Soil Denitrification Rates Research Articles

Activity and community structure of nitrifiers and denitrifiers in nitrogen-polluted rivers along a latitudinal gradient

Nitrogen (N) cycling in rivers is particularly active and dynamic due to excess nutrient inputs worldwide. However, the multidimensional spatial patterns of the activity and community structure of N-cycling microorganisms in rivers remain unclear, limiting our understanding of river ecological functions, especially N removal capacity. Here, we measured the nitrification and denitrification rates and identified nitrifying and denitrifying microorganisms using high-throughput sequencing of archaeal amoA, bacterial amoA, nirK, and nirS genes in channel sediments, riparian rhizosphere soils, and riparian bulk soils of 30 N-polluted rivers across China. Results showed that in the lateral dimension, nitrification rates in sediments did not differ significantly from those in rhizosphere and bulk soils, but denitrification rates were higher in sediments than in bulk soils. However, the archaeal amoA gene abundance in sediments was considerably lower than that in rhizosphere and bulk soils, and bacterial amoA gene abundance in sediments was greater than that in rhizosphere soils. In the vertical dimension, both nitrification and denitrification rates in riparian bulk soils decreased with soil depth, and topsoils harbored more nitrifying and denitrifying microbes than subsoils. Denitrification but not nitrification rates increased with latitude and altitude but decreased with increasing mean annual temperature and precipitation. Overall, these results provide new insights into the multidimensional spatial patterns of river N cycling at a large scale, which is crucial to evaluating the N removal function of global rivers.

Bacillus velezensis SQR9 inhibition to fungal denitrification responsible for decreased N2O emissions from acidic soils

Tropical and subtropical acidic soils are hotspots of global terrestrial nitrous oxide (N2O) emissions, with N2O produced primarily through denitrification. Plant growth-promoting microbes (PGPMs) may effectively mitigate soil N2O emissions from acidic soils, achieved through differential responses of bacterial and fungal denitrification to PGPMs. To test this hypothesis, we conducted a pot experiment and the associated laboratory trials to gain the underlying insights into the PGPM Bacillus velezensis strain SQR9 effects on N2O emissions from acidic soils. SQR9 inoculation significantly reduced soil N2O emissions by 22.6–33.5 %, dependent on inoculation dose, and increased the bacterial AOB, nirK and nosZ genes abundance, facilitating the reduction of N2O to N2 in denitrification. The relative contribution of fungi to the soil denitrification rate was 58.4–77.1 %, suggesting that the N2O emissions derived mainly from fungal denitrification. The SQR9 inoculation significantly inhibited the fungal denitrification and down-regulated fungal nirK gene transcript, dependent on the SQR9 sfp gene, which was necessary for secondary metabolite synthesis. Therefore, our study provides new evidence that decreased N2O emissions from acidic soils can be due to fungal denitrification inhibited by PGPM SQR9 inoculation.

Nanoplastics alter ecosystem multifunctionality and may increase global warming potential.

Although the presence of nanoplastics in aquatic and terrestrial ecosystems has received increasing attention, little is known about its potential effect on ecosystem processes and functions. Here, we evaluated if differentially charged polystyrene (PS) nanoplastics (PS-NH2 and PS-SO3 H) exhibit distinct influences on microbial community structure, nitrogen removal processes (denitrification and anammox), emissions of greenhouse gases (CO2 , CH4 , and N2 O), and ecosystem multifunctionality in soils with and without earthworms through a 42-day microcosm experiment. Our results indicated that nanoplastics significantly altered soil microbial community structure and potential functions, with more pronounced effects for positively charged PS-NH2 than for negatively charged PS-SO3 H. Ecologically relevant concentration (3 g kg-1 ) of nanoplastics inhibited both soil denitrification and anammox rates, while environmentally realistic concentration (0.3 g kg-1 ) of nanoplastics decreased the denitrification rate and enhanced the anammox rate. The soil N2 O flux was always inhibited 6%-51% by both types of nanoplastics, whereas emissions of CO2 and CH4 were enhanced by nanoplastics in most cases. Significantly, although N2 O emissions were decreased by nanoplastics, the global warming potential of total greenhouse gases was increased 21%-75% by nanoplastics in soils without earthworms. Moreover, ecosystem multifunctionality was increased 4%-12% by 0.3 g kg-1 of nanoplastics but decreased 4%-11% by 3 g kg-1 of nanoplastics. Our findings provide the only evidence to date that the rapid increase in nanoplastics is altering not only ecosystem structure and processes but also ecosystem multifunctionality, and it may increase the emission of CO2 and CH4 and their global warming potential to some extent.

