Potential Denitrification Rates Research Articles

Influence of Four Veterinary Antibiotics on Constructed Treatment Wetland Nitrogen Transformation.

The use of wetlands as a treatment approach for nitrogen in runoff is a common practice in agroecosystems. However, nitrate is not the sole constituent present in agricultural runoff and other biologically active contaminants have the potential to affect nitrate removal efficiency. In this study, the impacts of the combined effects of four common veterinary antibiotics (chlortetracycline, sulfamethazine, lincomycin, monensin) on nitrate-N treatment efficiency in saturated sediments and wetlands were evaluated in a coupled microcosm/mesocosm scale experiment. Veterinary antibiotics were hypothesized to significantly impact nitrogen speciation (e.g., nitrate and ammonium) and nitrogen uptake and transformation processes (e.g., plant uptake and denitrification) within the wetland ecosystems. To test this hypothesis, the coupled study had three objectives: 1. assess veterinary antibiotic impact on nitrogen cycle processes in wetland sediments using microcosm incubations, 2. measure nitrate-N reduction in water of floating treatment wetland systems over time following the introduction of veterinary antibiotic residues, and 3. identify the fate of veterinary antibiotics in floating treatment wetlands using mesocosms. Microcosms containing added mixtures of the veterinary antibiotics had little to no effect at lower concentrations but stimulated denitrification potential rates at higher concentrations. Based on observed changes in the nitrogen loss in the microcosm experiments, floating treatment wetland mesocosms were enriched with 1000 μg L-1 of the antibiotic mixture. Rates of nitrate-N loss observed in mesocosms with the veterinary antibiotic enrichment were consistent with the microcosm experiments in that denitrification was not inhibited, even at the high dosage. In the mesocosm experiments, average nitrate-N removal rates were not found to be impacted by the veterinary antibiotics. Further, veterinary antibiotics were primarily found in the roots of the floating treatment wetland biomass, accumulating approximately 190 mg m-2 of the antibiotic mixture. These findings provide new insight into the impact that veterinary antibiotic mixtures may have on nutrient management strategies for large-scale agricultural operations and the potential for veterinary antibiotic removal in these wetlands.

Wheat straw and microbial inoculants have an additive effect on N2O emissions by changing microbial functional groups

AbstractStraw‐decomposing microbial inoculants (MIs) have been increasingly applied to straw‐amended soils. However, the interactive effects and underlying microbial mechanisms of straw and decomposing MIs on nitrous oxide (N2O) emissions remain unclear. Here, a pot experiment with an Aquic Inceptisol was conducted to determine soil N2O emissions and the abundance and composition of microbial functional genes under six amendments: wheat straw (W), Bacillus subtilis MI (B), Streptomyces rochei MI (S), a combination of straw and B. subtilis MI (WB), a combination of straw and S. rochei MI (WS), and a control without wheat straw or MI (C). Compared with the control, the amendments of straw, decomposing MIs, and their combinations decreased soil N2O emissions by 43%, 22–30%, and 46–60%, respectively. Mechanistically, the positive relationship between ammonia‐oxidizing bacteria (AOB) and soil potential nitrification rate (PNR), along with decreased AOB abundance (−36%) following straw and decomposing MI amendments, further suggested that AOB predominated soil nitrification and was responsible for the suppressed nitrification. In addition, straw amendment increased nirK gene abundance (by 48%) and potential denitrification rate (by 11%), while the increase in nosZI gene abundance (+25%) involved in N2O consumption contributed to the decrease in N2O emissions. Overall, the additive effect of straw and decomposing MIs on soil N2O emissions was associated with two amendment‐induced changes in the abundance and composition of AOB and straw‐stimulated the abundance of the nosZI gene. Our results revealed the potential for mitigating N2O emissions following the amendments of straw and MI by influencing soil N2O‐related microbial groups.

