Potential Denitrification Rates Research Articles

The effect of leaf leachates addition on denitrification in subsurface flow constructed wetlands is shaped by the bed substrate type

In constructed wetlands (CWs), bed substrate and leaf leachates from vegetation may correct the low C/N ratio that constrains heterotrophic denitrification and nitrate removal from irrigated agricultural drainage water. However, the interactive effects of bed substrate type and leaf leachates on denitrification are still unknown. By focusing on a CWs pilot plant, we designed a laboratory experiment to evaluate i) wether denitrification potential rates varied among bed substrates: calcareous gravel (a conventional substrate), gravel+soil from a natural wetland (silty loam Solonchak, 1.5 % of organic C) and gravel+biochar from pyrolyzed ornamental plants (75 % of organic C); and ii) the response of denitrification within each bed substrate to the addition of their respective leaf leachates. We found that denitrification potential rates were lower in gravel beds (0.011 ± 0.006 μgN2O-N gDM−1 h−1) than those observed with the addition of biochar (0.06 ± 0.03) and especially soil (0.78 ± 0.04), with soil being the most advantageous option. Besides, leaf leachates addition boosted denitrification rates in all cases. Nevertheless, the effect of leachates was relatively higher in gravel beds than in the other substrates (15 times higher vs. 2 and 4 times with soil and biochar, respectively). Our outcomes highlight limited denitrification when using gravel substrate not only by low C but also due to essential macro- and micro-elements, and support the role of plant leaves as internal and self-sustainable source of nutrients.

Effects of warming and fertilization on nirK-, nirS- and nosZ-type denitrifier communities in paddy soil

The effects of fertilization on soil denitrifying microorganisms are well-documented. However, the impact of global warming on these microorganisms, particularly regarding the interaction with fertilization, remains poorly understood. Here, a 4-year field warming experiment that included experimental warming (ET) and ambient temperature control (AC), with nitrogen (N) fertilizer applied (CF) or without N fertilizer (CK), was employed to assess the response of the abundance and community of nirK-, nirS- and nosZ- type denitrifiers to warming and fertilization in paddies, and to understand their relationship with potential denitrification rate (PDR). The results showed that warming amplified the positive effect of fertilization on abundance of nirK and nirS genes, while the abundance of nosZ remained unaffected. The copies of nirK and nirS under the ET-CF treatment were notably higher than in the other treatments. In the terms of biodiversity, warming diminished the effect of fertilization on the α-diversity of nirK and nirS, but it did not influence the α-diversity of nosZ. Besides, warming intensified the effect of fertilization on the β-diversity of nirK, while the β-diversity of nirS and nosZ remained unchanged in response to fertilization. Additionally, the community structure of denitrifiers varied with warming and/or fertilization. Specifically, Mesorhizobium (nirK), Proteobacteria (nirS) and Rhizobiales (nosZ) were dominant in AC-CK treatment. In the AC-CF treatment, Proteobacteria (nirK/S), Rhizobiales (nosZ) were the main taxa. For the ET treatments (ET-CF, ET-CK), Bradyrhizobiaceae (nirK), Proteobacteria (nirS) and Alphaproteobacteria (nosZ) were predominant. Correlation analysis revealed that soil pH, carbon and N content were the primary factors influencing nirK-, nirS-and nosZ- type denitrifiers. Moreover, PDR showed a positive relationship with nirK abundance, α-diversity of nosZ, and SOC. Overall, the results demonstrate that warming can modify the response of denitrifiers to fertilization, subsequently affecting denitrification rates, a phenomenon that merits attention.

Seasonal wet–dry transition and denitrifying communities contributed to nitrous oxide emissions in the water-level fluctuating zone of the largest freshwater lake in China

