A novel dual-isotope labelling method for distinguishing between soil sources of N2O

Plastic particles affect N2O release via altering core microbial metabolisms in constructed wetlands

Constructed wetlands (CWs) have been proven to effectively immobilize plastic particles. However, little is known about the differences in the impact of varying sized plastic particles on nitrous oxide (N2O) release, as well as the intervention mechanisms in CWs. Here, we built a lab-scale wetland model and introduced plastic particles of macro-, micro-, and nano-size at 100 μg/L for 370 days. The results showed that plastic particles of all sizes reduced N2O release in CWs, with the degrees being the strongest for the Nano group, followed by Micro and Macro groups. Meanwhile, 15N- and 18O-tracing experiment revealed that the ammoxidation process contributed the most N2O production, followed by denitrification. While for every N2O-releasing process, the contributing proportion of N2O in nitrification-coupled denitrification were most significantly cut down under exposing to macro-sized plastics and had an obvious increase in nitrifier denitrification in all groups, respectively. Finally, we revealed the three mechanism pathways of N2O release reduction with macro-, micro-, and nano-sized plastics by impacting carbon assimilation (RubisCO activity), ammonia oxidation (gene amo abundance and HAO activity), and N-ion transmembrane and reductase activities, respectively. Our findings thus provided novel insights into the potential effects of plastic particles in CWs as an eco-technology.

Ammonium-derived nitrous oxide is a global source in streams

Global riverine nitrous oxide (N2O) emissions have increased more than 4-fold in the last century. It has been estimated that the hyporheic zones in small streams alone may contribute approximately 85% of these N2O emissions. However, the mechanisms and pathways controlling hyporheic N2O production in stream ecosystems remain unknown. Here, we report that ammonia-derived pathways, rather than the nitrate-derived pathways, are the dominant hyporheic N2O sources (69.6 ± 2.1%) in agricultural streams around the world. The N2O fluxes are mainly in positive correlation with ammonia. The potential N2O metabolic pathways of metagenome-assembled genomes (MAGs) provides evidence that nitrifying bacteria contain greater abundances of N2O production-related genes than denitrifying bacteria. Taken together, this study highlights the importance of mitigating agriculturally derived ammonium in low-order agricultural streams in controlling N2O emissions. Global models of riverine ecosystems need to better represent ammonia-derived pathways for accurately estimating and predicting riverine N2O emissions.

AOB Nitrosospira cluster 3a.2 (D11) dominates N2O emissions in fertilised agricultural soils

Ammonia-oxidation process directly contribute to soil nitrous oxide (N2O) emissions in agricultural soils. However, taxonomy of the key nitrifiers (within ammonia oxidising bacteria (AOB), archaea (AOA) and complete ammonia oxidisers (comammox Nitrospira)) responsible for substantial N2O emissions in agricultural soils is unknown, as is their regulation by soil biotic and abiotic factors. In this study, cumulative N2O emissions, nitrification rates, abundance and community structure of nitrifiers were investigated in 16 agricultural soils from major crop production regions of China using microcosm experiments with amended nitrogen (N) supplemented or not with a nitrification inhibitor (nitrapyrin). Key nitrifier groups involved in N2O emissions were identified by comparative analyses of the different treatments, combining sequencing and random forest analyses. Soil cumulative N2O emissions significantly increased with soil pH in all agricultural soils. However, they decreased with soil organic carbon (SOC) in alkaline soils. Nitrapyrin significantly inhibited soil cumulative N2O emissions and AOB growth, with a significant inhibition of the AOB Nitrosospira cluster 3a.2 (D11) abundance. One Nitrosospira multiformis-like OTU phylotype (OTU34), which was classified within the AOB Nitrosospira cluster 3a.2 (D11), had the greatest importance on cumulative N2O emissions and its growth significantly depended on soil pH and SOC contents, with higher growth at high pH and low SOC conditions. Collectively, our results demonstrate that alkaline soils with low SOC contents have high N2O emissions, which were mainly driven by AOB Nitrosospira cluster 3a.2 (D11). Nitrapyrin can efficiently reduce nitrification-related N2O emissions by inhibiting the activity of AOB Nitrosospira cluster 3a.2 (D11). This study advances our understanding of key nitrifiers responsible for high N2O emissions in agricultural soils and their controlling factors, and provides vital knowledge for N2O emission mitigation in agricultural ecosystems.

