification activity increased significantly in the amended sandy soil but not in the organic soil and only in the clay amended with CS. The results showed that fertilization with BR can increase NO emissions and that the size is dependent on the total N and organic C content of the slurry and on soil type. In conclusion, the two types of BR and the CS are not interchangeable regarding their effects on NO production in different soils, and, hence, matching fertilizer type to soil type could reduce NO emissions. For instance, it could be advisable to avoid fertilization of organic soils with CS containing high amounts or organic C and instead use BR. In clay soil, however, the risk of NO emissions could be lowered by choosing a CS.

Microbial community structure across a wastewater-impacted riparian buffer zone in the southeastern coastal plain

Riparian buffer zones are important for both natural and developed ecosystems throughout the world because of their ability to retain nutrients, prevent soil erosion, protect aquatic environments from excessive sedimentation, and filter pollutants. Despite their importance, the microbial community structures of riparian buffer zones remains poorly defined. Our objectives for this study were twofold: first, to characterize the microbial populations found in riparian buffer zone soils; and second, to determine if microbial community structure could be linked to denitrification enzyme activity (DEA). To achieve these objectives, we investigated the microbial populations of a riparian buffer zone located downslope of a pasture irrigated with swine lagoon effluent, utilizing DNA sequencing of the 16S rDNA, DEA, and quantitative PCR (qPCR) of the denitrification genes nirK, nirS, and nosZ. Clone libraries of the 16S rDNA gene were generated from each of twelve sites across the riparian buffer with a total of 986 partial sequences grouped into 654 operational taxonomic units (OTUs). The Proteobacteria were the dominant group (49.8% of all OTUs), with the Acidobacteria also well represented (19.57% of all OTUs). Analysis of qPCR results identified spatial relationships between soil series, site location, and gene abundance, which could be used to infer both incomplete and total DEA rates.

Soil nitrogen dynamics in a river floodplain mosaic

In their natural state, river floodplains are heterogeneous and dynamic ecosystems that may retain and remove large quantities of nitrogen from surface waters. We compared the soil nitrogen dynamics in different types of habitat patches in a restored and a channelized section of a Thur River floodplain (northeast Switzerland). Our objective was to relate the spatiotemporal variability of selected nitrogen pools (ammonium, nitrate, microbial nitrogen), nitrogen transformations (mineralization, nitrification, denitrification), and gaseous nitrogen emission (NO) to soil properties and hydrological processes. Our study showed that soil water content and carbon availability, which depend on sedimentation and inundation dynamics, were the key factors controlling nitrogen pools and processes. High nitrogen turnover rates were measured on gravel bars, characterized by both frequent inundation and high sediment deposition rates, as well as in low-lying alluvial forest patches with a fine-textured, nutrient-rich soil where anaerobic microsites probably facilitated coupled nitrification-denitrification. In contrast, soils of the embankment in the channelized section had comparatively small inorganic nitrogen pools and low transformation rates, particularly those related to nitrate production. Environmental heterogeneity, characteristic of the restored section, favors nitrogen removal by creating sites of high sedimentation and denitrification. Of concern, however, are the locally high NO efflux and the possibility that nitrate could leach from nitrification hotspots.

Anammox bacterial abundance, activity, and contribution in riparian sediments of the Pearl River estuary

The hypothesis of an anammox hotspot in river riparian zones was put forward based on our investigation on freshwater ecotones for over 25 years and previous anammox research. Here we used a complementary array of methods including isotope-pairing technique, quantitative PCR assays, and 16S rRNA and hydrazine synthase gene (hzsB) clone libraries to document the spatiotemporal evidence for a high abundance zone of anammox bacteria in river riparian sediment with observed abundance of 1.3-12 × 10(6) (summer) and 1.4-20 × 10(8) (winter) hydrazine synthase gene copies g(-1), which is the highest abundance in natural environments recorded so far. Meanwhile high anammox bacterial biodiversity were detected with 'Brocadia' and 'Kuenenia' dominating. However, the high anammox bacterial abundances were not related with high activities and contributions for nitrogen gas generation. The anammox activities ranged from 0.07 to 0.15 nmol N cm(-3) h(-1) (summer) to 1.0-2.6 nmol N cm(-3) h(-1) (winter) with high temporal heterogeneity. The retrieval of archaeal and bacterial amoA sequences indicated that nitrifying microbes might be the major source of nitrite for anammox bacteria in winter, while in summer the anaerobic nitrate reduction is more likely the main source. On the basis of (15)N tracing technology, it was estimated that a total loss of 0.67-9.62 g N m(-2) yr(-1) is linked to anammox in the riparian zone while denitrification contributed 96.2-170.3 g N m(-2) yr(-1) in Pearl River riparian sediments.

Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in a temperate re-connected floodplain

The relative magnitudes of, and factors controlling, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were measured in the soil of a re-connected temperate floodplain divided into four different land management zones (grazing grassland, hay meadow, fritillary meadow and a buffer zone). Soil samples were collected from each zone to measure their respective potentials for nitrate attenuation using 15N both at the surface and at depth in the soil column and additional samples were collected to measure the lability of the organic carbon. Denitrification capacity ranged between 0.4 and 4.2 (μmol N g(-1) dry soil d(-1)) across the floodplain topsoil and DNRA capacity was an order of magnitude lower (0.01-0.71 μmol N g(-1) d(-1)). Land management practice had a significant effect on denitrification but no significant effects were apparent for DNRA. In this nitrogen-rich landscape, spatial heterogeneity in denitrification was explained by differences in lability and the magnitude of organic carbon associated with different management practices (mowing and grazing). The lability of organic carbon was significantly higher in grazing grassland in comparison to other ungrazed areas of the floodplain, and consequently denitrification capacity was also highest in this area. Our results indicate that bacteria capable of DNRA do survive in frequently flooded riparian zones, and to a limited extent, compete with denitrification for nitrate, acting to retain and recycle nitrogen in the floodplain. Exponential declines in both denitrification and DNRA capacity with depth in the floodplain soils of a hay meadow and buffer zone were controlled primarily by the organic carbon content of the soils. Furthermore, grazing could be employed in re-connected, temperate floodplains to enhance the potential for nitrate removal from floodwaters via denitrification.

Decreased N2O reduction by low soil pH causes high N2O emissions in a riparian ecosystem

Quantification of harmful nitrous oxide (N(2)O) emissions from soils is essential for mitigation measures. An important N(2)O producing and reducing process in soils is denitrification, which shows deceased rates at low pH. No clear relationship between N(2)O emissions and soil pH has yet been established because also the relative contribution of N(2)O as the denitrification end product decreases with pH. Our aim was to show the net effect of soil pH on N(2)O production and emission. Therefore, experiments were designed to investigate the effects of pH on NO(3)(-) reduction, N(2)O production and reduction and N(2) production in incubations with pH values set between 4 and 7. Furthermore, field measurements of soil pH and N(2)O emissions were carried out. In incubations, NO(3)(-) reduction and N(2) production rates increased with pH and net N(2)O production rate was highest at pH 5. N(2)O reduction to N(2) was halted until NO(3)(-) was depleted at low pH values, resulting in a built up of N(2)O. As a consequence, N(2)O:N(2) production ratio decreased exponentially with pH. N(2)O reduction appeared therefore more important than N(2)O production in explaining net N(2)O production rates. In the field, a negative exponential relationship for soil pH against N(2)O emissions was observed. Soil pH could therefore be used as a predictive tool for average N(2)O emissions in the studied ecosystem. The occurrence of low pH spots may explain N(2)O emission hotspot occurrence. Future studies should focus on the mechanism behind small scale soil pH variability and the effect of manipulating the pH of soils.

N2O emission hotspots at different spatial scales and governing factors for small scale hotspots

Chronically nitrate-loaded riparian buffer zones show high N(2)O emissions. Often, a large part of the N(2)O is emitted from small surface areas, resulting in high spatial variability in these buffer zones. These small surface areas with high N(2)O emissions (hotspots) need to be investigated to generate knowledge on the factors governing N(2)O emissions. In this study the N(2)O emission variability was investigated at different spatial scales. Therefore N(2)O emissions from three 32 m(2) grids were determined in summer and winter. Spatial variation and total emission were determined on three different scales (0.3 m(2), 0.018 m(2) and 0.0013 m(2)) at plots with different levels of N(2)O emissions. Spatial variation was high at all scales determined and highest at the smallest scale. To test possible factors inducing small scale hotspots, soil samples were collected for slurry incubation to determine responses to increased electron donor/acceptor availability. Acetate addition did increase N(2)O production, but nitrate addition failed to increase total denitrification or net N(2)O production. N(2)O production was similar in all soil slurries, independent of their origin from high or low emission soils, indicating that environmental conditions (including physical factors like gas diffusion) rather than microbial community composition governed N(2)O emission rates.

Nitrous oxide accumulation in soils from riparian buffers of a coastal plain watershed carbon/nitrogen ratio control

Riparian buffers are used throughout the world for the protection of water bodies from nonpoint-source nitrogen pollution. Few studies of riparian or treatment wetland denitrification consider the production of nitrous oxide (N2O). The objectives of this research were to ascertain the level of potential N2O production in riparian buffers and identify controlling factors for N2O accumulations within riparian soils of an agricultural watershed in the southeastern Coastal Plain of the USA. Soil samples were obtained from ten sites (site types) with different agronomic management and landscape position. Denitrification enzyme activity (DEA) was measured by the acetylene inhibition method. Nitrous oxide accumulations were measured after incubation with and without acetylene (baseline N2O production). The mean DEA (with acetylene) was 59 microg N2O-N kg(-1) soil h(-1) for all soil samples from the watershed. If no acetylene was added to block conversion of N2O to N2, only 15 microg N2O-N kg(-1) soil h(-1) were accumulated. Half of the samples accumulated no N2O. The highest level of denitrification was found in the soil surface layers and in buffers impacted by either livestock waste or nitrogen from legume production. Nitrous oxide accumulations (with acetylene inhibition) were correlated to soil nitrogen (r2=0.59). Without acetylene inhibition, correlations with soil and site characteristics were lower. Nitrous oxide accumulations were found to be essentially zero, if the soil C/N ratios>25. Soil C/N ratios may be an easily measured and widely applicable parameter for identification of potential hot spots of N2O productions from riparian buffers.