Denitrification Rates Research Articles

Responses of nitrogen cycling and related microorganisms to brackish wetlands formed by evapotranspiration

Elevated evapotranspiration due to warmer air temperature could raise salinity and nutrient levels of some inland wetlands, potentially impacting nitrogen cycling. To characterize the impact of high evapotranspiration on soil microbial nitrogen cycling in inland wetlands, we compared freshwater and brackish marsh (or non-marsh) wetlands in terms of sediment ammonia-oxidizing rate (AOR), denitrifying rate (DR), and related microbial communities in a typical inland basin, the Hulun Lake basin, in China. Results showed that marsh ecosystems (ME) exhibited 31% higher AOR and 65% higher DR than non-marsh ecosystems (NE). For NE, freshwater non-marsh wetland exhibited 12% higher AOR than brackish non-marsh wetland. This was probably due to the inhibitory effects of high NH4+ and salinity levels on ammonia-oxidizing archaea in brackish non-marsh wetland. Conversely, DR in brackish non-marsh wetland was 23% higher than that in freshwater non-marsh wetland, with total organic carbon (TOC) significantly influencing this difference, suggesting that the higher DR in brackish non-marsh wetland was mainly due to its higher TOC level. For ME, due to the direct and indirect interference of salinity, brackish marsh wetland displayed 26% lower AOR and 19% lower DR than freshwater marsh wetland. Besides, brackish wetlands harbored distinct ammonia-oxidizing and denitrifying microbial communities compared to freshwater wetlands. The assembly of these communities was dominated by stochastic processes, while brackish wetlands exhibited more prominent deterministic processes than freshwater wetlands. Overall, high evapotranspiration altered activities and community characteristics of ammonia oxidizers and denitrifiers in inland brackish wetlands by enhancing salinity and nutrient levels, while emergent plants occurring in ME could mitigate the adverse effects of salt stress of inland brackish wetlands on nitrogen cycling.

Impact of reservoir operation policies on nitrogen cycling processes and water quality dynamics in a large water supply reservoir

Reservoir operation policies affect reservoir nitrogen (N) cycling and budgets through complex influences on physical and biogeochemical processes in water and sediment. However, the underlying influencing mechanisms, especially the impact of operations on sediment processes, remain unclear. This study combines a coupled hydrodynamic-eutrophication-sediment model and a reservoir operation model to clarify reservoir operation impact on N cycling processes in water and sediment as well as impact on stored and discharged water quality. Three typical reservoir operation policies (i.e., standard, hedging, and eco-friendly) were selected to create operation scenarios. The hydrodynamic-eutrophication-sediment model was used to simulate N cycling processes and N concentrations in stored and discharged water under each scenario. Using China's Danjiangkou Reservoir as a case study, results revealed that reservoir operation policies significantly affected spatiotemporal dynamics of N cycling processes and water quality. Specifically, reservoir sediment acted as a N source, contributing to annual net release of 9.5–16.1 Gg N. Higher water levels promoted sediment ammonium release through effects on reservoir submerged area and sediment temperature. Moreover, denitrification processes in water and sediment collectively contributed to annual N removal of 14.0–16.9 Gg N. Annual N removal was greater in the higher water level scenario despite its lower denitrification rate per unit area. Additionally, these processes and hydrodynamic conditions combine to affect N forms and concentrations. The N concentration in the lower water level scenario was generally higher, risking algal bloom outbreaks in the shallow-water zone. This study is intended to help guide reservoir operation policymaking to account for N management while also coordinating water quantity and quality targets.