Vallisneria spiralis L. adaptive capacity improves pore water chemistry and increases potential nitrification in organic polluted sediments

BackgroundMacrophytes may modify benthic biodiversity and biogeochemistry via radial oxygen loss from roots. This condition contrasts sediments anoxia, allows roots respiration, and facilitates aerobic microbial communities and processes in the rhizosphere. Simultaneously, the rhizosphere can stimulate anaerobic microorganisms and processes via exudates or by favoring the build-up of electron acceptors as nitrate. As eutrophication often results in organic enrichment in sediments and large internal nutrients recycling, an interesting research question is to investigate whether plants maintain the capacity to stimulate aerobic or anaerobic microbial communities and processes also under elevated organic pollution.MethodsA manipulative experiment was carried out under laboratory-controlled conditions. Microcosms containing bare sediments and sediments transplanted with the macrophyte Vallisneria spiralis L. were created. The effect of the plant was investigated on sediments with moderate (8%) and elevated (21%) organic matter content, after an acclimatization period of 30 days. Chemical and physical parameters, microbial community composition and the potential rates of nitrification, denitrification and nitrate ammonification were measured at two different depths (0–1 and 1–5 cm) after the acclimatization period to evaluate the role of roots.ResultsVallisneria spiralis grew and assimilated pore water nutrients at the two organic matter levels and vegetated sediments had always nutrient-depleted porewaters as compared to bare sediments. Nitrifying microbes had a lower relative abundance and diversity compared to denitrifying bacteria. However, regardless of the organic content, in vegetated sediments nitrifiers were detected in deeper horizons as compared to bare sediments, where nitrification was confined near the surface. In contrast, potential denitrification rates were not affected by the presence of roots, but probably regulated by the presence of nitrate and by root-dependent nitrification. Potential nitrate ammonification rates were always much lower (< 3%) than potential denitrification rates.ConclusionsVallisneria spiralis affects N-related microbial diversity and biogeochemistry at moderate and elevated organic matter content, smoothing bottom water–pore water chemical gradients and stimulating nitrification and nitrogen loss via denitrification. These results suggest the possibility to deploy V. spiralis as a nature-based solution to counteract eutrophication in freshwater systems impacted by high loads of organic matter, for example, downstream of wastewater treatment plants.

Relationship between eutrophication and greenhouse gases emission in shallow freshwater lakes

In shallow lakes, there are complex relationships between lake eutrophication and greenhouse gas emissions that deserve to be studied, which are important for solving lake eutrophication, slowing down climate warming, and reducing carbon emissions. In order to explore the relationship and mechanism between eutrophication and greenhouse gases (GHGs), the net GHGs emission flux and transformation of carbon, and nitrogen in 45 shallow freshwater lakes were investigated from May to September 2022. Eutrophication facilitated potential denitrification rate (Dt) without increasing nitrous oxide (N2O) production based on the significantly positive relationship between eutrophication and Dt. This should be attributed to the shift from incomplete (N2O producing process) to complete denitrification (N2 producing process). Compared to NarG mediating nitrate (NO3−) to nitrite (NO2−), fewer eutrophication indicators showed a positive relationship with NosZ mediating N2O to N2, suggesting that more stringent conditions are required for complete denitrification, which was achieved in the lakes we investigated. Optimal reduction in net carbon dioxide (CO2) emissions occurs at high levels of primary productivity, as indicated by the V-shaped relationship between chlorophyll a (Chl a) and CO2 emissions. However, in hyper-eutrophic lakes, there is an upward trend in CO2 production. The possible explanations should include CO2 production and fixation as well as methane (CH4) oxidation. The bell-shaped relationship between the net flux of CH4 emission and Chl a could be explained that CH4 was heavily oxidized due to sufficient oxygen caused by algal bloom. This fact gave evidence for the increase of the net flux of CO2 emission in high primary productivity lakes. Therefore, the relationship and mechanism between net GHGs emission flux and eutrophication remained complex and various.

Biomass and the potential denitrification rate of the epiphyton on three kinds of submerged plants in Lake Taihu

Biomass and the potential denitrification rate of the epiphyton on three kinds of submerged plants in Lake Taihu

Elevated temperature and nutrients lead to increased N2O emissions from salt marsh soils from cold and warm climates