Stronger effects of environmental factors than denitrifying genes on soil denitrification under a subtropical land use change

Despite the importance of nitrogen (N) cycling in terrestrial ecosystems, the underlying mechanisms of changes in the soil denitrification rate and microbial function under land use change remain unclear. Here we conducted a field experiment to explore soil denitrification rate, abundance and diversity of denitrifying genes (nirS, nirK, and nosZ) across summer and winter for three land use types (abandoned land, shrubland, and woodland) in subtropical China. The highest denitrification rate was observed in abandoned land (0.31 ± 0.03 mg m-2h−1) while the lowest was in woodland (0.24 ± 0.01 mg m-2h−1); however, denitrification enzyme activity was higher in both abandoned land (6.68 ± 0.18 ng g-1h−1) and woodland (6.35 ± 0.46 ng g-1h−1) than in shrubland (5.45 ± 0.24 ng g-1h−1). Both the denitrification rate and denitrification enzyme activity in all land use types were lower in summer than in winter. Notably, the alpha diversities of the denitrifying genes did not change significantly among the land use types. But the beta diversity of nirS significantly differed among the land use types, that of nirK did not significantly vary with seasons, and that of nosZ changed significantly with both different land use types and seasons. Overall, our findings reveal that soil moisture, soil organic carbon, and NO3–-N concentration predominantly regulate denitrification under different land use types, whereas soil moisture and denitrifying genes primarily control variations in denitrification between seasons, thereby providing novel insights into predicting the effects of land use change on N cycling and belowground microbial function.

Relationships between denitrification rates and functional gene abundance in a wetland: The roles of single- and multiple-species plant communities

Wetland soil denitrification removes excess inorganic nitrogen (N) and prevents eutrophication in aquatic ecosystems. Wetland plants have been considered the key factors determining the capacity of wetland soil denitrification to remove N pollutants in aquatic ecosystems. However, the influences of various plant communities on wetland soil denitrification remain unknown. In the present study, we measured variations in soil denitrification under different herbaceous plant communities including single Phragmites karka (PK), single Paspalum thunbergia (PT), single Zizania latifolia (ZL), a mixture of Paspalum thunbergia plus Phragmites karka (PTPK), a mixture of Paspalum thunbergia plus Zizania latifolia (PTZL), and bare soil (CK) in the Estuary of Nantiaoxi River, the largest tributary of Qingshan Lake in Hangzhou, China. The soil denitrification rate was significantly higher in the surface (0–10 cm) than the subsurface (10–20 cm) layer. Wetland plant growth increased the soil denitrification rate by significantly increasing the soil water content, nitrate concentration, and ln(nirS) + ln(nirK). A structural equation model (SEM) showed that wetland plants indirectly regulated soil denitrification by altering the aboveground and belowground plant biomass, nitrate concentration, abundances of denitrifying functional genes, and denitrification potential. There was no significant difference in soil denitrification rates among PT, PK and ZL. The soil denitrification rate was significantly lower in PTZL than PTPK. Two-plant communities did not necessarily enhance the denitrification rate compared to single planting, the former had a greater competitiveness on N uptake and consequently reduced the amount of nitrate available for denitrification. As PTPK had the highest denitrification rate, co-planting P. thunbergia and P. karka could effectively improve N removal efficiency and help mitigate eutrophication in adjacent aquatic ecosystems. The results of this investigation provide useful information guiding the selection of appropriate wetland herbaceous plant species for wetland construction and the removal of N pollutants in aquatic ecosystems.