Nitrate attenuation potential in karst conduits and aquifer matrix

A critical determinant of the effective denitrification potential in karst aquifers is the relative contribution of matrix and non-matrix (e.g., conduits) to the total groundwater flow. This work tests the hypothesis that observed karst aquifer-scale denitrification rates represent the superposition of the effects from matrix zones, which have low denitrification rates and long residence times, and non-matrix zones, which have higher denitrification rates and short residence times. To better evaluate karst whole-aquifer denitrification capacity, this study examined groundwater chemical data from 69 individual wells across the springshed of Silver Springs, FL, and conducted five push–pull tracer tests (PPTTs) in matrix and non-matrix aquifer sections. Mean residence times and mean zero-order denitrification rates (K0) were determined using reduced-complexity analytical models for matrix and non-matrix zones. Significant nitrate degradation at the local scale was observed in wells that sampled matrix portions of the aquifer (K0 ≈ 5 to 7 × 103 μmol N/L/d), indicating denitrification hotspots comparable to previous results employing the same PPTT method. However, PPTTs in matrix portions of the aquifer found no evidence of aquifer denitrification, likely due to the short residence times experienced during PPTTs. The analytical models coupled with the measured nitrate concentrations and excess N2 data in groundwater discharged at the spring vents showed whole-aquifer estimates of residence times tavg = 4.2 years and aquifer denitrification rate K0,avg = 0.01 ± 0.01 μmol N/L/d that were consistent with previous studies. However, the average denitrification rate for non-matrix zones K0,nm = 1.29 ± 0.92 μmol N/L/d was two orders of magnitude higher than that for matrix zones K0,m = 0.002 ± 0.001 μmol N/L/d. Whole-aquifer denitrification measures thus reflect the weighted contributions of low denitrification in non-matrix zones, which constitute the majority of the groundwater flow, and high-denitrification but low-flow matrix zones. The modeling approach with non-matrix networks from this study can provide insight into the catchment-scale N budget and transport in karst aquifers.

Remote Sensing Soil Freeze‐Thaw Status and North American N2O Emissions From a Regional Inversion

AbstractNorth American nitrous oxide (N2O) emissions over 2011–2018 are estimated using the CarbonTracker‐Lagrange regional inversion framework. Emissions are strongest in the Midwestern corn/soybean belt and display a distinct dual maxima seasonal pattern. The first maximum occurs in late winter/early spring, suggestive of freeze‐thaw (FT) effects on denitrification rates and associated N2O emissions. The second maximum occurs in late spring/early summer, consistent with a growing season nitrogen fertilizer‐driven source, although fertilizer applied in late fall may contribute to the FT pulse as well. Interannual variability in the first maximum correlates significantly to soil freeze thaw status derived from remote sensing data. A requisite frozen period in the preceding early winter appears necessary to create conditions for the N2O pulse after thawing. The FT pulse is a prominent feature of the annual cycle in Canadian cropland, where it may be of comparable magnitude to growing season emissions. In contrast, the growing season peak in N2O dominates the FT peak in the Midwestern Corn‐Soybean region of the United States.

Enhanced nitrogen removal in immersed rotating self-aerated biofilm reactor: Nitrogen removal pathway and microbial mechanism

To achieve energy-efficient treatment of the rural wastewater with satisfying performance, a novel immersed rotating self-aerated biofilm reactor (iRSABR) was proposed in this study. The iRSABR system showed better biofilm renewal performance and higher microbial activity. The effect of different regulation strategies on the iRSABR system was investigated in this study. The 70% immersion ratio and 4 r/min rotation speed (stage III) exhibited the best performance, with a total nitrogen removal efficiency of 86% and a simultaneous nitrification–denitrification (SND) rate of 76%, along with the highest electron transport system activity. The nitrogen removal pathway revealed that the SND was achieved through autotrophic/heterotrophic nitrification and aerobic/anoxic denitrification. The regulation strategy in the iRSABR system established a synergistic microbial community with main functional bacteria of nitrification (Nitrosomonas), anoxic denitrification (Flavobacterium, Pseudoxanthomonas), and aerobic denitrification (Thauera). This study highlighted the feasibility and adaptability of the iRSABR system toward energy-efficient rural wastewater treatment.