Salt marshes can attenuate nutrient pollution and store large amounts of ‘blue carbon’ in their soils, however, the value of sequestered carbon may be partially offset by nitrous oxide (N2O) emissions. Global climate and land use changes result in higher temperatures and inputs of reactive nitrogen (Nr) into coastal zones. Here, we investigated the combined effects of elevated temperature (ambient + 5℃) and Nr (double ambient concentrations) on nitrogen processing in marsh soils from two climatic regions (Quebec, Canada and Louisiana, U.S.) with two vegetation types, Sporobolus alterniflorus (= Spartina alterniflora) and Sporobolus pumilus (= Spartina patens), using 24-h laboratory incubation experiments. Potential N2O fluxes increased from minor sinks to major sources following elevated treatments across all four marsh sites. One day of potential N2O emissions under elevated treatments (representing either long-term sea surface warming or short-term ocean heatwaves effects on coastal marsh soil temperatures alongside pulses of N loading) offset 15–60% of the potential annual ambient N2O sink, depending on marsh site and vegetation type. Rates of potential denitrification were generally higher in high latitude than in low latitude marsh soils under ambient treatments, with low ratios of N2O:N2 indicating complete denitrification in high latitude marsh soils. Under elevated temperature and Nr treatments, potential denitrification was lower in high latitude soil but higher in low latitude soil as compared to ambient conditions, with incomplete denitrification observed except in Louisiana S. pumilus. Overall, our findings suggest that a combined increase in temperature and Nr has the potential to reduce salt marsh greenhouse gas (GHG) sinks under future global change scenarios.

Effects of fresh and aged biochar on soil N2O emission from a poplar plantation

Nitrous oxide (N2O) emissions are a significant concern when excessive nitrogen (N) fertilizers are applied to plantation systems to enhance tree growth. Although biochar can improve soil fertility and mitigate soil N losses, our understanding of its interaction with N fertilizer and its long-term effects remains limited owning to experimental constraints. In this study, we performed two microcosm incubation experiments to evaluate the effect of fresh biochar compared to eight-year field-aged biochar application in a poplar plantation on soil N2O emissions triggered by biogas slurry application. The experiments incorporated three biochar levels (B0: 0, B2: 6%, and B3: 9% by mass) and four biogas slurry application rates, each with three replicates. The results demonstrated that fresh and field-aged biochar significantly reduced soil cumulative N2O emissions by 31%–61% and 75%–99%, respectively, over seven days following biogas slurry application. However, these mitigating effects diminished over time. Fresh biochar application significantly reduced the available organic carbon levels and potential denitrification rates, suggesting that it primarily suppressed soil N2O emissions by limiting the supply of electron donors. In contrast, field-aged biochar had minimal impact on soil available organic carbon, and generally enhanced the relative abundance of bacterial amoA, nirS, nirK, and nosZ genes. This suggests that the aged biochar potentially suppressed soil N2O emissions by promoting complete denitrification. Partial least squares structure equation models (PLS-SEM) corroborated the two different mechanisms regulating the inhibitory influence of fresh and aged biochar on soil N2O emissions. The lower R2 of PLS-SEM for aged biochar (R2 = 0.256) compared to that of fresh biochar (R2 = 0.798) indicates that other factors, such as biochar properties, potentially affect soil N2O emissions and warrant further investigation. This study highlights the need to evaluate the long-term effects of biochar on soil N2O emissions, owing to the dynamic changes in biochar and soil properties over time.

Mechanism of additional carrier with seasonal temperature changes enhanced ecological floating beds for non-point source pollution water treatment

The continuous performance and denitrification characteristics of carriers were investigated in two modified enhanced ecological floating beds (EFBs), one with only ceramsite and the other with ceramsite and extra additional stereo-elastic packing. Over a period of more than 414 days, the extra carrier was found to improve nitrogen removal while enhancing the system's resistance to seasonal temperature variations. The denitrification of all carriers in EFBs was inhibited in practice by seasonal temperature change, especially temperature rose from below 20 °C to above 20 °C and the inhibition rate of nitrous nitrogen (NO2−-N) reduction was consistently above 91%, which was higher than that of nitrate nitrogen (NO3−-N). However, the denitrification process including the rate and the resistance to temperature changes of ceramsite in the same EFBs with stereo-elastic packing at different temperatures, was consistently improved. The removal rate of NO3−-N and NO2−-N increased by up to 23.5% and 19.5%, respectively. The potential denitrification rates of all carriers increased with time which was also evidenced by in PICRUSt results, which showed that the abundances of predicted functional genes encoding NO3−-N and NO2−-N reductase increased over time. The dominant denitrifier also differed over time due to seasonal temperature changes.