Response of soil N2O production pathways to biochar amendment and its isotope discrimination methods

Reducing nitrous oxide (N2O) emission from farmland is crucial for alleviating global warming since agriculture is an important contributor of atmospheric N2O. Returning biochar to agricultural fields is an important measure to mitigate soil N2O emissions. Accurately quantifying the effect of biochar on the process of N2O production and its driving factors is critical for achieving N2O emission mitigation. Recently, stable isotope techniques such as isotope labeling, natural abundance, and site preference (SP) value, have been widely used to distinguish N2O production pathways. However, the different isotope methods have certain limitations in distinguishing N2O production in biochar-amended soils where it is difficult to identify the relative contribution of individual pathways for N2O production. This paper systematically reviews the pathways of soil N2O production (nitrification, nitrifier denitrification, bacterial denitrification, fungal denitrification, coupled nitrification-denitrification, dissimilatory nitrate reduction to ammonium and abiotic processes) and their response mechanism to the addition of biochar, as well as the development history and advantages of isotopes in differentiating N2O production pathways in biochar-amended soils. Moreover, the limitations of current research methods and future research directions are proposed. These results will help resolve how biochar affects different processes that lead to soil N2O generation and provide a scientific basis for sustainable agricultural carbon sequestration and the fulfilment of carbon neutrality goals.

Pathways and biological mechanisms of N2O emission reduction by adding biochar in the constructed wetland based on 15N stable isotope tracing

Constructed wetlands (CWs) added with biochar were built to study pollutant removal efficiencies, nitrous oxide (N₂O) emission characteristics, and biological mechanisms in nitrogen transformation. The results showed that biochar addition enhanced the average removal rates of ammonium (NH₄⁺-N), total nitrogen, and chemical oxygen demand by 4.03-18.5%, 2.90-4.99%, and 2.87-5.20% respectively while reducing N₂O emissions by 25.85-83.41%. Based on ¹⁵N stable isotope tracing, it was found that nitrification, denitrification, and simultaneous nitrification and denitrification were the main processes contributing to N₂O emission. The addition of biochar resulted in maximum reduction rates of 71.50%, 80.66%, and 73.09% for these three processes, respectively. The relative abundance of nitrogen-transforming microbes, such as Nitrospira, Dechloromonas, and Denitratisoma, increased after the addition of biochar, promoting nitrogen removal and reducing N₂O emissions. Adding biochar could increase the functional gene copy number and enzyme activity responsible for nitrogen conversion, which helped achieve efficient NH₄⁺-N oxidation and eliminate nitrite accumulation, thereby reducing N₂O emissions.

Nitrifiers Cooperate to Produce Nitrous Oxide in Plateau Wetland Sediments

The thawing of dormant plateau permafrost emits nitrous oxide (N₂O) through wetlands; however, the N₂O production mechanism in plateau wetlands is still unclear. Here, we used the ¹⁵N-¹⁸O double tracer technique and metagenomic sequencing to analyze the N₂O production mechanism in the Yunnan-Kweichow and Qinghai-Tibet plateau wetlands during the summer of 2020. N₂O production activity was detected in all 16 sediment samples (elevation 1020-4601 m: 2.55 ± 0.42-26.38 ± 3.25 ng N g^-1 d^-1) and was promoted by nitrifier denitrification (ND). The key functional genes of ND (amoA, hao, and nirK) belonged to complete ammonia oxidizing (comammox) bacteria, and the key ND species was the comammox bacterium Nitrospira nitrificans. We found that the comammox bacterial species N. nitrificans and the ammonia oxidizing bacterial (AOB) species Nitrosomonas europaea cooperate to produce N₂O in the plateau wetland sediments. Furthermore, we inferred that environmental factors (elevation and total organic matter (TOM)) influence the cooperation pattern via N. nitrificans, thus affecting the N₂O production activity in the plateau wetland sediments. Our findings advance the mechanistic understanding of nitrifiers in biogeochemical cycles and global climate change.