Drought-Induced Salinity Intrusion Affects Nitrogen Removal in a Deltaic Ecosystem (Po River Delta, Northern Italy)

In the summer of 2022, the Po River Delta (Northern Italy), a eutrophication hotspot, was severely affected by high temperatures, exceptional lack of rainfall and saline water intrusion. The effect of saline intrusion on benthic nitrogen dynamics, and in particular the N removal capacity, was investigated during extreme drought conditions. Laboratory incubations of intact sediment cores were used to determine denitrification and DNRA rates at three sites along a salinity gradient in the Po di Goro, an arm of the Po River Delta. Denitrification was found to be the main process responsible for nitrate reduction in freshwater and slightly saline sites, whereas DNRA predominated in the most saline site, highlighting a switch in N cycling between removal and recycling. These results provide evidence that salinity is a key factor in regulating benthic N metabolism in transitional environments. In a climate change scenario, salinity intrusion, resulting from long periods of low river discharge, may become an unrecognized driver of coastal eutrophication by promoting the dissimilatory nitrate reduction to ammonium and N recycling of bioactive nitrogen within the ecosystem, rather than its permanent removal by denitrification.

Long-term effects of Ag NPs on denitrification in sediment: Importance of Ag NPs exposure ways in aquatic ecosystems

The widespread use of silver nanoparticles (Ag NPs) inevitably leads to their increasing release into aquatic systems, with studies indicating that the mode of Ag NPs entry into water significantly affects their toxicity and ecological risks. However, there is a lack of research on the impact of different exposure ways of Ag NPs on functional bacteria in sediment. This study investigates the long-term influence of Ag NPs on denitrification process in sediments by comparing denitrifies responses to single (pulse injection of 10 mg/L) and repetitive (1 mg/L × 10 times) Ag NPs treatments over 60-day incubation. Results showed that a single exposure of 10 mg/L Ag NPs caused an obvious toxicity on activity and abundance of denitrifying bacteria on the first 30 days, reflecting by the decreased NADH amount, ETS activity, NIR and NOS activity, and nirK gene copy number, which resulted in a significant decline of denitrification rate in sediments (from 0.59 to 0.64 to 0.41–0.47 μmol15N L−1 h−1). While inhibition was mitigated with time and denitrification process recovered to the normal at the end of the experiment, the accumulated nitrate generated in the system showed that the recovery of microbial function did not mean the restoration of aquatic ecosystem after pollution. Differently, the repetitive exposure of 1 mg/L Ag NPs exhibited the evident inhibition on metabolism, abundance, and function of denitrifiers on Day 60, due to the accumulated amount of Ag NPs with the increased dosing number, indicating that the accumulated toxicity on functional microorganic community of repetitive exposure in less toxic concentration. Our study highlights the importance of Ag NPs entry pathways into aquatic ecosystem on their ecological risks, which affected dynamic responses of microbial function to Ag NPs.

Process optimization of a moving bed bioreactor undergoing simultaneous nitrification and denitrification for wastewater in the absence of organic carbon

ABSTRACT Moving bed bioreactors are efficient for the removal of nitrogen effluents by simultaneous nitrification and denitrification (SND), which is particularly challenging for effluents with low C/N ratio due to insufficient carbon available for denitrification. The addition of external organic carbon in such cases increases treatment costs. Endogenous degradation of biomass offers a cost-effective and persistent source of carbon. Internal carbon leads to low denitrification rates and optimizing the biomass content leads to a sustainable solution for treating such effluents. The present study aims to optimize the process of SND using endogenous carbon by investigating the effect of operational parameters like dissolved oxygen, influent ammonium (NH4–N) loading, hydraulic retention time (HRT) and carrier filling ratio (CFR). The synthetic wastewater used in the study does not inherently contain any organic carbon, thus indicating the responsible carbon for denitrification to be of internal origin. Using a novel carrier for multispecies biofilm development, initial NH4–N concentration variation showed an insignificant effect on SND efficiency. At higher loading, nitrite accumulation due to incomplete denitrification was observed. The effect of CFR was investigated in a range of 20–35%, beyond which non-homogeneous mixing was caused in the reactor. Under controlled DO and temperature of about 5 mg/L and 28 ± 2°C, respectively, an optimum filling ratio of 35% was finalised, resulting in an SND efficiency of 97.9% at 22 h HRT for high influent NH4–N concentration of 500 mg/L. Bacteriological species identification revealed the coexistence of nitrogen-fixing, aerobic nitrifying and anaerobic denitrifying bacteria in the biofilm growing on the biocarrier. Mechanism of SND occurring in aerobic moving bed bioreactors: Laboratory-scale aerobic Moving Bed Bioreactor setup for a continuous study evaluating the influence of operational parameters on SND Occurrence of aerobic and anoxic zones in biofilm grown on the surface of novel carriers due to diffusional gradient of dissolved oxygen