The Ability of Nitrification Inhibitors to Decrease Denitrification Rates in an Arable Soil

A nitrification inhibitor is an effective tool that can be used to reduce the loss of nitrogen (N) and improve crop yields. Most studies have focused on the changes in the soil N mineralization process that may influence the dynamics of soil inorganic N and the soil N cycle. However, the effects of the inhibitors on denitrification rates remain largely unclarified. Therefore, in this study, we monitored the dynamics in annual denitrification rates affected by nitrification inhibitors from a maize field for the first time. Treatments included inorganic fertilizer (NPK), cattle manure, a combination of NPK and DMPP (3,4-dimethylpyrazole phosphate), and a combination of manure and DMPP, applied to brown soils in a no-tillage maize field. The findings demonstrated that the denitrification rate and denitrifying enzyme activity (DEA) were highly variable and there were no significant decreases in all treatment groups after the addition of DMPP. Compared to the control soils, the ammonium (NH4+-N) concentration was significantly increased, while the nitrate (NO3−-N) level was significantly decreased in the DMPP-amended soils less than 30 days after treatment application, indicating that nitrification was partially inhibited. The formation of NO3−-N and the nitrification rates could be markedly reduced by DMPP, while NO3−-N availability did not affect the denitrification rates. Complete degradation of DMPP was observed in the soil on day 70 after DMPP addition, and its half-life was 10 days. Our study may ultimately help to clarify the characteristics of denitrification rates affected by nitrification inhibitors from different N fertilizer types applied to soils and explore the influencing factors of the dynamics in annual denitrification rates. However, more field studies evaluating the effectiveness of nitrification inhibitors in reducing denitrification under different sites and climate conditions, and the molecular mechanisms driving denitrification rate changes, need to be performed in the future.

Effects of flooding outweigh those of vegetation restoration on key processes of carbon and nitrogen cycling in a degraded riparian zone

Active revegetation (AR) and natural regeneration (NR) are two restoration approaches widely used to rehabilitate degraded riparian ecosystems, which are subjected to frequent hydrological fluctuations owing to climate change and human activities. Nevertheless, the impacts of restoration approaches and hydrological changes on the key processes of carbon and nitrogen cycling, particularly in degraded riparian ecosystems, remain unclear. In this study, we conducted hydrological experiments, which began in 2016, at three flooding zones (extreme flooding zone, severe flooding zone, and moderate flooding zone) of AR and NR areas in the Three Gorges Reservoir to investigate the effects of restoration approach and hydrological changes on soil properties, vegetation diversity, microbial characteristics, denitrification rates, and greenhouse gas fluxes. The results showed that compared with NR, AR did not change vegetation diversity, microbial diversity, and soil denitrification rates, but decreased soil ammonium and total carbon contents. The vegetation coverage of each flooding zone was more than 90%, and vegetation diversity was relatively high in the moderate flooding zone. Moreover, the maximum denitrification rates and CO2 fluxes were observed in the extreme flooding zone along with a high soil nutrient content. Additionally, the effect of flooding zone superseded the impacts of restoration approaches since AR could only significantly increase the CO2 flux (p < 0.05). Overall, this study elucidates the impacts of restoration approaches and hydrological changes on C and N cycling, and aids in developing restoration strategies for degraded riparian ecosystems.

Spatial variability of denitrification enzyme activity and actual denitrification emissions on Missouri claypan soils