The water-level fluctuating zone (WLFZ) is the alternating dry/wet zone in freshwater lakes, and plays a crucial role in nitrogen (N) biogeochemical cycling and nitrous oxide (N2O) emissions. However, N2O emission fluxes, the underlying microbial mechanisms, and their feedbacks to global warming in the extensive WLFZ of the freshwater lakes with dramatic water-level variations remain unclear. Here, we sampled through both wet and dry seasons and compared the N removal rates, N2O emission fluxes, composition and co-occurrence network properties of the denitrifying microbial communities of the WLFZ, continuously flooded, and non-flooded zones covered with the dominant plant (Carex. cinerascens) in Poyang Lake, the largest freshwater lake of China. We observed higher potential denitrification (1.129–5.657 nmol N g-1h−1) and anammox (0.047–0.756 nmol N g-1h−1) rates in the WLFZ for both wet and dry seasons than those in the other two zones. A large seasonal N2O sink-source transition (−2.297 and 0.297 μg m-2h−1 for wet and dry season, respectively) was observed in WLFZ. Moreover, the composition of nirK and nirS communities significantly varied in WLFZ between the wet and dry seasons. Especially, several keystone taxa (e.g., Mesorhizobium and Rhodanobacter) were enriched in the WLFZ and were responsible for denitrification and N2O emissions under drought stress in the dry season. Variations in denitrification rates and N2O fluxes were driven by pH, C and N substrates, as well as the co-occurrence network properties, including natural connectivity and small-world coefficients of the nirK and nirS communities. Our results suggested that WLFZ in freshwater lakes under future climate change scenarios would be a substantial N2O source and aggravate climate, which would result in the positive feedback.

Microbial nitrogen cycling in Microcystis colonies and its contribution to nitrogen removal in eutrophic Lake Taihu, China

Cyanobacterial blooms induced by excessive loadings of nitrogen (N) and other nutrients are a severe ecological problem in aquatic ecosystems. Previous studies of N removal have primarily focused on sediment-water interface, yet the role of cyanobacterial colonies has recently been attracting more research attention. In this study, N cycling processes were quantified for cyanobacterial colonies (primarily Microcystis colonies) and their contribution to N removal was estimated for a large, shallow eutrophic lake in China, Lake Taihu. Various N cycling processes were determined via stable 15N isotope, together with 16S rRNA gene sequencing and quantitative microbial element cycling (QMEC) chip. Denitrification was found to be the most prominent process, estimated to be 36.63, 9.85, 3.35, and 3.15 times higher than dissimilatory nitrate reduction to ammonium (DNRA), nitrification, ammonium (NH4+) uptake and nitrate (NO3−) uptake rates, respectively. Denitrifiers accounted for a large part of the bacterial taxa (35.50 ± 24.65%), and the nirS gene was the most abundant among N cycling-related genes, with (2.54 ± 0.51) × 109 copies g−1Microcystis colonies. A field investigation revealed a positive correlation between the potential denitrification rate and the Chl-a concentration (mostly derived from Microcystis colonies). Based on a multiple stepwise regression model and historical data from 2007 to 2015 for Lake Taihu, the total amount of N removed via denitrification by Microcystis colonies was estimated at 171.72 ± 49.74 t yr−1; this suggests that Microcystis colonies have played an important role in N removal in Lake Taihu since the drinking water crisis in 2007. Overall, this study revealed the importance of denitrification within Microcystis colonies for N removal in eutrophic lakes, like Lake Taihu.

Thinning alters nitrogen transformation processes in subtropical forest soil: Key roles of physicochemical properties

Thinning—a widely used forest management practice—can significantly influence soil nitrogen (N) cycling processes in subtropical forests. However, the effects of different thinning intensities on nitrification, denitrification, and their relationships with soil properties and microbial communities remain poorly understood. Here, we conducted a study in a subtropical forest in China and applied three thinning treatments, i.e., no thinning (0 %), intermediate thinning (10–15 %), and heavy thinning (20–25 %), and investigated the effects of thinning intensity on the potential nitrification rate (PNR), potential denitrification rate (PDR), and microbial communities. Moreover, we explored the relationships among soil physicochemical properties, microbial community structure, and nitrogen transformation rates under different thinning intensities. Our results showed that intermediate and heavy thinning significantly increased the PNR by 87 % and 61 % and decreased the PDR by 31 % and 50 % compared to that of the control, respectively. Although the bacterial community structure was markedly influenced by thinning, the fungal community structure remained stable. Importantly, changes in microbial community composition and diversity had minimal impacts on the nitrogen transformation processes, whereas soil physicochemical properties, such as pH, organic carbon content, and nitrogen forms, were identified as the primary drivers. These findings highlight the critical role of managing soil physicochemical properties to regulate nitrogen transformations in forest soils. Effective forest management should focus on precisely adjusting the thinning intensity to enhance the soil physicochemical conditions, thereby promoting more efficient nitrogen cycling and improving forest ecosystem health in subtropical regions.

