Denitrification Enzyme Activity Research Articles

Newly plant-derived carbon in the deeper vadose zone of a sandy agricultural soil does not stimulate denitrification

Analysis of stable carbon (C) isotopic signatures (δ13C) in various soil C pools provides useful information on soil C sources, transport, and availability. Understanding the extent of deeper soil (below 40 cm) sequestration and transport of plant derived C is of particular interest as this provides a source for microorganisms to drive biological denitrification (as indicated by denitrification enzyme activity, DEA) and hence mitigate the nitrate leaching to groundwater. Meanwhile, studies on deeper soil C sequestration are rare due to methodological constraints. This study was done in deeper vadose zone (0–160 cm) of a sandy agricultural soil in a humid and temperature zone after a C3-C4 vegetation change taking advantage of the marked isotopic differences between C3 and C4 plants. It took place in a site previously grown with C3 crops (beet, barley, grass), but where C4 crops (maize) were grown continuously for the last 20 years. The other site where C3 crops were continuously grown was used for comparison. Specifically, the δ13C signature in top and deeper soil layers was used to distinguish between old C3– and newly C4-plant derived C in four C pools, i.e., bulk soil C, hot- and cold-water extractable C, and respired CO2-C. The δ13C signature between C3 soils and C3-C4 shifted soils was similar for bulk soil C but significantly different for water extractable and respired C pools. Hence, we estimated that the contribution of newly derived C to bulk soil C was negligible, whereas the contributions to the other C pools amounted up to 28.4 % along the soil profile. This emphasizes the importance of simultaneously analysing δ13C signature in various soil C pools to accurately assess C vertical transport and distribution. The concentrations of cold-DOC and values of specific ultraviolet visible absorbance of the wavelengths 254 and 280 nm decreased from 50 to 130 cm soil depths, while they increased below these depths. However, this suggested rise in C chemical quality at the deepest soil depths did not cause an increase in soil respiration activity or DEA, which was attributed to the protective effects of iron and aluminium oxides on C decomposition. Upon the application of labile C and N substrates, the deepest soil layers displayed a significantly increased DEA, suggesting the presence of a relatively abundant population of active denitrifying organisms. Overall, this study documents the presence of plant-derived C in the deeper vadose zone. Meanwhile, this particular C pool might not be an important substrate to drive deep-soil denitrification due to constraints imposed by the protection by metal oxides.

Straw management adopted by large farms sustains grain yield but mitigates greenhouse gas emissions

ContextStraw incorporation is a globally recommended strategy for enhancing soil fertility. The methods for straw incorporation adopted by large farms usually differ from those used by small households, primarily because of variations in cost affordability. However, the effects of straw management on rice yields and greenhouse gas (GHG) emissions have not been well explored at different farming scales. ObjectivesThis study aims to compare the grain yield and greenhouse gas emissions under different straw management adopted by large farms than small households. MethodsA three-year field experiment using split-plot design, with straw management as the main plot and preceding tillage as the sub-plot, was conducted in a wheat-rice rotation system. Four treatments were given as rotary tillage with straw removal (RT-S), rotary tillage with straw incorporation (RT+S; small households), ploughing tillage with straw removal (PT-S), and ploughing tillage with straw incorporation (PT+S, large farms) in the preceding wheat season. Soil tillage in the rice season was the same, which was rotary tillage after irrigation. ResultsStraw incorporation significantly increased methane (CH4) and nitrous oxide (N2O) emissions, and 60%-62% of CH4 was emitted during the transplanting to jointing stages while 45–73% of N2O was emitted during the jointing to the booting stage. The stimulation of CH4 and N2O emissions by straw incorporation was closely correlated to enhanced soil organic carbon (SOC) and denitrification enzyme activity, respectively. In the last year, PT+S significantly reduced the global warming potential (GWP) compared to RT+S in the whole rice season, which can be attributed to the dramatically decline of CH4 emission, even though N2O emission was increased. Additionally, the decreased CH4 emission was mainly due to lower methanogenic activity, and the increased N2O emission was mainly resulted from lower nosZ abundance under PT+S compared to RT+S. Straw incorporation increased rice yield by 15.73% on average in the last two years, and rice yield of PT+S in the last year was significantly enhanced by 27.50%, resulting in a lower greenhouse gas intensity (GHGI) by 25.54% compared to RT+S. ConclusionStraw management practices adopted by large farms (PT+S) have the potential to synchronize high rice yields with lower GHG emissions compared to small households (RT+S). ImplicationsOur findings provide some evidence that large-scale agricultural organization may be helpful for sustainable rice production in China.