Microbial regulation of nitrous oxide emissions from chloropicrin-fumigated soil amended with biochar

The microbial mechanism underpinning biochar's ability to reduce emissions of the potent greenhouse gas nitrous oxide (N₂O) is little understood. We combined high-throughput gene sequencing with a dual-label ¹⁵N-¹⁸O isotope to examine microbial mechanisms operative in biochar made from Crofton Weed (BC1) or pine wood pellets (BC2) and the N₂O emissions from those biochar materials when present in chloropicrin (CP)-fumigated soil. Both BC1 and BC2 reduced N₂O total emissions by 62.9-71.9% and 48.8-52.0% in CP-fumigated soil, respectively. During the 7-day fumigation phase, however, both BC1 and BC2 increased N₂O production by significantly promoting nirKS and norBC gene abundance, which indicated that the N₂O emission pathway had switched from heterotrophic denitrification to nitrifier denitrification. During the post-fumigation phase, BC1 and BC2 significantly decreased N₂O production as insufficient nitrogen was available to support rapid population increases of nitrifying or denitrifying bacteria. BC1 and BC2 significantly reduced CP's inhibition of nitrifying archaeal bacteria (AOA, AOB) and the denitrifying bacterial genes (nirS, nirK, nosZ), which promoted those bacterial populations in fumigated soil to similar levels observed in unfumigated soil. Our study provided insight on the impact of biochar and microbes on N₂O emissions.

Effects and mechanisms of land-types conversion on greenhouse gas emissions in the Yellow River floodplain wetland

The mechanism and extent of changes in greenhouse gas (GHG) emissions from seasonal river-floodplain wetlands subjected to land-type conversion are unknown. We monitored GHG fluxes and characterized soil microbial communities in four types of wetland (Riverside lower-beach wetland (RLW), Riverside higher-beach wetland (RHW), Cultivated wetland (CW), Mesophytic wetland (MW)) in the Yellow River flood land. Results revealed that land reclamation activities altered the distribution patterns of carbon (C) and nitrogen (N) in soil, as well as the structure and activities of microbial communities, leading to changes in the GHG emissions. Cumulative CO₂ and N₂O emissions were highest in CW, which were 2.10-10.71 times and 3.19-8.61 times greater than the other three wetlands, respectively, whereas cumulative CH₄ emissions were highest in RLW (1850.192 mg·m^-2). CW exhibited the highest 100-years-scale Global Warming Potential (GWP₁₀₀-CO₂-eq) (81.175 t CO₂-eq·ha^-1), which was 9.93, 3.12, and 2.11 times greater than RLW, RHW, and MW. Moreover, reclaiming riverside wetland as farmland will increase CO₂ and N₂O emission fluxes by 54.546-72.684 t·ha^-1 and 2.615-2.988 kg·ha^-1, respectively. 16S rRNA high throughput sequencing revealed that bacterial community composition changed significantly overtime and seasons. GHG fluxes showed a significant positive linear correlation with bacterial OTUs (y = 0.71x-319.4, R² = 0.304) and Shannon index (y = 228.62x-796.6, R² = 0.336). Structure equation models indicated that soil C, N and moisture content were the primary factors influencing bacterial community evolution, which had an impact on GHG fluxes. Actinomycetes were significantly affected by total carbon (TC) content, dissolved organic carbon (DOC), and C/N, while ammonia oxidizing and nitrifying bacteria were greatly influenced by NO₃^--N rather than TN and NH₄⁺-N content. Opportunities exist to reduce GHG emissions and mitigate climate change by maintaining the original state of riverside wetland or restoring cultivated land to wetland in the Yellow River floodplain wetland.

Time-resolved characteristics and production pathways of simulated landfilling N2O emission under different oxygen concentrations

Nitrous oxide (N₂O), an important greenhouse gas, is emitted from landfill reservoirs, especially in the working face, where nitrification and denitrification occur under different O₂ concentrations. In order to explore the effects of O₂ concentration on N₂O emissions and production pathways, the production of N₂O from simulated fresh waste landfilling under 0%, 5%, 10%, and 21% (vol/vol) O₂ concentrations were examined, and ¹⁵N isotopes were used as tracers to determine the contributions of nitrification (NF), heterotrophic denitrification (HD), and nitrification-coupled denitrification (NCD) to N₂O production over a 72-h incubation period. Equal amounts of total nitrogen consumption occurred for all studied O₂ concentration and the simulated waste tended to release more N₂O under 0% and 21% O₂. Heterotrophic denitrification was the main source of N₂O release at the studied oxygen concentrations, contributing 90.51%, 69.04%, 80.75%, and 57.51% of N₂O under O₂ concentrations of 0%, 5%, 10%, and 21%, respectively. Only denitrification was observed in the simulated fresh waste when the oxygen concentration of the bulk atmosphere was 0%. The nitrate reductase (nirS)-encoding denitrifiers in the simulated landfill were also studied and significant differences were observed in the richness and diversity of the denitrifying community at different taxonomic levels. It was determined that optimising the O₂ content is a crucial factor in N₂O production that may allow greenhouse gas emissions and N turnover during landfill aeration to be minimised.