Epiphytic microorganisms of submerged macrophytes effectively contribute to nitrogen removal

Submerged macrophytes play important roles in nutrient cycling and are widely used in ecological restoration to alleviate eutrophication and improve water quality in lakes. Epiphytic microbial communities on leaves of submerged macrophytes might promote nitrogen cycling, but the mechanisms and quantification of their contributions remain unclear. Here, four types of field zones with different nutrient levels and submerged macrophytes, eutrophic + Vallisneria natans (EV), eutrophic + V. natans + Hydrilla verticillata, mesotrophic + V. natans + H. verticillata, and eutrophic without macrophytes were selected to investigate the microbial communities that involved in nitrification and denitrification. The alpha diversity of bacterial community was higher in the phyllosphere than in the water, and that of H. verticillata was higher compared to V. natans. Bacterial community structures differed significantly between the four zones. The highest relative abundance of dominant bacterioplankton genera involved in nitrification and denitrification was observed in the EV zone. Similarly, the alpha diversity of the epiphytic ammonia-oxidizing archaea and nosZI-type denitrifiers were highest in the EV zone. Consist with the diversity patterns, the potential denitrification rates were higher in the phyllosphere than those in the water. Higher potential denitrification rates in the phyllosphere were also found in H. verticillata than those in V. natans. Anammox was not detected in all samples. Nutrient loads, especially nitrogen concentrations were important factors influencing potential nitrification, denitrification rates, and bacterial communities, especially for the epiphytic nosZI-type taxa. Overall, we observed that the phyllosphere harbors more microbes and promotes higher denitrification rates compared to water, and epiphytic bacterial communities are shaped by nitrogen nutrients and macrophyte species, indicating that epiphytic microorganisms of submerged macrophytes can effectively contribute to the N removal in shallow lakes.

Patterns and determinants of nitrification and denitrification potentials across 24 rice paddy soils in subtropical China

Nitrification and denitrification processes are major pathways for nitrogen (N) losses in agricultural soils, and highly depend on soil properties. Improved understanding of soil nitrification and denitrification processes is essential for better N management in agroecosystems. However, the effects of geochemical properties on nitrification and denitrification potentials remain largely uncertain, especially in paddy soils. Here, we investigated the patterns and controlling factors of nitrification and denitrification potentials across 24 paddy soil sampling sites (varied in soil texture, pH, and contents of soil organic carbon (SOC), nutrients, and exchangeable metals) in major rice-producing areas of Hubei Province, China. The results showed substantial variations in the potential rates of nitrification (0.46–17.65 mg N kg−1 soil d−1) and denitrification (6.31–18.12 mg N kg−1 soil d−1) across the paddy soil sampling sites. Soil pH and SOC were the predominant factors regulating nitrification and denitrification potentials, respectively. Nitrification potential strongly increased with soil pH at value above 6.2, while showed an insignificant correlation with pH at value below 6.2. Increased soil pH stimulated nitrification potential via enhanced NH4+ availability, likely due to increased soil active iron (Fe) and manganese (Mn) oxides facilitating chemisorption. Denitrification potential strongly positively correlated with SOC at value below 15 g kg−1, dissolved organic carbon and nitrate contents, but had little relationship with active Fe and Mn properties and clay contents. The potentials of nitrification uncoupled with denitrification, possibly resulting from their distinct controlling factors across the paddy soils. Overall, these findings could help improve the understanding of the key factors controlling potential nitrification and denitrification across paddy soils, and provide a theoretical basis for optimizing agricultural management to mitigate N losses.

Cultivation increased soil potential denitrification rates by modifying denitrifier communities in desert‐oasis ecotone