Repeated Solid Digestate Amendment Increases Denitrifying Enzyme Activity in an Acid Clayey Soil

The use of organic fertilizers to replace chemically synthesized fertilizers has assumed an important role in managing plant nutrition and soil fertility. The various organic matrices currently available as organic byproducts and digestates are relatively abundant and have shown promising effects in terms of plant-available nutrients. However, like mineral fertilizers, organic fertilizers must be carefully managed to avoid negative effects on the environment, especially when they are repeatedly applied over time. The aim of the present study was to assess the effect of the single (DIG) and repeated application (DIGP) of solid anaerobic digestates compared to an unamended control (CTR) on the denitrifying enzymatic activity (DEA), which is responsible for nitrous oxide emissions into the atmosphere, and some related soil properties, such as total soluble nitrogen (TSN), nitrate (NO3−-N), extractable carbon (Cextr), microbial biomass carbon (MBC), and basal respiration (Rbas), for a period of ~3 months after application. The application of solid anaerobic digestates progressively boosts N and C concentrations in the soil, with the degree of enhancement directly correlated with the frequency of application over the sampling period. Depending on the textural properties of soils, there was a notable rise in denitrification enzyme activity (DEA), particularly during the DIGP treatment, suggesting that clay soils are highly susceptible to denitrification under suitable conditions. The results of this study recommend the careful management of soils subjected to repeated digestate amendment to prevent the occurrence of conditions conducive to denitrification and the promotion of N2O emissions.

Efficacy of grape seed procyanidins for inhibiting denitrification varies by source, soil texture, and cropping history

Plant-derived denitrification inhibitors, such as procyanidins derived from grape seed extract (GSE), hold potential in mitigating nitrous oxide (N2O) emissions, a potent greenhouse gas and stratospheric ozone depleter. A deeper understanding of the effects of GSE – considering aspects like rate, source, application timing, storage, and interactions with soil properties and cropping histories – on denitrification inhibition remains elusive. We conducted a series of laboratory microcosm experiments to assess these effects using denitrification enzyme activity (DEA) and measurements of N2O and carbon dioxide (CO2) production rates under both anaerobic and aerobic conditions, soil inorganic nitrogen (N), and other major soil properties. Our results revealed that one commercial GSE brand (USP GSE) inhibited DEA, while another source was ineffective. Two non-procyanidin compounds in commercial GSE mixtures, tartaric acid and gallic acid, actually enhanced DEA. Inhibition efficiency of the USP GSE varied significantly across soils with differing crop histories and physical properties, exhibiting a positive dose-response in three out of the five soils examined. Greater efficiency was observed in sandy soils compared to silt loam soils and in soils with a history of continuous corn production as opposed to soybean. The GSE application timing relative to urea addition and a one-week room temperature storage had minor effects on denitrification inhibition. In corn-cultivated soil, co-application of GSE and urea reduced cumulative N2O production by 27 % compared to soils treated with urea alone. The CO2 production and nitrate levels remained largely unaffected by GSE application. These findings supported the potential of GSE as a soil amendment to reduce N2O emissions in corn-cultivated soils. However, challenges including purity of procyanidins, cost-effectiveness, and inconsistent performance under various systems warrant consideration for broader field applications.

Unraveling the impact of human perturbation on nitrogen cycling in terrestrial ecosystems of lower Himalaya, Pakistan.