Hot spots and hot moments of nitrogen removal from hyporheic and riparian zones: A review

The Earth is experiencing excessive nitrogen (N) input to its various ecosystems due to human activities. How to effectively and efficiently remove N from ecosystems has been, is and will be at the center of attention in N research. Hyporheic and riparian zones are widely acknowledged for their buffering capacity to reduce contaminants (especially N) transport downstream. However, these zones are usually misunderstood that they can remove N at all spots and at any moments. Here pathways of N removal from hyporheic and riparian zones are reviewed and summarized with an emphasize on their hot spots and hot moments. N is biogeochemically removed by denitrification, anammox, nitrifier denitrification, denitrifying anaerobic methane oxidation, Feammox and Sulfammox. Hot moments of N removal are mainly triggered by precipitation, fire and snowmelt. Finally, some research needs are outlined and discussed, such as developing approaches for multiscale sampling and monitoring, quantifying the effects of hot spots and hot moments at hyporheic and riparian zones and evaluating the impacts of human activities on hot spots and hot moments, to inspire more research on hot spots and hot moments of N removal. By this review, we hope to bring awareness of the heterogeneity of hyporheic and riparian zones to catchment managers and policy makers when tackling N pollution problems.

Dual isotope ratio normalization of nitrous oxide by bacterial denitrification of USGS reference materials

We measured the δ values of N₂O using gas chromatography isotope ratio mass spectrometry with a preconcentrator (precon-GC-IRMS). The instrumental precision of the mass spectrometer was restricted to below the shot noise limit, which agreed with the theoretical and experimental results of 0.02‰ (δ¹⁵N) and 0.04‰ (δ¹⁸O), respectively. The precision of the measured δ values was significantly improved by the temperature regulation protocol of the LN₂ preconcentrator, which was monitored by various temperature sensors placed along the U-trap. The reproducibility of the He-diluted N₂O gas measurements resulted in 0.063‰ (δ¹⁵N) and 0.075‰ (δ¹⁸O) due to additional sources of uncertainty in the vials used for autosampling and in the general preconcentration process. Multipoint normalization of the dual δ values of the measured N₂O samples was conducted using United States Geological Survey reference materials denitrified by Pseudomonas aureofaciens. Kaiser's ion correction method, based on International Atomic Energy Agency parameters, exhibited low bias for the atomic isotope ratio reduction of the nitrate reference material, for which the oxygen anomaly was considerably high. Dedicated corrections for net isotope fractionation and water exchange were important in improving uncertainties in the procedure for normalizing the oxygen isotope ratio. Blank measurements for correcting biases in isotope ratios caused by pre-dissolved nitrate and nitrite ions in the water solvent led to further improvements, i.e. beyond unevenly controlled net isotope fractionation, throughout the bacterial denitrification process. The uncertainty evaluation revealed that three-point normalization can significantly improve the normalization accuracy compared with two-point normalization. In addition, an alternative strategy was suggested for assigning δ¹⁸O using a CO₂ lab tank, allowing its use as a reference material for N₂O gas tanks.