AbstractOases soils in northwestern China are used widely for agricultural production, but low soil moisture and fertility necessitate high volumes of irrigation and fertilization, with significant losses of water via evaporation and nitrogen via denitrification. The dynamics of denitrifying communities and their responses to potential denitrification rate (PDR) in continuously irrigated oases remain unclear. In this study, we examined the dynamics of nirK and nirS denitrifying communities in three distinct areas, an old oasis field (OOF, 54 years of cultivation), a young oasis field (YOF, 20 years) and an adjacent uncultivated sandy land (USL, 0 years), and used the partial least squares path model (PLS‐PM) to predict how and to what extent soil properties and denitrifying communities may be responsible for changes in PDR. Our findings indicate that cultivation, compared with the USL treatment, improved soil structure and fertility and increased the abundance and diversity of denitrifying microbes, resulting in a further elevation of soil PDR in YOF and OOF. Additionally, our analysis highlights the potential dominance of the nirK gene in denitrification. PLS‐PM revealed that soil chemical properties and microbial biomass indirectly affected soil PDR by regulating the abundance and diversity of nirK and nirS genes. Conversely, soil physical properties had a direct negative impact on PDR. Alterations in PDR were, in part, attributed to changes in abundance, richness and beta‐diversity, but not correlated with changes in alpha‐diversity. Notably, the standardized total effect demonstrated that the denitrifier community exhibited greater responsiveness to changes in PDR than did soil properties. Overall, our findings suggest that denitrifying communities may play a more important role than soil properties in PDR, and an increased understanding of denitrifying communities allows PDR prediction during conversion of oasis to cultivated land.

Regulation of potential denitrification rates in sediments by microbial-driven elemental coupled metabolisms

Microbial driven coupled processes between denitrification and methane/sulfur metabolism play a very substantial role in accelerating nitrogen removal in river sediments. Until now, little is known about how element coupling processes alter nitrogen metabolism by the microbial functional communities. The primary objective of this research was to clarify the contributory role of microbial-mediated coupled processes in controlling denitrification. Specifically, the study sought to identify the key bioindicators (or metabolic pathway) for preferably regulating and predicting potential denitrification rate (PDR). Here, a total of 40 sediment samples were collected from the inflow rivers of Chaohu Lake under nitrogen stress. The results revealed the ecological importance of methanogens and sulfate reducing bacteria in the microbial interaction network. Correlations between quantitative or predicted genes showed that the methanogenic gene (mcrA) was synergistic with denitrifying genes, further unraveling that the key role of methanogenesis in denitrification process for facilitating nitrogen removal. The PDR of sediments ranged from 0.03 to 133.21 μg N·g−1·h−1. The study uncovered specific environmental factors (NH4+ and OM) and microbial indicators (nosZ, mcrA, Paracoccus, Thauera, Methanobrevibacter and Desulfomicrobium) as potential contributors to the variations in PDR. Structural Equation Model (SEM) analysis revealed a significant direct effect of NH4+ on PDR, evidenced by a standardized coefficient (λ) of 0.77 (P < 0.001). Additionally, the findings also emphasized the salient role of methanogens (Methanobrevibacter) and methanogenic gene (mcrA) in indicating PDR. The research's aforementioned findings shed light on the substantial consequences of methanogenesis on nitrogen metabolism in coupled processes, enabling improved control of nitrogen pollution in river sediments. This study provided fresh perspectives on the effects of multiple functional taxa on denitrification, and reinforces the significance of coupling processes for nitrogen removal.

Soil biota associated with soil N cycling under multiple anthropogenic stressors in grasslands

Soil nitrogen (N) processes and statuses are two essential components for soil N cycling in terrestrial ecosystems. Although studies have estimated the effects of multiple anthropogenic stressors (e.g., nutrient enrichment and soil acidification) on soil N cycling, how multiple trophic levels of soil biota regulate the responses of soil N processes and statuses under multiple anthropogenic stressors has not yet been studied. We established an N and phosphorus (P) enrichment and acid addition experiment in a semi-arid grassland on the Mongolian Plateau. After five years, our results showed that soil acidification strongly inhibited soil N processes (via decreasing N mineralization rate, potential nitrification rate, and potential denitrification rate) and soil N statuses (via decreasing dissolved organic N, NO3−-N, and microbial biomass N). Acid addition-induced declines in soil N processes or N statuses were mainly associated with the inhibition of the abundance/richness of soil N-related bacteria and nematodes. Compared with soil acidification, N or P enrichment had relatively minor effects on soil N processes or N statuses, except for the N enrichment-induced decline in the potential nitrification rate and increase in soil NH4+-N and P enrichment-induced decline in soil NO3−-N. Furthermore, we found that the responses of soil N processes or statuses to nutrient enrichment were different between non-acidified soils and acidified soils. Overall, our results evidenced that soil biota strongly mediated the responses of soil N processes and statuses to nutrient enrichment and acid addition in semi-arid grasslands, and soil acidification weakened the associations between soil biota and soil N cycling. These findings indicate that we should concentrate more on the importance of soil biota in maintaining ecosystem N functions and services against a background of global change.