Stream restoration can reduce nitrate levels in agricultural landscapes

The EU Water Framework Directive (WFD) has emphasized that altered stream/river morphology and diffuse pollution are the two major pressures faced by European water bodies at catchment scales. Increasing efforts have been directed toward restoration to meet WFD standards for ecological health, but this work has achieved limited success. One challenge is that little is known about how morphological changes (i.e., re-meandering) may affect nitrate retention within whole stream networks. We investigated this issue in the well-monitored Bode catchment (3200 km2) in central Germany. First, we implemented a fully distributed process-based mHM-Nitrate model, exploring its performance over the period from 2015 to 2018. Second, we simulated the effects of restoring more natural stream morphology (i.e., increasing sinuosity) on nitrate retention. The mHM-Nitrate model performed well in replicating daily discharge and nitrate concentrations (median Kling-Gupta values of 0.78 and 0.74, respectively). Within the stream network, mean and standard deviation (SD) of gross nitrate retention efficiency was 5.1 ± 0.61 % and 74.7 ± 23.2 % in the winter and summer, respectively; this measure took into account both denitrification and assimilatory uptake. In the summer, the denitrification rate was about twice as high in a lowland sub-catchment dominated by agricultural lands as in a mountainous sub-catchment dominated by forested areas (median ± SD of 204 ± 22.6 and 102 ± 22.1 mg N m−2 d−1, respectively). Similarly, in the same season, the assimilatory uptake rate was approximately five times higher in streams surrounded by lowland agricultural areas than in streams in higher-elevation, forested areas (median ± SD of 200 ± 27.1 and 39.1 ± 8.7 mg N m−2 d−1, respectively). This suggests that restoration strategies targeted at lowland agricultural areas may have a greater potential for increasing nitrate retention. In our simulation, restoring stream sinuosity was found to increase net nitrate retention efficiency by up to 25.4 ± 5.3 %; greater effects were seen in small streams. Taken together, our results indicate that restoration efforts should consider augmenting stream sinuosity to increase nitrate retention and decrease nitrate concentrations at the catchment scale.

Chemolithoautotrophic denitrification intensifies nitrogen loss in the Eastern Arabian Sea Shelf waters during sulphidic events

The Eastern Arabian Sea Shelf i.e. Western Indian Continental Shelf (WICS) – a known biogeochemical hotspot is characterized by monsoonal upwelling, seasonal O2 deficiency, extremely high N2O build-up and sulphidic events. The frequency and duration of the sulphidic events have increased over the last two decades, but their impact on the pelagic N cycling, N budget, and N2O dynamics is poorly constrained. Thus, to address these problems and assess their implications on WICS biogeochemistry, we carried out physico-chemical measurements, 15N-labeled incubations and bag incubations on five transects over the shelf during the sulphidic event (September-October) of 2011. We observed very high rates of sulphide-driven chemolithotrophic denitrification (1885–5825 nM N2 d−1) in the sulphidic, nitrate-depleted waters, and its potential occurrence in the sulphide-free, nitrate-replete waters (460–3137 nM N2 d−1), along with high transient N2O production, and comparably low rates of anammox (0–119 nM N2 d−1) and DNRA (0–45 nM N d−1). Despite the predominant cloud cover during the monsoon season, we could for the first time show the satellite image of a large colloidal sulphur (S0) plume associated with the sulphidic event off Western India providing further evidence of extensive sulphide oxidation coupled to denitrification. Sulphide-driven denitrification (mean rate = 2697 nM N2 d−1) appeared to be the dominant N loss process during the anoxic regime (September-October) replacing the chemoorganotrophic (i.e. heterotrophic) denitrification (342 nM N2 d−1) that predominates during the preceding suboxic regime (July-August). Overall, the highest sulphide-driven denitrification rate over the WICS was found to be the second highest among the anoxic coastal systems of the world. Furthermore, simultaneous consumption of NOx− and S2− at a ratio close to the theoretical value in the anaerobic incubations of chemocline waters indicated that the sulphide-driven denitrifiers were fixing carbon. The estimated dark C production (0.21 g C m−2 d−1) due to chemolithoautotrophic denitrification was 18% of the photoautotrophic production and accounted for 15% of the total column productivity. Based on our conservative estimates, the chemolithoautotrophic denitrification was responsible for the removal of 0.4 Tg of fixed N and 0.57 Tg of sulphide, and fixation of 0.1 Tg of carbon annually in the shelf waters. Thus, the sulphidic event impacted the biogeochemistry and ecology, and modulated the N loss pathways and rates over the WICS. With the expansion and intensification of OMZs induced by global climate change, and the spreading of dead zones due to increasing anthropogenic activities, chemolithoautotrophic denitrification is likely to become increasingly significant in oceanic N cycle and impact the N budgets of shallow marine systems particularly.