What can we learn from N2 O isotope data? - Analytics, processes and modelling

The isotopic composition of nitrous oxide (N2 O) provides useful information for evaluating N2 O sources and budgets. Due to the co-occurrence of multiple N2 O transformation pathways, it is, however, challenging to use isotopic information to quantify the contribution of distinct processes across variable spatiotemporal scales. Here, we present an overview of recent progress in N2 O isotopic studies and provide suggestions for future research, mainly focusing on: analytical techniques; production and consumption processes; and interpretation and modelling approaches. Comparing isotope-ratio mass spectrometry (IRMS) with laser absorption spectroscopy (LAS), we conclude that IRMS is a precise technique for laboratory analysis of N2 O isotopes, while LAS is more suitable for in situ/inline studies and offers advantages for site-specific analyses. When reviewing the link between the N2 O isotopic composition and underlying mechanisms/processes, we find that, at the molecular scale, the specific enzymes and mechanisms involved determine isotopic fractionation effects. In contrast, at plot-to-global scales, mixing of N2 O derived from different processes and their isotopic variability must be considered. We also find that dual isotope plots are effective for semi-quantitative attribution of co-occurring N2 O production and reduction processes. More recently, process-based N2 O isotopic models have been developed for natural abundance and 15 N-tracing studies, and have been shown to be effective, particularly for data with adequate temporal resolution. Despite the significant progress made over the last decade, there is still great need and potential for future work, including development of analytical techniques, reference materials and inter-laboratory comparisons, further exploration of N2 O formation and destruction mechanisms, more observations across scales, and design and validation of interpretation and modelling approaches. Synthesizing all these efforts, we are confident that the N2 O isotope community will continue to advance our understanding of N2 O transformation processes in all spheres of the Earth, and in turn to gain improved constraints on regional and global budgets.

Enhanced N2O Production Induced by Soil Salinity at a Specific Range

Nitrous oxide (N₂O) as a by-product of soil nitrogen (N) cylces, its production may be affected by soil salinity which have been proved to have significant negative effect on soil N transformation processes. The response of N₂O production across a range of different soil salinities is poorly documented; accordingly, we conducted a laboratory incubation experiment using an array of soils bearing six different salinity levels ranging from 0.25 to 6.17 dS m⁻¹. With ammonium-rich organic fertilizer as their N source, the soils were incubated at three soil moisture (θ) levels—50%, 75% and 100% of field capacity (θfc)—for six weeks. Both N₂O fluxes and concentrations of ammonium, nitrite and nitrate (NH₄⁺-N, NO₂⁻-N and NO₃⁻-N) were measured throughout the incubation period. The rates of NH₄⁺-N consumption and NO₃⁻-N accumulation increased with increasing soil moisture and decreased with increasing soil salinity, while the accumulation of NO₂⁻-N increased first then decreased with increasing soil salinity. N₂O emissions were significantly promoted by greater soil moisture. As soil salinity increased from 0.25 to 6.17 dS m⁻¹, N₂O emissions from soil first increased then decreased at all three soil moisture levels, with N₂O emissions peaking at electric conductivity (EC) values of 1.01 and 2.02 dS m⁻¹. N₂O emissions form saline soil were found significantly positively correlated to soil NO₂⁻-N accumulation. The present results suggest that greater soil salinity inhibits both steps of nitrification, but that its inhibition of nitrite oxidation is stronger than that on ammonia oxidation, which leads to higher NO₂⁻-N accumulation and enhanced N₂O emissions in soil with a specific salinity range.

Discrepancies in N2O emissions between household waste and its food waste and non-food waste components during the predisposal stage

Nitrous oxide (N₂O) is a greenhouse gas (GHG) and an ozone-depleting substance. Municipal solid waste (MSW) management and treatment activities are some of the sources of GHG emissions. However, the biogenic GHG emissions during the predisposal stage of MSW management, during which waste is transferred to garbage cans and then transported to disposal sites, have received little attention. In this study, household waste was divided into food and non-food waste, and the effects of these types of waste and different oxygen concentrations (21%, 10%, and 1%) on N₂O emissions were investigated. A¹⁵N-labeled isotope experiment was conducted over three days to determine the contributions of nitrification and denitrification to N₂O emissions. The results showed that the N₂O fluxes first increased and then decreased during the three-day tests at different O₂ concentrations. The maximum N₂O flux of 1469.59 ± 1004.32 μg N·kg^-1 wet waste·h^-1 occurred during the predisposal of food waste at an O₂ concentration of 21%, with the total N₂O emissions reaching 20.26 ± 10.87 mg N·kg^-1 wet waste, which exceeds the emissions from some waste disposal processes, such as composting and landfills. The N₂O emissions decreased in the following order: food waste > household waste > non-food waste. For food waste, the peak value and total amount of N₂O emissions decreased significantly as the O₂ concentration decreased. In contrast, the N₂O emissions from non-food waste increased as the O₂ concentration decreased. Denitrification was the predominant biogenic source of N₂O emissions; it accounted for over 60% of N₂O production in all treatments. Nitrification also played an important role in N₂O emissions during the early predisposal stage.