Do NO, N2O, N2 and CO2 fluxes differ in soils sourced from cropland and varying riparian buffer vegetation? An incubation study.

Riparian buffers are expedient interventions for water quality functions in agricultural landscapes. However, the choice of vegetation and management affects soil microbial communities, which in turn affect nutrient cycling and the production and emission of gases such as nitric oxide (NO), nitrous oxide (N2O), nitrogen gas (N2) and carbon dioxide (CO2). To investigate the potential fluxes of the above-mentioned gases, soil samples were collected from a cropland and downslope grass, willow and woodland riparian buffers from a replicated plot scale experimental facility. The soils were re-packed into cores and to investigate their potential to produce the aforementioned gases via potential denitrification, a potassium nitrate (KNO3 -) and glucose (labile carbon)-containing amendment, was added prior to incubation in a specialized laboratory DENItrification System (DENIS). The resulting NO, N2O, N2 and CO2 emissions were measured simultaneously, with the most NO (2.9 ± 0.31 mg NO m-2) and N2O (1413.4 ± 448.3 mg N2O m-2) generated by the grass riparian buffer and the most N2 (698.1 ± 270.3 mg N2 m-2) and CO2 (27,558.3 ± 128.9 mg CO2 m-2) produced by the willow riparian buffer. Thus, the results show that grass riparian buffer soils have a greater NO3 - removal capacity, evidenced by their large potential denitrification rates, while the willow riparian buffers may be an effective riparian buffer as its soils potentially promote complete denitrification to N2, especially in areas with similar conditions to the current study.

Influence of seasonal water-level fluctuations on depth-dependent microbial nitrogen transformation and greenhouse gas fluxes in the riparian zone

Greenhouse gas (GHG) emissions from riparian zones affect the global GHG balance, contribute to global warming and climate change, and generate adverse effects on ecological health as well as environmental, economic and social sustainable development. However, the influence of water-level fluctuations on depth-dependent soil properties, nitrogen transformation and GHG emissions in riparian zone ecosystems during wetting–drying cycles remains unclear. In this study, we analyzed seasonal nitrogen dynamics under seasonal water-level fluctuations at seven different depths (0–20, 20–40, 40–60, 60–80, 80–100, 100–120 and 120–140 cm) along a typical transect in the Jianghan Plain, Central China for 2018–2019. Changes in depth-dependent soil physical and chemical properties, soil textural characteristics and mineralogy, potential nitrification rate (PNR) and potential denitrification rate (PDR), as well as soil GHG emission rates were determined across a range of meteorological conditions and surface water and groundwater level fluctuations. Groundwater levels varied significantly between seasons with rising water-levels during the wet seasons and falling water-levels during the dry seasons. The physicochemical properties and chemical composition in the three studied riparian soil textures revealed higher spatiotemporal variability during the wet seasons as compared to the dry seasons. Our results showed that groundwater level depth was a crucial influencing factor controlling the microbial nitrogen transformation in the soil profiles. The PNR values were characterized by substantial seasonal variation with the same or higher metabolic activity in the dry season than in the wet season owing to water-level fluctuations, while they declined with depth. Compared to the wet season, the PDR values in the studied riparian zone were typically lower in the dry season due to extremely low soil water content constrained by fluctuating water-levels. Besides, the PDR values in the silt loam layers were higher than that in the sandy loam layer for most of the monitoring period. The depth-dependent GHG flux rates highlighted that the riparian zone ecosystem was a key source of CO2 and N2O and a sink for CH4. In summary, our findings suggest that fluctuating water table management in riparian ecosystems must be considered to more effectively control seasonal fluctuations of nitrogen dynamics and mitigate GHG emissions. Thus, a suitable riparian groundwater quality control and management scheme is urgently necessary to bring awareness of seasonal water-level fluctuations of riparian zones to catchment managers and policy makers.