AbstractDenitrification in agricultural soils is responsible for the majority of anthropogenic N2O production. The objective of this research was to assess denitrification potential and the spatial dependence of soil N2O flux from claypan soils in the Central Claypan Region of Missouri. Surface soil samples (0–10 cm) were collected on a 90‐m grid from two claypan fields, one conventionally managed in a corn (Zea maysL.)–soybean [Glycine max(L.) Merr.] rotation with tillage and without cover crops (Field 1), and the other managed as a corn–soybean–wheat (Triticum aestivumL.) rotation with no‐till and cover crops (Field 3). The denitrification enzyme activity (DEA) protocol was used to measure denitrification potential and actual soil denitrification rates of N2O and N2were measured with the N‐free atmospheric recirculation method (N‐FARM). In both fields, DEA fluxes were highest in the toeslope, with a rate of 1.4 kg N ha–1d–1in Field 1, and 1.8 kg N ha–1d–1in Field 3. Actual denitrification was dominated by N2(&gt;85%) rather than N2O emissions, and it was considerably lower than DEA. Actual N2, N2O, and total denitrification (N2+ N2O) fluxes were not different between fields; however, only total denitrification flux showed an effect of landscape position similar to that of DEA in both fields. Although the high smectitic clay content of upland soils provides environmental conditions suitable for denitrification, these results indicate that the accumulation of soil C in the toeslope position controlled DEA and actual denitrification rates.

Soil denitrification rates are more sensitive to hydrological changes than restoration approaches in a unique riparian zone

Abstract Riparian zones, an aquatic‐terrestrial interface, can intercept more than half of nitrogen (N) exported from terrestrial ecosystems to adjacent rivers, primarily by denitrification processes. However, damming has disrupted natural patterns and processes of flooding and vegetation community assemblages, and yet little is known about how hydrological changes and ecosystem restoration affect the biogeochemical functioning in the riparian ecosystems. We conducted an in situ experiment to evaluate the effects of hydrological change (e.g. altering flooding intensity and frequency) and restoration approaches (e.g. natural regeneration and active revegetation) on denitrification rates and the abundance of denitrifier genes in the riparian zone of the Three Gorges Reservoir, China. Our results showed that active revegetation did not significantly increase denitrification rates compared to the natural regeneration, but their underlying mechanism was different. At the natural regeneration area, the denitrification rate was primarily regulated by soil properties and abundance of nosZ gene, while at the active revegetation area, it was controlled merely by the abundance of nosZ gene. In addition, vegetation types showed little effect on the soil denitrification process, and the denitrification rate decreased with flooding intensity by reducing denitrifier gene abundance. The periodic flooding treatment doubled the denitrification rate compared with the no flooding treatment, which might be attributed to the enhancement of soil carbon availability. Our results suggest that in terms of N removal via denitrification processes, natural regeneration is a priority approach to restoring degraded riparian ecosystems. Read the free Plain Language Summary for this article on the Journal blog.

Variations and controlling factors of soil denitrification rate.

The denitrification process profoundly affects soil nitrogen (N) availability and generates its byproduct, nitrous oxide, as a potent greenhouse gas. There are large uncertainties in predicting global denitrification because its controlling factors remain elusive. In this study, we compiled 4301 observations of denitrification rates across a variety of terrestrial ecosystems from 214 papers published in the literature. The averaged denitrification rate was 3516.3±91.1µgNkg-1 soil day-1 . The highest denitrification rate was 4242.3±152.3µgNkg-1 soil day-1 under humid subtropical climates, and the lowest was 965.8±150.4µgNkg-1 under dry climates. The denitrification rate increased with temperature, precipitation, soil carbon and N contents, as well as microbial biomass carbon and N, but decreased with soil clay contents. The variables related to soil N contents (e.g., nitrate, ammonium, and total N) explained the variation of denitrification more than climatic and edaphic variables (e.g., mean annual temperature (MAT), soil moisture, soil pH, and clay content) according to structural equation models. Soil microbial biomass carbon, which was influenced by soil nitrate, ammonium, and total N, also strongly influenced denitrification at a global scale. Collectively, soil N contents, microbial biomass, pH, texture, moisture, and MAT accounted for 60% of the variation in global denitrification rates. The findings suggest that soil N contents and microbial biomass are strong predictors of denitrification at the global scale.