Terrestrial ecosystems are under the enormous pressure of land use management regimes through human disturbances, resulting in the disruption of biogeochemical cycles and associated ecosystem services. Nitrogen (N) in soil ecosystems is of vital importance for primary productivity, hence estimating the extent of these human interventions on N-cycling processes becomes imperative from economic and environmental perspectives. This work investigated the impacts of variable anthropogenic activities on N cycling in three different terrestrial ecosystems (arable, grassland, and forest) in three regions of lower Himalaya, Pakistan. Potential nitrification (PNA) and denitrification (DEA) enzyme activities, relative distribution of inorganic N species (NH4, NO3), and the role of inherent edaphic factors were assessed. Results revealed high nitrification potentials and increased nitrous oxide (N2O) emissions in the incubated soil microcosms, in the order as arable > grassland > forest ecosystems. Notably, higher rates of both studied processes (~ 30-50%) and elevated soil mineral nitrogen pool were observed in arable ecosystems. Forest soils, assumed as pristine ecosystems relying mainly on natural N fixation, produced (de)nitrification rates relatively lower than grasslands, followed by arable soils which were moderately disturbed through long-term fertilization and intensive land-use regimes. Linear regression modeling revealed that the inorganic N species (particularly NO3), and inherent edaphic factors were the key determinants of high (de)nitrification rates, hence warn of accelerated N losses in these ecosystems. The study highlights that elevated PNA and DEA being proxies for the altered N cycling in the studied terrestrial ecosystems are of great ecological relevance in view of predicted N2O budget in the lower Himalaya.

Exploring the Use of Living Shorelines for Stabilization and Nutrient Mitigation in New England.

As salt marsh habitats face challenges due to sea level rise, storm events, and coastal development, there is an effort to use nature-based approaches such as living shorelines to enhance salt marshes and provide coastal protection. A living shoreline restoration and seasonal monitoring was conducted between July 2016 and October 2018 at an eroding salt marsh on Martha's Vineyard, Massachusetts, Northeastern USA to assess changes in two essential ecosystem services: shoreline stabilization and nitrogen removal. Neither the living shoreline nor unaltered sites demonstrated significant sediment deposition at the marsh edge or on the marsh platform between 2017 and 2018. While we expected nitrogen removal via denitrification to improve at the living shoreline sites over time as abiotic and biotic conditions became more favorable, we found limited support for this hypothesis. We found higher rates of denitrification enzyme activity (DEA) at the living shoreline sites when compared to unaltered sites, but these rates did not increase over time. This study also provides a qualitative assessment of our living shoreline structural integrity through the years, particularly following storm events that greatly challenged our restoration efforts. We demonstrate that living shorelines fortified solely with natural materials may not be the most effective approach to maintain these ecosystem services for Northeastern USA salt marshes exposed to intense northeasterly storms. We suggest the restoration of salt marshes to improve major functions be a priority among managers and restoration practitioners. Initiatives promoting the use of nature-based restoration solution where environmental conditions permit should be encouraged.

Efficient aerobic denitrification without nitrite accumulation by Pseudomonas mendocina HITSZ-D1 isolated from sewage sludge

A highly efficient aerobic denitrifying microbe was isolated from sewage sludge by using a denitrifier enrichment strategy based on decreasing carbon content. The microbe was identified as Pseudomonas mendocina HITSZ-D1 (hereafter, D1). Investigation of the conditions under which D1 grew and denitrified revealed that it performed good growth and nitrate removal performance under a wide range of conditions. In particular, D1 rapidly removed all types of inorganic nitrogen without accumulation of the intermediate products nitrite and nitrous oxide. Overall, D1 showed a total nitrogen removal efficiency >96% at a C/N ratio of 8. The biotransformation modes and fates of three typical types of inorganic nitrogen were also assessed. Moreover, D1 had significantly higher denitrification efficiency and enzyme activities than other aerobic denitrifying microbes (Paracoccus denitrificans, Pseudomonas aeruginosa, and Pseudomonas putida). These results suggest that D1 has great potential for treating wastewater containing high concentrations of nitrogen.