Effects of dam building on N2O-producing and N2O-reducing community structures in river sediments and the associated N2O emission potential

Dam construction is regarded as the greatest anthropogenic disturbance in aquatic ecosystems, and it promotes denitrification, through which large N2O emissions occur. However, the effect of dams on N2O producers and other N2O-reducing microorganisms (especially for nosZ II), and the associated denitrification rates remain poorly understood. This study systematically investigated the spatial variation of potential denitrification rates in dammed river sediments in winter and summer and the microbial processes driving N2O production and reduction. Sediments in the transition zone of dammed rivers were found to be critical for N2O emission potential, with lower potential denitrification rate and N2O production rate in winter than in summer. In dammed river sediments, the dominant N2O-producing microorganisms and N2O-reducers were nirS-harboring bacteria and nosZ I-harboring bacteria, respectively. Diversity analysis showed that diversity of N2O-producing did not differ significantly between upstream and downstream sediments, whereas the population size and diversity of N2O-reducing microbial communities in upstream sediments significantly decreased, leading to biological homogenization. Further ecological network analysis revealed that the ecological network of nosZ II microbes was more complex than that of nosZ I microbes, and both exhibited more cooperation in the downstream sediments than in the upstream sediments. Mantel analysis showed that the potential N2O production rate was mainly influenced by electrical conductivity (EC), NH4+, and TC content, and that higher nosZ II/nosZ I ratios contributed to improved N2O sinks in dammed river sediments. Moreover, the Haliscomenobacter genus from the nosZ II-type community in the downstream sediments contributed significantly to N2O reduction. Collectively, this study elucidates the diversity and community distribution of nosZ-type denitrifying microorganisms influenced by dams, and also highlights the non-negligible role played by nosZ II-containing microbial groups in mitigating N2O emissions from dammed river sediments.

Refining the role of nitrogen mineralization in mycorrhizal nutrient syndromes

Forest stands dominated by ectomycorrhizal (ECM) associated trees often have more closed nitrogen (N) cycling than stands dominated by arbuscular mycorrhizal (AM) associated trees, with slower N mineralization in ECM stands thought to suppress inorganic N cycling. However, most estimates of N mineralization come from measurements of net processes, which can lead to an incomplete view of ecosystem N retention and loss. To explore the mechanisms driving mycorrhizal N cycling syndromes, we measured gross N production and assimilation rates and net and potential N flux rates in paired N addition (from NH4SO4 and NaNO3) and control plots within ECM and AM-dominated stands. We observed greater gross N mineralization and microbial ammonium assimilation in ECM compared to AM stands, suggesting that increased microbial N demand drove lower net N mineralization rates in ECM stands. We found lower nitrification rates in ECM compared to AM stands and no effect of N addition on nitrification in ECM stands. Therefore, the low soil pH or high C:N ratios found in those stands, not limited ammonium supply, may have suppressed nitrification. Finally, potential denitrification rates and nitrous oxide fluxes were lower in ECM compared to AM stands with no effect of N addition, suggesting that denitrification is controlled by the endogenous supply of nitrate from nitrification, not exogenous nitrate inputs. Overall, we conclude that N mineralization may not play a central role in forming mycorrhizal nutrient syndromes, and that acidic conditions in ECM stands may ultimately control nitrification and the potential for ecosystem N loss.

Nitrification and denitrification processes in a zero-water exchange aquaculture system: characteristics of the microbial community and potential rates

​The zero-water exchange aquaculture has been identified as a promising method of farming to decrease the environment pressure of aquaculture and to increase profits. However, the ecological functions (e.g., nitrogen removal) and microbial biodiversity of the zero-water exchange pond aquaculture system are relatively understudied. In the present study, the zero-water exchange pond aquaculture system were constructed to investigated the microbial communities, sediment potential nitrification and denitrification production rates. And five functional genes (AOA amoA, AOB amoA, nirS, nosZ, and hzsB) were used to quantify the abundance of nitrifying and denitrifying microorganisms. The results showed that the sediment of the system had excellent potential nitrification-denitrification performance, with potential nitrification and denitrification rate were 149.77-1024.44 ng N g−1 h−1 and 48.32-145.01 ng N g−1 h−1, respectively. The absolute copy numbers of nitrogen functional genes and total bacterial 16S rRNA were 1.59×105-1.39×109 and 1.55×1010-2.55×1010copies g−1, respectively, with the dominant phyla, i.e., Proteobacteria, Actinobacteriota, Chloroflexi, Cyanobacteria, and Firmicutes. The relative abundances of the genera related to nitrification and denitrification, varied from 0.01% to 0.79% and from 0.01% to 15.54%, respectively. The potential nitrification rate was positively related to the sediment TOC concentration; and the potential denitrification rate had a positive correlation with sediment nitrate concentration. The genera Bacillus positively correlated with sediment NO3‐-N concentration, whereas Flavobacterium and Shewanella positively correlated with sediment NH4+-N concentration, which could be the functional bacteria for nitrogen removal. These findings may shed light on quantitative molecular mechanisms for nitrogen removal in zero-water exchange ponds, providing a sustainable solution to nitrogen pollution problem in the freshwater aquaculture ecosystems.