A global synthesis of soil denitrification: Driving factors and mitigation strategies

Dinitrogen (N2) and nitrous oxide (N2O) produced via denitrification may represent major nitrogen (N) loss in terrestrial ecosystems. A global assessment of soil denitrification rate, N2O/(N2O+N2) ratio, and their driving factors and mitigation strategies is lacking. We conducted a global synthesis using 225 studies (3367 observations) to fill this knowledge gap. We found that daily N loss through soil denitrification varied with ecosystems and averaged 0.25 kg N ha−1. The average emission factor of denitrification (EFD) was 4.8%. The average N2O/(N2O+N2) ratio from soil denitrification was 0.33. Soil denitrification rate was positively related to soil water-filled pore space (WFPS) (p < 0.01), nitrate (NO3-) content (p < 0.05) and soil temperature (p < 0.01), and decreased with higher soil oxygen content (p < 0.01). N2 emissions increased with latitude (p < 0.05), WFPS (p < 0.01) and soil mineral N (p < 0.05) but decreased with soil oxygen content (p < 0.05). The N2O/(N2O+N2) ratio increased with soil oxygen content (p < 0.01) but decreased with organic carbon (C) (p < 0.05), C/N ratio (p < 0.01), soil pH (p < 0.05) and WFPS (p < 0.01). We also found that optimizing N application rates, using ammonium-based fertilizers compared to nitrate-based fertilizers, biochar amendment, and application of nitrification inhibitors could effectively reduce soil denitrification rate and associated N2 emissions. These findings highlight that N loss via soil denitrification and N2 emissions cannot be neglected, and that mitigation strategies should be adopted to reduce N loss and improve N use efficiency. Our study presents a comprehensive data synthesis for large-scale estimations of denitrification and the refinement of relevant parameters used in the submodels of denitrification in process-based models.

Effects of water content on gross nitrogen transformation rates in forest land and grassland soils

Soil water content is an important factor driving microbial activities related to soil nitrogen (N) transformation. In this study, 15N pair tracing technique combined with the numerical model FLUAZ was used to investigate the gross N mineralization, immobilization, nitrification, and denitrification rates in grassland and forest land soils from Beian City, Heilongjiang Province, China under laboratory condition [60% or 100% water holding capacity (WHC)]. The responses of soil gross N transformation rates to soil water content changes, and the mechanisms of N production, consumption, and conservation in soil under different water conditions and its environmental effects were elucidated. The results showed that changes of soil water content did not affect gross rates of N mineralization and NH4+ immobilization in the forest land and grassland soils. Increasing soil water content from 60% WHC to 100% WHC significantly increased soil gross nitrification rate in forest land soil, but not affect that in grassland soil. Gross denitrification rates in grassland and forest land soils were close to zero under 60% WHC, and significantly increased under 100% WHC. The rate in grassland soil was significantly lower than that in forest land soil. In forest land soil, the ratio of gross nitrification to NH4+ immobilization rates (gn/ia) and N2O emissions under 100% WHC were significantly higher than that under 60% WHC. N2O emission from grassland soil under 100% WHC was significantly higher than that under 60% WHC, but without significant difference in gn/ia between the two water conditions. Our results indicated that increasing soil water content in the short-term may increase the negative environmental effects of nitrogen cycling in grassland and forest land soils, especially in forest land soil.

Denitrification Rates and nirS-type Denitrifying Bacteria Community Structure Characteristics of Bulk and Rhizosphere Soil in Spring and Summer in the Alpine Wetlands of the Qinghai-Tibet Plateau