Hydrodynamic controls on nitrogen distribution and removal in aquatic ecosystems

The impact of nitrogen (N) on water eutrophication is well-known, but the specific influence of hydrodynamic factors on N occurrence in aquatic systems has remained unclear. This lack of understanding has hindered our ability to assess the self-purification function of aquatic ecosystems and address water pollution problem. Here, we collected overlying water and sediment samples from different aquatic ecosystems (ditch, pond, river, and reservoir) in the Danjiangkou Reservoir area and compared the variation characteristics of various N components, and further conducted an incubation experiment to investigate the rate of N removal. We found that the concentration of total N and its N components decreased from ditches and ponds to rivers and reservoirs, indicating that N removal occurred during water flow, with up to 43% of total N concentration reduction rate. Additionally, we observed higher heterogeneity in eco-stoichiometric characteristics of N components in ditches and ponds compared to rivers and reservoirs. Interestingly, the ditches and ponds exhibited stronger interactions between overlying water and sediment, with higher rates of denitrification and anaerobic ammonium oxidation (anammox). Our findings highlight the need to focus on the upper reaches of agricultural catchments, such as ditches and ponds, for N removal and emphasize the importance of developing region-specific conservation strategies to mitigate N pollution and protect water resources.

Microbe-mediated simultaneous nitrogen reduction and sulfamethoxazole/N-acetylsulfamethoxazole removal in lab-scale constructed wetlands

Constructed wetlands (CWs) are increasingly used to treat complex pollution such as nitrogen and emerging organic micropollutants from anthropogenic sources. In this study, the denitrification, anaerobic ammonium oxidation, dissimilatory nitrate reduction to ammonium, and nitrous oxide release rates following exposure to the frequently detected sulfonamides sulfamethoxazole (SMX) and its human metabolite, N-acetylsulfamethoxazole (N-SMX), were investigated in lab-scale CWs. Over a period of 190 d, the denitrification rates were noticeably inhibited in the SMX and N-SMX groups at week 5. Subsequently, the denitrification rates recovered, accompanied by an increase in the relevant nitrogen reduction and antibiotic resistance genes (ARGs). The composition of the microbial community also changed during this process. After the denitrification rates recovered, Burkholderia_Paraburkholderia and Gordonia exhibited a significant positive correlation with SMX exposure, which simultaneously reduced nitrate concentrations and degraded antibiotics. Burkholderia_Paraburkholderia is a key carrier of ARGs. Finally, nitrogen reduction (> 90%) and antibiotic removal (> 80%) also recovered in both SMX- and N-SMX-exposed lab-scale CWs during the operation, which revealed the interaction of SMX or N-SMX removal and nitrogen reduction.