Integrating electrochemical pre-treatment with carrier-based membrane bioreactor for efficient treatment of municipal waste transfer stations leachate

An integrated process of electrochemical pre-treatment with carrier-based membrane bioreactor (MBR) was constructed for fresh leachate from waste transfer stations with high organic and NH4+-N content. Results showed that within a hydraulic retention time 40 h, the removal efficiencies of chemical oxygen demand (COD), NH4+-N, suspended solids (SS) and total phosphorus (TP) were over 98.5%, 91.2%, 98.3% and 98.4%, respectively, with the organic removal rate of 18.7 kg/m3. The effluent met the Grade A Standard of China (GB/T31962-2015). Pre-treatment contributed about 70 % of the degraded refractory organics and almost all the SS, with the transformation of the humic-like acid to readily biodegradable organics. Biotreatment further removed over 50% of nitrogen pollutants through simultaneous nitrification and denitrification (SND) and consumed about 30% of organics. Meanwhile, the addition of carriers in the oxic MBR enhanced the attached biomass and denitrification enzyme activity, alleviating membrane fouling.

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.

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(>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.

Vegetation zones as indicators of denitrification potential in salt marshes.

Salt marsh vegetation zones shift in response to large‐scale environmental changes such as sea‐level rise (SLR) and restoration activities, but it is unclear if they are good indicators of soil nitrogen removal. Our goal was to characterize the relationship between denitrification potential and salt marsh vegetation zones in tidally restored and tidally unrestricted coastal marshes, and to use vegetation zones to extrapolate how SLR may influence high marsh denitrification at the landscape scale. We conducted denitrification enzyme activity assays on sediment collected from three vegetation zones expected to shift in distribution due to SLR and tidal flow restoration across 20 salt marshes in Connecticut, USA (n = 60 sampling plots) during the summer of 2017. We found lower denitrification potential in short‐form Spartina alterniflora zones (mean, 95% CI: 4, 3–6 mg N h−1 m−2) than in S. patens (25, 15–36 mg N h−1 m−2) and Phragmites australis (56, 16–96 mg N h−1 m−2) zones. Vegetation zone was the single best predictor and explained 52% of the variation in denitrification potential; incorporating restoration status and soil characteristics (soil salinity, moisture, and ammonium) did not improve model fit. Because denitrification potential did not differ between tidally restored and unrestricted marshes, we suggest landscape‐scale changes in denitrification after tidal restoration are likely to be associated with shifts in vegetation, rather than differences driven by restoration status. Sea‐level‐rise‐induced hydrologic changes are widely observed to shift high marsh dominated by S. patens to short‐form S. alterniflora. To explore the implications of this shift in dominant high marsh vegetation, we paired our measured mean denitrification potential rates with projections of high marsh loss from SLR. We found that, under low and medium SLR scenarios, predicted losses of denitrification potential due to replacement of S. patens by short‐form S. alterniflora were substantially larger than losses due to reduced high marsh land area alone. Our results suggest that changes in vegetation zones can serve as landscape‐scale predictors of the response of denitrification rates to rapid changes occurring in salt marshes.

Fungi and Archaea Control Soil N2O Production Potential in Chinese Grasslands Rather Than Bacteria.

Nitrous oxide (N2O) is a powerful greenhouse gas and the predominant stratospheric ozone-depleting substance. Soil is a major source of N2O but remains largely uncertain due to the complicated processes of nitrification and denitrification performed by various groups of microbes such as bacteria, fungi, and archaea. We used incubation experiments to measure the total fungal, archaeal, and bacterial N2O production potential and the microbial functional genes in soils along 3,000 km Chinese grassland transect, including meadow steppe, typical steppe, desert steppe, alpine meadow, and alpine steppe. The results indicated that fungi, archaea, and bacteria contributed 25, 34, and 19% to nitrification and 46, 29, and 15% to denitrification, respectively. The AOA and AOB genes were notably correlated with the total nitrification enzyme activity (TNEA), whereas both narG and nirK genes were significantly correlated with total denitrification enzyme activity (TDEA) at p < 0.01. The correlations between AOA and ANEA (archaeal nitrification enzyme activity), AOB and BNEA (bacterial nitrification enzyme activity), and narG, nirK, and BDEA (bacterial denitrification enzyme activity) showed higher coefficients than those between the functional genes and TNEA/TDEA. The structural equation modeling (SEM) results showed that fungi are dominant in N2O production processes, followed by archaea in the northern Chinese grasslands. Our findings indicate that the microbial functional genes are powerful predictors of the N2O production potential, after distinguishing bacterial, fungal, and archaeal processes. The key variables of N2O production and the nitrogen (N) cycle depend on the dominant microbial functional groups in the N-cycle in soils.