Denitrification is a key process in the nitrogen cycle of ecosystems. Most existing studies of nitrogen emissions and denitrifying bacterial communities are carried out in ecosystems with significant human interference, yet few focus in natural ecosystems with low human disturbance. Here, the denitrification rates and characteristics of nirS-type denitrifying bacterial communities in rhizosphere and bulk soils from alpine wetland plants at different altitudes(Tangke, Jiuzhi, Maduo, and Dari) and seasons(spring and summer) in the Yellow River source region of the Qinghai-Tibet Plateau were investigated. The 15N isotope tracer technique was used to estimate the denitrification rates, and high-throughput sequencing technology was used to determine the composition and relative abundance of nirS-type denitrifying bacterial communities. We also investigated the environmental factors(temperature and altitude) and soil physical and chemical properties(pH, soil organic carbon, ammonia, nitrate, and nitrite) controlling the denitrification and related microorganisms. The results show that the denitrification rates of alpine wetland soils ranged from 0.80 to 14.98 nmol·(g·h)-1, and the contribution to the total N2 production ranged from 11.23% to 71.16%. The soil samples from Tangke, Jiuzhi, and Dari showed higher denitrification rates in rhizosphere soils than the corresponding bulk soils(P<0.05). Proteobacteria was the most dominate denitrifying bacteria phylum. At the genus level, unclassified Proteobacteria(2.86%-29.41%) showed the highest relative abundance, indicating that unique unidentified bacteria may dominate denitrification in these wetland soils. The genera with the next highest relative abundances were Pseudomonas(2.45%-26.52%) and Cupriavidus(0%-34.14%). Distance-based redundancy analysis showed that the community structure of the nirS-type denitrifying bacteria was mainly affected by altitude, pH, and nitrite concentrations; Pearson correlation analysis showed that denitrification rates and the Shannon index are significantly negatively correlated with soil pH(P<0.05), and the relative abundance of the main denitrifying bacterial genera were influenced by temperature and soil pH(P<0.05). This study provides valuable insights for understanding the nitrogen cycle in the unique alpine wetlands of the Qinghai-Tibet Plateau.

Soil Characteristics and Hydromorphological Patterns Control Denitrification at the Floodplain Scale

Nitrate pollution in aquatic ecosystems is still a major problem in Germany. There is a great potential to permanently remove nitrate from aquatic systems through denitrification as a relevant ecosystem function. However, the controlling factors and the dimension of the denitrification potential are still not fully understood due to the high complexity of the process. This study presents the combined assessment of potential soil denitrification rates, physical and chemical soil parameters, and hydrological parameters from six floodplains of four large German rivers, namely the Rhine, the Elbe, the Weser, and the Main. Based on multivariate statistics, results show that the denitrification potential of soil was almost solely controlled by soil pH. The lab assays showed mean soil denitrification potentials of 6.4–11.4 mg N m−2h−1(pH &amp;lt; 7) and 23.0–30.5 mg N m−2h−1(pH &amp;gt; 7). We contend that when upscaling these estimates to annual rates of potential denitrification, the duration of average inundation should be incorporated, as this accounts for water saturation and nutrient supply − the major controlling variables for denitrification. Results provide evidence that the denitrification potential can only be fully exploited in frequently inundated floodplains. Thus, despite favorable soil conditions for denitrification, floodplains that have suffered from anthropogenic impacts, lose their importance in nitrate removal for the river system. We conclude that pH and lateral hydrological connectivity are likely to be key factors that should be considered when estimating denitrification as an ecosystem function.

Anaerobic ammonium oxidation (anammox) is the main microbial N loss pathway in alpine wetland soils of the Qinghai-Tibet Plateau

Anaerobic ammonium oxidation (anammox) is an important process in the biogeochemical cycle of nitrogen (N). However, knowledge remains limited on the anammox bacterial community, controlling factors, anammox rates and contributions to N loss in alpine wetland ecosystems with low temperatures and deficient oxygen concentrations. This study focused on the distribution, abundance and biodiversity of anammox bacteria and their contribution to N2 production in the rhizosphere and bulk soils of alpine wetlands from the Qinghai-Tibet Plateau along an altitude gradient and discussed their potential relationship with environmental variables. A high diversity of anammox bacteria was found in alpine wetland soils, with most of the detected anammox bacteria classified as unknown species. The anammox bacterial abundance based on the 16S rRNA gene ranged from 1.35 × 103 to 2.52 × 104 copies·g−1 in all the samples, and the anammox process generally contributed 62.66%–88.77% of the N2 production in rhizosphere and bulk soils among the sites at different altitudes, except for the summer bulk soil in Jiuzhi (30.52%) and Maduo (28.84%). The anammox bacterial community was closely linked to the NO2− concentration and water content, while the abundance and activity of anammox bacteria showed a significantly positive correlation with the NO3− concentrations and denitrification rates in both bulk and rhizosphere soils (p < 0.05). These results emphasized the key role of anammox in N cycling in the high-altitude alpine wetlands of Qinghai-Tibet, and extended our understanding of the community dynamics of anammox bacteria and their environmental importance.