Nitrogen dynamics affected by biochar and irrigation level in an onion field

Soil amended with biochar has many potential environmental benefits, but its influence on the fate of nitrogen (N) under irrigated conditions is unclear. The objective of this research was to determine the effects of biochar and interactions with irrigation on N movement in soil, gas emissions, and leaching. A three-year study was conducted in an onion field with three main irrigation treatments (50, 75, and 100% of a reference that provided sufficient water for plant growth) and three biochar amendment rates (0 or control, low char - applied first year at 29 Mg ha^-1, and high char - added both first and second year for a total 58 Mg ha^-1) as sub-treatments in a split-plot design. Nitrogen fertilizer was applied three times during first year growing season, but weekly the second year. Ammonia (NH₃) volatilization, nitrous oxide (N₂O) emission, and nitrate (NO₃^-) in soil pore water were monitored during growing season, and annual N (total and NO₃^-) changes in soil profile were determined for first two years. Nitrate leaching was measured in the third year. Ammonia volatilization was affected by fertilization frequency with higher loss (5-8% of total applied) when fertilizer was applied in large doses during the first year compared to the second year (4-5%). Nitrous oxide emissions were ≤0.1% of applied N for both years and not affected by any treatments or fertilization frequency. Nitrate concentration in soil profile increased significantly as irrigation level dropped, but most of the NO₃^- was leached by winter rain. There was no significant biochar effect on total N gas emissions or soil NO₃^- accumulation, but significant irrigation effect and interaction with biochar were determined on soil NO₃^- accumulation. High leaching was associated with biochar amendment and higher irrigation level. Irrigation strategies are the key to improving N management and developing the best practices associated with biochar.

Is wood ash amendment a suitable mitigation strategy for N2O emissions from soil?

Wood ash, the by-product of biomass combustion to energy, can return important nutrients back to the soil and counteract acidification. However, the application of wood ash may affect the emission of greenhouse gases. Here, the effect of wood ash application on nitrous oxide (N₂O) emissions from different soil environments were investigated in a 40 days incubation experiment comprising ten different soil types amended with five different wood ash concentrations (0, 3, 9, 20, and 54 t ash ha^-1). The emitted N₂O was measured continuously, and initial soil properties without ash application (carbon (C), nitrogen (N), ammonium (NH₄⁺), nitrate (NO₃^-), and pH) and resulting soil properties (pH, NH₄⁺, and NO₃^-) were measured prior and after the incubation period, respectively. The Random Forests (RF) model was used to identify which factors (initial and resulting soil properties, vegetation, management, wood ash doze, and respiration rate) were the most important to predict the development of emitted N₂O after ash application. Wood ash either increased, decreased, or had no effect on the amount of emitted N₂O depending on soil type and ash dose. The RF model identified the final resulting pH as the most important factor for the prediction of emitted N₂O. The results suggest that wood ash can mitigate N₂O emissions from soil, however, this effect depends on soil type where a mitigating effect of wood ash application was observed mainly in low pH soils with high soil organic matter whereas an increase in N₂O emissions was observed in mineral soils that had previously received N fertilization. This study emphasises the importance of pH manipulation in regards to N₂O emissions from soil.

From Understanding to Sustainable Use of Peatlands: The WETSCAPES Approach

Of all terrestrial ecosystems, peatlands store carbon most effectively in long-term scales of millennia. However, many peatlands have been drained for peat extraction or agricultural use. This converts peatlands from sinks to sources of carbon, causing approx. 5% of the anthropogenic greenhouse effect and additional negative effects on other ecosystem services. Rewetting peatlands can mitigate climate change and may be combined with management in the form of paludiculture. Rewetted peatlands, however, do not equal their pristine ancestors and their ecological functioning is not understood. This holds true especially for groundwater-fed fens. Their functioning results from manifold interactions and can only be understood following an integrative approach of many relevant fields of science, which we merge in the interdisciplinary project WETSCAPES. Here, we address interactions among water transport and chemistry, primary production, peat formation, matter transformation and transport, microbial community, and greenhouse gas exchange using state of the art methods. We record data on six study sites spread across three common fen types (Alder forest, percolation fen, and coastal fen), each in drained and rewetted states. First results revealed that indicators reflecting more long-term effects like vegetation and soil chemistry showed a stronger differentiation between drained and rewetted states than variables with a more immediate reaction to environmental change, like greenhouse gas (GHG) emissions. Variations in microbial community composition explained differences in soil chemical data as well as vegetation composition and GHG exchange. We show the importance of developing an integrative understanding of managed fen peatlands and their ecosystem functioning.