Chemical identity of carbon substrates drives differences in denitrification and N2O reduction within agricultural soils

Rates of nitrous oxide (N2O) production from agricultural soils are highly variable across space and time. Improving predictions of N2O emissions will require improving our understanding of the drivers of denitrification and the sources of variability in the rates of N2O production between soils and over time. While the amount of available carbon (C) is a known control on denitrification and N2O reduction, relatively little attention has been paid to the effect of the chemical identity of C substrates on rates of denitrification and N2O reduction. We investigated the effects of twelve different C-substrate additions on the production and reduction of N2O in five soils taken from two distinct agricultural locations in Michigan under multiple land uses. We provided additions of glucose, cellulose, N-acetyl-glucosamine, chitin, amino acids, protein, vanillyl alcohol, lignin, citrate, succinate, methanol, and water in laboratory denitrification potential assays to determine the effects of denitrifier C preference on denitrification rates. We found that amino acids, protein, and organic acids stimulated the greatest rates of denitrification potential across all land uses. Similarly, we found these same substrates caused the most N2O reduction, resulting in the lowest net concentrations of N2O. Soils from agricultural rotations without cover crops had overall lower rates of denitrifier activity, leading to less net N2O production compared to soils from other land uses. In general, C-utilization patterns were similar among all soils, and C-substrate identity had a much stronger effect than land use. Here, we demonstrate that the chemical identity of available C gives rise to wide variability in rates of denitrification and N2O reduction.

Autonomous observations of biogenic N2 in the Eastern Tropical North Pacific using profiling floats equipped with gas tension devices

Oxygen Deficient Zones (ODZs) of the world’s oceans represent a relatively small fraction of the ocean by volume (<0.05% for suboxic and<5% for hypoxic) yet are receiving increased attention by experimentalists and modelers due to their importance in ocean nutrient cycling and predicted susceptibility to expansion and/or contraction forced by global warming. Conventional methods to study these biogeochemically important regions of the ocean have relied on well-developed but still relatively high cost and labor-intensive shipboard methods that include mass-spectrometric analysis of nitrogen-to-argon ratios (N2/Ar) and nutrient stoichiometry (relative abundance of nitrate, nitrite, and phosphate). Experimental studies of denitrification rates and processes typically involve either in-situ or in-vitro incubations using isotopically labeled nutrients. Over the last several years we have been developing a Gas Tension Device (GTD) to study ODZ denitrification including deployment in the largest ODZ, the Eastern Tropical North Pacific (ETNP). The GTD measures total dissolved gas pressure from which dissolved N2 concentration is calculated. Data from two cruises passing through the core of the ETNP near 17 °N in late 2020 and 2021 are presented, with additional comparisons at 12 °N for GTDs mounted on a rosette/CTD as well as modified profiling Argo-style floats. Gas tension was measured on the float with an accuracy of< 0.1% and relatively low precision (< 0.12%) when shallow (P< 200 dbar) and high precision (< 0.03%) when deep (P > 300 dbar). We discriminate biologically produced N2 (ie., denitrification) from N2 in excess of saturation due to physical processes (e.g., mixing) using a new tracer – ‘preformed excess-N2’. We used inert dissolved argon (Ar) to help test the assumption that preformed excess-N2 is indeed conservative. We used the shipboard measurements to quantify preformed excess-N2 by cross-calibrating the gas tension method to the nutrient-deficit method. At 17 °N preformed excess-N2 decreased from approximately 28 to 12 µmol/kg over σ0 = 24–27 kg/m3 with a resulting precision of ±1 µmol N2/kg; at 12 °N values were similar except in the potential density range of 25.7< σ0< 26.3 where they were lower by 1 µmol N2/kg due likely to being composed of different source waters. We then applied these results to gas tension and O2 (< 3 µmol O2/kg) profiles measured by the nearby float to obtain the first autonomous biogenic N2 profile in the open ocean with an RMSE of ± 0.78 µM N2, or ± 19%. We also assessed the potential of the method to measure denitrification rates directly from the accumulation of biogenic N2 during the float drifts between profiling. The results suggest biogenic N2 rates of ±20 nM N2/day could be detected over >16 days (positive rates would indicate denitrification processes whereas negative rates would indicate predominantly dilution by mixing). These new observations demonstrate the potential of the gas tension method to determine biogenic N2 accurately and precisely in future studies of ODZs.