Mineralizable nitrogen and denitrification enzyme activity drive nitrate concentrations in well-drained stony subsoil under lucerne (Medicago sativa L.)

Nitrogen (N) inputs to agricultural systems contribute substantially to soil nitrate (NO3−) concentrations, which increase NO3− leaching and contamination of groundwater. The influence of soil microbes in regulating NO3− concentrations in the topsoil are well studied but it is often assumed that microbial regulation of NO3− concentrations in the subsoil is negligible. The aim of this study was to test this assumption by determining the relationships between microbial properties and NO3− concentrations in both the subsoil and the topsoil. We measured the size of the mineralizable N (Nm) pool, microbial properties (microbial biomass, bacterial richness), nitrifier gene abundance (amoA gene copy number), denitrifier gene abundance (nirK and nirS gene copy number), denitrifier enzyme activity and NO3− concentrations in the topsoil and the subsoil in a well-drained stony soil under an established lucerne crop. We used structural equation modelling (SEM) to identify and compare the linkages of microbial properties with NO3− concentrations at each depth. In the topsoil, we found higher Nm, gene abundance, denitrification enzyme activity, bacterial richness, and microbial biomass than those in the subsoil, but there were no relationships between these variables and NO3− concentrations in the topsoil (the SEM model explained 0.06% of the variability in NO3− concentrations). In contrast, in the subsoil, NO3− concentrations were strongly correlated with bacterial amoA abundance and denitrification enzyme activity, with both variables associated significantly with Nm. We found that bacterial richness was also associated with Nm in the subsoil. Our findings highlight that microbial properties are associated with NO3− concentrations in the subsoil (the SEM model explained 82% the variability in NO3− concentrations) and this suggest that nitrification and denitrification may contribute to regulating NO3− concentrations in the subsoil. Our findings also suggest that denitrification contributes to reducing NO3− concentrations in the subsoil. We conclude that studies addressing drivers of NO3− leaching need to consider the potential for microbially-mediated attenuation (or an increase) in NO3− concentrations throughout the soil profile.

Analysis of denitrification performance and microbial community structure in a bio-electrochemical reactor under different current densities with wheat-rice stone powder

Abstract Nitrate widely exists in water pollution and is the most stable form of nitrogen pollution. This study investigated the effect of the current density (CD) and the wheat-rice stone powder (WP) on denitrification performance, microbial diversity and enzyme activity in a bio-electrochemical reactor (BER). It was found that an optimum CD of 200 mA/m2 and the addition of WP significantly improved the nitrate removal rate constant compared with the control group (12.28 d−1 versus 9.75 d−1) and remarkably reduced the intermediate accumulation of nitrite. The application of both optimum CD and WP enhanced the microbial diversity and catalytic activity of nitrate reductase (Nar) and nitrite reductase (Nir). The most dominant microbial taxa in our reactor were Methyloversatilis, Methylotenera, and an unclassified genus of the family Methylophilaceae. Moreover, WP allowed the denitrifiers to better withstand the stress of high CD. This study presented results supporting the use of an optimum CD and natural mineral addition to improving the performance of the denitrification process within a BER.