Responses of Alpine Soil Nitrification and Denitrification Rates to Nitrogen Addition Gradient—The Role of Functional Genes

Increases in nitrogen (N) deposition affect the nitrification and denitrification processes that are regulated by microorganisms. With the aim of understanding what happened to the soil nitrification and denitrification rates under the N application gradients, we set a field experiment treated with N at 6 different rates (0, 2, 4, 8, 16, and 32 g N·m−2·yr−1) in 2014. We determined the physico-chemical properties, abundances and community structures of the nitrifiers and denitrifiers, the net nitrification rate (NNR) and the potential denitrification rate (PDR) of soil samples that were collected in 2020. We found that the abundances of ammonia-oxidizing bacteria amoA (AOB amoA), the sum of ammonia-oxidizing archaea amoA (AOA amoA) and AOB amoA increased with the increase in N application rate, but the abundances of nosZ decreased with that. The microorganisms that dominate the ammonia oxidation process could shift from AOA to AOB under high N application rates. Furthermore, the soil microorganisms respond to the N addition preferentially with the abundance changes rather than the community composition changes. Moreover, the NNR increased with the N input, while the decrease in the PDR was due to the decrease in the pH value caused by high N application. The results also showed that the amoA gene abundance explained most (46.3%) of the variation in the NNR. Moreover, the soil moisture and pH explained 44.0% and 27.1% of the variation in the PDR, respectively. The results demonstrated that the NNR and PDR were mainly explained by functional genes abundances and environmental factors, respectively, in alpine meadow soil under sustained N deposition.

Trade-offs on carbon and nitrogen availability lead to only a minor effect of elevated CO2 on potential denitrification in soil

The global surplus of reactive nitrogen (Nr) and elevated atmospheric CO2 are both major threats to current and future ecosystem integrity. Denitrification is the primary sink for Nr by the conversion to inert N2 and is also the predominant source of N2O emission. Yet, it is unclear how and to what extent soil denitrification would respond to elevated CO2 (eCO2), which is a major part of the uncertainties on the estimation of terrestrial nitrogen (N) cycle budget under the influence of climate changes. Herein, we provided quantitative results of a global meta-analysis with 127 observations that included data on soil potential denitrification rates (PDR) and denitrifying gene abundance, and 305 observations that included data on N2O emission and soil available N content under eCO2. Averaged across all studies, we found that globally eCO2 had an overall minor effect on soil PDR, but showed a large variation with different edaphic and environmental factors. The response was primarily explained by soil organic carbon (SOC) and mean annual precipitation (MAP), i.e., the higher SOC content or MAP, the lower response of soil PDR to eCO2. Further, eCO2 significantly decreased soil NO3- content (-15.3%) and denitrifying bacteria (-31.3%), but increased nirS (14.7%), napA (24.4%) genes and N2O emission (34.1%). We speculate that eCO2 has both positive and negative effects on soil potential denitrification, i.e., a higher soil C input induced by eCO2 supplies more energy for denitrifiers and thus increases denitrification in N-rich ecosystems, while unbalanced C:N input decreases soil NO3- content, causing substrate deficiency in N-limiting ecosystems under eCO2. Whilst the response of soil PDR to eCO2 is governed by trade-offs on soil C and N availability, this may serve as a buffer on soil available N content in terrestrial ecosystems under eCO2. Collectively, based on our findings, we built a comprehensive conceptual model defining the key regulators, which has significant implications for expanding the understanding and predicting the responses of the terrestrial N cycle to eCO2 in future scenario evaluation.