Enhanced electron recovery by optimizing sandwich structure agricultural waste corncob filled anode in microbial electrochemical system to facilitate wastewater denitrification

Microbial electrochemical system autotrophic denitrification has attracted much attention due to its cost-efficiency and clean advantages. The autotrophic denitrification rate highly depends on the input electrons to the cathode. In this study, agricultural waste corncob was filled into sandwich structure anode as low-cost carbon source for electron production. The COMSOL software was used to guide the construction of sandwich structure anode to control carbon source release and enhance electron collection, including suitable pore size (4 mm) and current collector arrangement (five branches). Optimized sandwich structure anode system with the help of 3D printing obtained a higher denitrification efficiency (21.79 ± 0.22 gNO3--N/m3d) than anodic systems without pore and current collector. Statistical analysis showed that enhanced autotrophic denitrification efficiency was the responsible for enhanced denitrification performance of the optimized anode system. This study provides a strategy to improve the autotrophic denitrification performance of the microbial electrochemical system by optimizing the anode structure.

Climate change effects on denitrification performance of woodchip bioreactors treating agricultural tile drainage

Denitrifying woodchip bioreactors (WBRs) are a nature-based technology that are increasingly used to control nonpoint source nitrate (NO3−) pollution in agricultural catchments. The treatment effectiveness of WBRs depends on temperature and hydraulic retention time (HRT), both of which are affected by climate change. Warmer temperatures will increase microbial denitrification rates, but the extent to which the resulting benefits to treatment performance may be offset by intensified precipitation and shorter HRTs is not clear. Here, we use three years of monitoring data from a WBR in Central New York State to train an integrated hydrologic-biokinetic model describing links among temperature, precipitation, bioreactor discharge, denitrification kinetics, and NO3− removal efficiencies. Effects of climate warming are assessed by first training a stochastic weather generator with eleven years of weather data from our field site, and then adjusting the distribution of precipitation intensities according to the Clausius-Clapeyron relationship between water vapor and temperature. Modeling results indicate, in our system, faster denitrification rates will outweigh the influence of intensified precipitation and discharge under warming, leading to net improvements in NO3− load reductions. Median cumulative NO3− load reductions at our study site from May - October are projected to increase from 21.7% (interquartile range 17.4%–26.1%) under baseline hydro-climate to 41.0% (interquartile range 32.6–47.1%) with a + 4 °C change in mean air temperature. This improved performance under climate warming is driven by strong nonlinear dependence of NO3− removal rates on temperature. Temperature sensitivity may increase with woodchip age and lead to stronger temperature-response in systems like this one with a highly aged woodchip matrix. While the impacts of hydro-climatic change on WBR performance will depend on site-specific properties, this hydrologic-biokinetic modeling approach provides a framework for assessing climate impacts on the effectiveness of WBRs and other denitrifying nature-based systems.

Feasibility and optimization of a novel upflow denitrification reactor using denitrifying granular sludge for nitric acid pickling wastewater treatment

Stainless steel is highly valued for its superior resistance to corrosion. However, the pickling process involved in stainless steel production generates abundant NO3−-N, causing health and environmental risks. To address this issue, this study proposed a novel solution utilizing an up-flow denitrification reactor and denitrifying granular sludge for treating NO3–-N pickling wastewater under high NO3–-N loading. It was found that, the denitrifying granular sludge exhibited stable denitrification performance with the highest denitrification rate of 2.79 gN/(gVSS·d) and average removal rates of NO3−-N and TN of 99.94% and 99.31%, respectively, under optimal operating conditions of pH 6–9, temperature 35 °C, C/N ratio 3.5, hydraulic retention time (HRT) 11.1 h and ascending flow rate 2.75 m/h. This process reduced carbon source usage by 12.5–41.7% as compared to traditional denitrification methods. These findings demonstrate the efficacy of combining granular sludge and an up-flow denitrification reactor for treating nitric acid pickling wastewater.