Microbial community structure from southern High Plains beef cattle feedyard manure and relationship with nitrous oxide emissions

AbstractModern molecular techniques enable characterization of the microbial biome in livestock manure, from which there is particular concern over emission of greenhouse gases. This study evaluated how sampling depth, time, temperature, and artificial rainfall affected microbial community structure in feedyard manure, and relationships between the manure biome and known parameters related to nitrous oxide (N2O) emissions. In three large incubation chambers, maintained at different temperatures that received two applications of artificial rainfall, we evaluated manure microbiome composition and abundance of N2O‐producing enzymes (nirK and nirS) using quantitative polymerase chain reaction (qPCR). These data were used with previously published data from the same study on N2O emissions and assessment of manure physicochemical properties, denitrification enzyme activity (DEA), and nitrification activity (NA). Microbiome composition was Firmicutes (50%), followed by 32% Actinobacteria, 11% Proteobacteria, 5% Bacteroidetes, 1% Chloroflexi, and small populations (&lt;0.5%) of Planctomycetes, Deinococcus‐Thermus, Gemmatimonadetes, Verrucomicrobia, Tenericutes, and other organisms. Average bacterial populations varied largely as a function of sampling depth and time. Firmicutes increases tended to coincide with high N2O emissions. Overall, the largest change observed was increased Proteobacteria at 5–10 cm, where relative abundance increased from 10% (17.2 °C) to 24% (46.2 °C) over time and with increased temperature. Firmicutes and Actinobacteria predominated the microbial community of manure, but favorable conditions may lead to increases in Bacteroidetes, Proteobacteria, and Chloroflexi, which could influence N cycling and N2O emissions from feedyards. Copy numbers of nirS at the beginning of the experiment were higher than nirK. Differences in concentrations of nirK and nirS indicated that denitrifying enzymes in feedyard manure, particularly nirS, were sensitive to environmental changes.

Seasonal changes in the abundance and activity of bacterial and fungal denitrifying communities associated with different compost amendments.

Composts can be efficient organic amendments in potato culture as they can supply carbon and nutrients to the soil. However, more information is required on the effects of composts on denitrification and nitrous oxide emissions (N2O) and emission-producing denitrifying communities. The effects of three compost amendments (municipal source separated organic waste compost (SSOC), forestry waste mixed with poultry manure compost (FPMC), and forestry residues compost (FRC)) on fungal and bacterial denitrifying communities and activity was examined in an agricultural field cropped to potatoes during the fall, spring, and summer seasons. The denitrification enzyme activity (DEA), N2O emissions, and respiration were measured in parallel. N2O emission rates were greater in FRC-amended soils in the fall and summer, whereas soil respiration was highest in the SSOC-amended soil in the fall. A large number of nirK denitrifying fungal transcripts were detected in the fall, coinciding with compost application, while the greatest nirK bacterial transcripts were measured in the summer when plants were actively growing. Denitrifying community and transcript levels were poor predictors of DEA, N2O emissions, or respiration rates in compost-amended soil. Overall, the sampling date was driving the population and activity levels of the three denitrifying communities under study.

Interactive regulation of root exudation and rhizosphere denitrification by plant metabolite content and soil properties

AimsRoot exudates are known to shape microbial activities in the rhizosphere and to be of fundamental importance for plant-soil-microbe-carbon–nitrogen interactions. However, it remains unclear how and to what extent the amount and composition of root exudation affects rhizosphere denitrification.MethodsIn this study root exudation patterns and rhizosphere denitrification enzyme activity of three different grass species grown on two agricultural soils under two different soil water contents were investigated under controlled conditions.ResultsWe found that root exudation of primary metabolites largely depends on plant species, soil type, soil moisture and root exudation medium. In dependence of soil properties and soil moisture levels, plants largely controlled amount and quality of root exudation. Exudates affected denitrification activity and plant–microbe competition for nitrate. Specifically, exudation of organic acids stimulated denitrifying activity while the sugar lyxose exhibited an inhibitory effect.ConclusionWe show that interactive effects of physicochemical soil properties and species-specific effects of plant metabolism on root exudation act as a dominant control of rhizosphere denitrification, thereby explaining more than half of its variance.

Riparian Forest Cover Modulates Phosphorus Storage and Nitrogen Cycling in Agricultural Stream Sediments.