PBAT microplastics exacerbates N2O emissions from tropical latosols mainly via stimulating denitrification

Biodegradable plastic mulching films are regarded as promising substitutes for their non-degradable counterparts to ameliorate serious plastic pollution in agriculture. Inevitably, biodegradable microplastics (BMs) can be formed and profoundly affect the function of soil ecosystems. However, little is known about the effects of BMs loading on soil nitrogen (N) dynamics and loss in tropical agricultural soils. Hereby, a 120-day soil incubation trial was performed to evaluate the effects of polybutylene adipate co-terephthalate (PBAT) microplastics on nitrous oxide (N2O) emissions, N transformation, and bacterial community structure in latosols. The results showed that PBAT addition raised soil ammonium nitrogen (NH4+–N) and dissolved organic carbon (DOC) contents, but lowered nitrate nitrogen (NO3––N) contents, accompanied with a smaller potential nitrification rate and a greater potential denitrification rate. The microbial biomass nitrogen and nrfA gene abundance were increased by PBAT, suggesting the processes of microbial N immobilization and dissimilatory nitrate reduction to ammonia were largely promoted. Positive correlations existed between N2O emissions and denitrification-related enzymatic activities and functional gene abundances (narG, nirK/S and Fungal nirK). PBAT amendment stimulated N2O emissions by 54.8 − 317.3 %, accompanied with increased values of those denitrification-related parameters, suggesting that stimulated denitrification was responsible for the exacerbated N2O emissions. Moreover, PBAT addition changed bacterial alpha diversity and bacterial community composition, where the relative abundances of Proteobacteria increased while those of Nitrospirae and Acidobacteria decreased. These findings are beneficial to understanding the fate of N in microplastics-polluted soil, which will provide a new insight into the impact of BMs on soil functions and environmental risks.

Evidence of nitrogen inputs affecting soil nitrogen purification by mediating root exudates of salt marsh plants

Salt marsh has an important ‘purification’ role in coastal ecosystems by removing excess nitrogen that could otherwise harm aquatic life and reduce water quality. Recent studies suggest that salt marsh root exudates might be the ‘control centre’ for nitrogen transformation, but empirical evidence is lacking. Here we sought to estimate the direction and magnitude of nitrogen purification by salt marsh root exudates and gain a mechanistic understanding of the biogeochemical transformation pathway(s). To achieve this, we used a laboratory incubation to quantify both the root exudates and soil nitrogen purification rates, in addition to the enzyme activities and functional genes under Phragmites australis populations with different nitrogen forms addition (NO3−, NH4+ and urea). We found that NO3− and urea addition significantly stimulate P. australis root exudation of total acids, amino acids, total sugars and total organic carbon, while NH4+ addition only significantly increased total acids, amino acids and total phenol exudation. High total sugars, amino acids and total organic carbon concentrations enlarged nitrogen purification potential by stimulating the nitrogen purifying bacterial activities (including enzyme activities and related genes expression). Potential denitrification rates were not significantly elevated under NH4+ addition in comparison to NO3− and urea addition, which should be ascribed to total phenol self-toxicity and selective inhibition. Further, urea addition stimulated urease and protease activities with providing more NH4+ and NO2− substrates for elevated anaerobic ammonium oxidation rates among the nitrogen addition treatments. Overall, this study revealed that exogenous nitrogen could increase the nitrogen purification-associated bacterial activity through accelerating the root exudate release, which could stimulate the activity of nitrogen transformation, and then improve the nitrogen removal capacity in salt marsh.

Particulate organic matter predicts spatial variation in denitrification potential at the field scale

High spatiotemporal variability in soil nitrous oxide (N2O) fluxes challenges quantification and prediction of emissions to evaluate the climate change mitigation outcomes of sustainable agricultural practices. Triggers for large, short-lived N2O emission pulses, such as rainfall and fertilization, alter soil oxygen (O2) and nitrate (NO3–) availability to favor N2O production via denitrification. However, the organic C (OC) needed to fuel denitrification may exhibit subfield variation that constrains the potential for high denitrification rates to occur, leading to spatial variation in N2O hot moments. We tested the hypothesis that the particulate organic matter (POM) fraction of soil organic matter (SOM) controls subfield variation in denitrification potential by regulating availability of dissolved organic C (DOC), the form of OC used by denitrifiers. Among 38 soil samples collected across a maize field in central Illinois, USA, we found that potential denitrification rate was best predicted by POM C concentration (R2 = 0.35). Using multiple linear regression analysis that included other soil properties as explanatory variables, we found that POM C fraction of bulk soil (mg POM C/g SOC) was the most important predictor based on regression coefficient size (P < 0.01). Our results, which provide support for our hypothesis, suggest that consideration of the link between C and N cycling may be a key to predicting spatiotemporal variation in soil N2O emissions when denitrification is the dominant N2O source process.

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.