Effects of Sea Animal Activities on Tundra Soil Denitrification and nirS- and nirK-Encoding Denitrifier Community in Maritime Antarctica.

In maritime Antarctica, sea animals, such as penguins or seals, provide a large amount of external nitrogen input into tundra soils, which greatly impact nitrogen cycle in tundra ecosystems. Denitrification, which is closely related with the denitrifiers, is a key step in nitrogen cycle. However, effects of sea animal activities on tundra soil denitrification and denitrifier community structures still have received little attention. Here, the abundance, activity, and diversity of nirS‐ and nirK-encoding denitrifiers were investigated in penguin and seal colonies, and animal-lacking tundra in maritime Antarctica. Sea animal activities increased the abundances of nirS and nirK genes, and the abundances of nirS genes were significantly higher than those of nirK genes (p < 0.05) in all tundra soils. Soil denitrification rates were significantly higher (p < 0.05) in animal colonies than in animal-lacking tundra, and they were significantly positively correlated (p < 0.05) with nirS gene abundances instead of nirK gene abundances, indicating that nirS-encoding denitrifiers dominated the denitrification in tundra soils. The diversity of nirS-encoding denitrifiers was higher in animal colonies than in animal-lacking tundra, but the diversity of nirK-encoding denitrifiers was lower. Both the compositions of nirS‐ and nirK-encoding denitrifiers were similar in penguin or seal colony soils. Canonical correspondence analysis indicated that the community structures of nirS‐ and nirK-encoding denitrifiers were closely related to tundra soil biogeochemical processes associated with penguin or seal activities: the supply of nitrate and ammonium from penguin guano or seal excreta, and low C:N ratios. In addition, the animal activity-induced vegetation presence or absence had an important effect on tundra soil denitrifier activities and nirK-encoding denitrifier diversities. This study significantly enhanced our understanding of the compositions and dynamics of denitrifier community in tundra ecosystems of maritime Antarctica.

Investigation of Fe(II) and Mn(II) involved anoxic denitrification in agricultural soils with high manganese and iron contents

Soil denitrification rates and the roles Mn(II) and Fe(II) played as alternative electron donors were investigated by performing anoxic incubation experiments, using agricultural soils from Guangxi Autonomous Region, South China. The dynamic of Mn(II) and Fe(II), electron contributions of Mn(II) and Fe(II) to denitrification, potential denitrification capacity, and gaseous-N composition were investigated. The results indicated that Mn(II) and Fe(II) act as electron donors during the denitrification of paddy soils with high Mn and Fe contents. Coupling of denitrification and Fe(II) oxidation and coupling of denitrification and Mn(II) oxidation was found in paddy soils from the villages of Xialei. The maximum electron contributions of Mn(II) and Fe(II) to denitrification were similar, both reaching 18%, although the total Mn content was lower than the total Fe content. On the contrary, the roles and contributions of Mn(II) and Fe(II) to denitrification in soils from upland fields were not proved, even though all the studied agricultural soils had high Mn and Fe contents. The results suggested that Mn(II) is an important but previously ignored electron donor for denitrification and that Mn(II) plays a vital role in denitrification in paddy soils.

Variation in surface and subsurface nitrogen cycling in headwater floodplain wetlands due to soil type and wetland condition