Watershed land cover affects in-stream water quality and sediment nutrient dynamics. The presence of natural land cover in the riparian zone can reduce the negative effects of agricultural land use on water quality; however, literature evaluating the effects of natural riparian land cover on stream sediment nutrient dynamics is scarce. The objective of this study was to assess if stream sediment phosphorus retention and nitrogen removal varies with riparian forest cover in agricultural watersheds. Stream sediment nutrient dynamics from 28 sites with mixed land cover were sampled three times during the growing season. Phosphorus dynamics and nitrification rates did not change considerably throughout the study period. Sediment total phosphorus concentrations and nitrification rates decreased as riparian forest cover increased likely due to a decline in fine, organic material. Denitrification rates were strongly correlated to surface water nitrate concentrations. Denitrification rate and denitrification enzyme activity decreased with an increase in forest cover during the first sampling period only. The first sampling period coincided with the greatest connectivity between the watershed and in-stream processing, indicating that riparian forest cover indirectly decreased denitrification rates by reducing the concentrations of dissolved nutrients entering the stream. This reduction in load may allow the sediment to maintain greater nitrogen removal efficiency, because bacteria are not saturated with nitrogen. Riparian forest cover also appeared to lessen the effect of agriculture in the watershed by decreasing the amount of fine material in the stream, resulting in reduced phosphorus storage in the stream sediment.

Nitrogen species control the interaction between NO3--N reduction and aniline degradation and microbial community structure in the oxic-anoxic transition zone.

Contrary to the fact that NO3--N can serve as electron acceptor to promote organics degradation, it was also found NO3--N reduction does not necessarily promote organics degradation. We speculate nitrogen (N) species may control the interaction between NO3--N reduction and organics degradation via shifting related microbial community structure. To prove the hypothesis, oxic-anoxic transition zone (OATZ) microcosms simulated by lake water and sediment were conducted with the addition of N species (NO3--N, NO2--N, and NH4+-N) and aniline as typical organics. High-throughput sequencing was used to analyze the microbial community structure and functional enzyme in the microcosms. Results show that, NO2--N inhibited NO3--N reduction while enhanced aniline degradation. For NH4+-N, it promoted NO3--N reduction when NH4+-N/NO3--N concentration ratio ≤ 2 and inhibited aniline degradation when NH4+-N/aniline concentration ratio ≥ 0.5. The presence of NO2--N or NH4+-N weakened the interaction between NO3--N reduction and aniline degradation, which might be caused by significant changes in the diversity and abundance of microbial communities controlled by N species. The microbial mechanism indicates that NO2--N weakened the interaction by affecting both denitrification enzyme activity and electron transfer capability, while NH4+-N weakened the interaction mainly by affecting electron transfer capability. These results imply that N species, as well as other electron acceptors and donors, in the contaminated OATZ should be fully considered, when performing in situ remediation technology of NO3--N reduction.

Earthworm functional groups are related to denitrifier activity in riparian soils

Riparian buffers, located in the transition zone between terrestrial and aquatic ecosystems, are a hotspot for nitrogen (N) removal through denitrification. Earthworms are abundant in riparian buffers and may enhance denitrification. This study investigated earthworm demographics of three earthworm functional groups (anecic, epigeic, and endogeic) and denitrifier activity in temporarily flooded and non-flooded riparian soils from April to October 2012 in southern Quebec, Canada. Nine earthworm species, mostly endogeic, were found in the temporarily flooded soil, while only six earthworm species were found in the non-flooded soil. On average, there were 11.7 times more earthworms with 12.4 times greater biomass (P < 0.05) found in the temporarily flooded soil than in the non-flooded soil. The denitrification enzyme activity (DEA) was of similar magnitude in temporarily flooded and non-flooded soils, with temporal variation associated with rainfall patterns. Endogeic earthworm biomass was positively correlated (P < 0.05) with DEA, while epigeic earthworm biomass was positively correlated (P < 0.05) with 16S rRNA gene copies and nosZ gene copies from bacteria, indicating an association between earthworm functional groups and denitrifier activity in riparian soils. Stepwise multiple regressions showed that DEA in riparian soils could be predicted using soil moisture, inorganic N concentration, and earthworm functional groups, suggesting that endogeic and epigeic earthworms contributed to denitrifier activity in riparian soils.