Soil N2O emissions and functional genes in response to grazing grassland with livestock: A meta-analysis

Livestock grazing affects nitrous oxide (N2O) emissions from grassland ecosystems by altering soil physical, chemical and biological properties. However, how soil N2O emissions related to nitrogen process rates and functional genes are unexplored. We compiled 83 published studies of soil N2O emissions, potential nitrification and denitrification rates, and the abundance of nitrogen functional genes to uncover their associations with varying intensities of livestock grazing. Compared to ungrazed condition, heavy and moderate grazing reduced N2O emissions by 22–25%, nitrification rate by 23–37%, and denitrification rate by 44–48%, respectively, while light grazing had no effect. Furthermore, moderate to heavy grazing intensities decreased the abundances of ammonia-oxidizing bacteria ammonia monooxygenase (AOB amoA) by 40–47%. Heavy grazing also simultaneously decreased ammonia-oxidizing archaea (AOA amoA) by 43%. Additionally, grazing significantly decreased the abundance of nitrate reductase (narG) and nitrite reductase (nirS) and by 28% and 35%, respectively, but did not affect the abundance of nitrous oxide reductase (nosZ). Overall, potential nitrification rate was positively correlated with AOB amoA and AOA amoA abundances. This global-scale assessment demonstrates that moderate to heavy livestock grazing can reduce grassland N2O emissions, and such reductions were linked to decreased abundances of amoA genes with decreasing soil moisture and inorganic N (NO3– and NH4+) availabilities. Considering that heavy grazing may increase the risk of grassland degradation, we recommend that livestock grazing at an appropriately moderate intensity is important for sustaining livestock production while contributing to greenhouse gas mitigation.

Long-Term Pollution Does Not Inhibit Denitrification and DNRA by Adapted Benthic Microbial Communities

Denitrification in sediments is a key microbial process that removes excess fixed nitrogen, while dissimilatory nitrate reduction to ammonium (DNRA) converts nitrate to ammonium. Although microorganisms are responsible for essential nitrogen (N) cycling, it is not yet fully understood how these microbially mediated processes respond to toxic hydrophobic organic compounds (HOCs) and metals. In this study, we sampled long-term polluted sediment from the outer harbor of Oskarshamn (Baltic Sea), measured denitrification and DNRA rates, and analyzed taxonomic structure and N-cycling genes of microbial communities using metagenomics. Results showed that denitrification and DNRA rates were within the range of a national reference site and other unpolluted sites in the Baltic Sea, indicating that long-term pollution did not significantly affect these processes. Furthermore, our results indicate an adaptation to metal pollution by the N-cycling microbial community. These findings suggest that denitrification and DNRA rates are affected more by eutrophication and organic enrichment than by historic pollution of metals and organic contaminants.

Quantitative study of biofilm yield and biomass distribution of a multi-stage moving-bed biofilm system

Abstract A multi-stage anoxic/oxic (A/O) moving-bed biofilm reactor (MBBR) system with multiple chambers was established for municipal wastewater treatment. The active biomass quantity, bioactivity, and biomass yield of a pilot-scale multi-stage MBBR were investigated in this study. The microbial activity and heterotrophic yield coefficients (YH) were measured using respirometric techniques in each chamber at different temperature conditions. Meanwhile, the growth, nitrification, and denitrification rates of functional biomass were also quantified as specific respiration rate (SOUR). The total active biomass in the multi-stage A/O-MBBR system was 0.71–1.68 g COD/m2 for the aerobic reactor and 0.39–1.44 g COD/m2 for the anoxic reactor at 10–19 °C. The YH values for the anoxic reactors were 0.61–0.69, which were comparable to the recommended value of the activated sludge model (ASM1). The correlation coefficient between Nitrospira and the autotrophic specific respiration rate (SOURA) was 0.82. Meanwhile, denitrifying genera showed a significant correlation with the heterotrophic specific respiration rate (SOURH) and the active heterotrophic biomass (XH). This study provided insights into biomass distribution and the corresponding kinetic parameters for the multi-stage MBBR systems, which may serve as a reference for process design and trouble shooting.