Headwater wetlands intercept, store, and transform nitrogen inputs from the landscape. However, uncertainties in rate estimates for processes such as denitrification in surface and subsurface soils hinder our ability to quantitatively predict the downstream water quality benefits of these headwater systems. We measured nitrogen pools and fluxes from surface (0 to 10 cm) and subsurface soils (~ 50 cm) of six headwater wetlands of low and high ecological condition (based on anthropogenic stressors) across three hydric soil types (Atkins, Holly, Udifluvent-Dystrochrept Complex). We quantified net nitrification (− 0.027 to 0.113 mg N kg soil−1 day−1), net ammonification (− 0.159 to 1.683 mg N kg soil−1 day−1), and potential denitrification rates (0.12 to 17.05 mg N kg soil−1 day−1) in surface soils during the fall. We also quantified potential denitrification rates in subsurface soils at 4 of the 6 sites in fall and spring (0.005 to 0.177 mg N kg soil−1 day1). Potential denitrification rates in surface soils varied with soil type, and rates were positively correlated with ground cover and the percentage of time the water level was between 0 and 10 cm below the surface. Potential denitrification rates of subsurface soil did not vary between condition groups or seasons; however, nitrous oxide yield was higher in the spring compared to the fall. At the plot scale, potential denitrification rates in surface soils were correlated with ammonification rates, and rates in subsurface soils were correlated with conductivity and ammonium levels. This work demonstrates the variability in surface and subsurface soil nitrogen cycling and highlights the need to investigate nutrient dynamics at multiple soil depths to accurately quantify the role of headwater wetlands in nitrogen removal at regional scales where soil properties are highly variable.

Responses of Denitrification Rate and Denitrifying Bacterial Communities Carrying nirS and nirK Genes to Grazing in Peatland

Improving knowledge on how the denitrifier community responds to grazing activity is essential to better understanding whether peatland nitrous oxide (N2O) emissions will react to grazing. To quantify the effects of grazing type on the denitrifier community, an experiment was conducted to determine how grazing types (pig uprooting, PU; yak tramping, YT) influence denitrification and the bacterial community in peatland. Soil samples were collected from control and two grazing types in the peatland of Southwest China. Soil physicochemical properties, denitrification rate (DR), and reductase assay activities were analyzed. Molecular methods were applied to determine the community of nirS and nirK genes. The relationship among DR, physicochemical properties, enzymatic activity, and denitrifier community was analyzed by Pearson correlation analysis, structural equation modeling (SEM), and canonical correspondence analysis (CCA). YT had a more significant influence on the ACE index, while PU affected the diversity index obviously. Grazing reduced unclassified Betaproteobacteria but increased unclassified Proteobacteria for nirS, while increased Bradyrhizobiaceae and decreased Phyllobacteriaceae for nirK. nirK was more sensitive to grazing disturbances than nirS. YT increased Bradyrhizobium and unclassified Proteobacteria, but Hyphomicrobiaceae disappeared in YT, and a reverse trend was observed in PU. Unclassified Chloroflexi and Nitrosomonadaceae were new denitrifier species that appeared in PU. Pig uprooting has a greater potential effect on denitrification than yak tramping in the peatland ecosystem. The alteration of soil environmental conditions is the dominant mechanism by which grazing type influences soil denitrification rate and the denitrifier community.

Spartina alterniflora invasion impacts denitrifying community diversity and functioning in marsh soils

This study aimed to investigate soil denitrifying bacterial and fungal community structures and potential denitrification rates in the coastal marsh soils of Eastern China across Spartina alterniflora invasive gradients (i.e. native (Nat), native-Spartina coexisted (Ecot) and Spartina-dominated (Mono) vegetations, respectively). Plant biomass and soil chemical properties were also analyzed to understand the patterns of spatial variability of structure and function of denitrifier communities across the studied gradients. Results indicated that total, bacterial and fungal potential denitrification rates (TDR, BDR and FDR) were 5.5, 3.5 and 1.5 times greater respectively in the Mono soil than in the Nat soil. Denitrifying bacterial richness, evenness and α-diversity were 2.0, 1.3 and 1.3 times greater in the Mono soil than those in the Nat soil, while only the richness among denitrifying fungal community parameters was 1.4 times greater in the Mono soil than that in the Nat soil. Plant biomass, soil organic carbon and pH were the most important factors influencing denitrifying bacterial and fungal community structures and functions. Denitrifying bacterial community composition was related closely to BDR and TDR, while denitrifying fungal community composition was not related to FDR and TDR, thus showing that the denitrifying bacterial community is a better predictor of denitrification function following Spartina invasion.