Can Incorporating Brassica Tissues into Soil Reduce Nitrification Rates and Nitrous Oxide Emissions?

New Zealand agriculture is composed predominantly of pastoral grazing systems; however, forage crops have been increasingly used to supplement the diet of grazing animals. Excreta from grazing animals has been identified as a main contributor of NO emissions. Some forage crops, such as brassicas ( spp.), contain secondary metabolites that have been identified to inhibit soil N cycling processes, and nitrification in particular. Our objective was to determine if secondary metabolites released from brassica tissues inhibited nitrification and reduced NO emissions when incorporated into soil, which was amended with a large amount of urea N (such as derived from urine patches deposited during grazing). Three brassica tissues (kale [ L.], turnip [ L.] bulb, and turnip leaf and stem) and ryegrass ( L.) tissue were incorporated into soil with and without urea solution, and NO, NO, and NH were measured during a 52-d incubation. All brassica tissues reduced urea-derived NO emissions relative to ryegrass tissues when incorporated into soil. According to the mineral N and microbial community data, this reduction, however, could not be attributed to inhibition of nitrification. Although there was less NO from urea in the brassica treatments, total NO emissions increased after incorporation of all tissue residues into soil, so this tradeoff must be explored if brassica tissues are to be considered as a tool for NO reduction.

Linking N2O Emissions from Biofertilizer-Amended Soil of Tea Plantations to the Abundance and Structure of N2O-Reducing Microbial Communities

Nitrous oxide (N₂O) contributes up to 8% of global greenhouse gas emissions, with approximately 70% from terrestrial sources; over one-third of this terrestrial emission has been linked to increased agricultural fertilizer use. Much of the nitrogen in fertilizers is converted to N₂O by microbial processes in soil. However, the potential mechanism of biofertilizers and the role of microbial communities in mitigating soil N₂O emissions are not fully understood. Here, we used a greenhouse-based pot experiment with tea plantation soil to investigate the effect of Trichoderma viride biofertilizer on N₂O emission. The addition of biofertilizer reduced N₂O emissions from fertilized soil by 67.6%. Quantitative PCR (qPCR) analysis of key functional genes involved in N₂O generation and reduction ( amoA, nirK, nirS, and nosZ) showed an increased abundance of nirS and nosZ genes linked to the pronounced reduction in N₂O emissions. High-throughput sequencing of nosZ showed enhanced relative abundance of nosZ-harboring denitrifiers in the T. viride biofertilizer treatments, thus linking greater N₂O reduction capacity to the reduced emissions. Our findings showed that biofertilizers can affect the microbial nitrogen transformation process and reduce N₂O emissions from agroecosystems.

Evaluation of potassium thiosulfate as a nitrification inhibitor to reduce nitrous oxide emissions

Potassium thiosulfate (KTS, K₂S₂O₃) has been shown to function as a nitrification inhibitor, thus has the potential to reduce nitrous oxide (N₂O) emissions and play an important role in effective N management. The objective of this research was to determine the potential effects of KTS on N₂O emissions and N transformation processes in comparison with commercial N transformation inhibitors (stabilizers). A laboratory incubation experiment was conducted using urea and ammonium nitrate (UAN) applied at 150mgNkg^-1 in a Hanford sandy loam soil (coarse-loamy, mixed, superactive, nonacid, thermic Typic Xerorthents). Treatments included three rates of KTS (26, 51, and 102mgS₂O₃^2--Skg^-1), a urease and nitrification inhibitor (Agrotain® Plus), a nitrification inhibitor (N-Serve® 24), and an untreated control. Nitrous oxide emission, soil pH, and mineral N species were monitored for 35days. Total N₂O emissions were reduced significantly by all KTS treatments as a function of KTS rate. At 102mgS₂O₃^2--Skg^-1, KTS reduced N₂O emissions by 48% (0.18% of total inorganic N), which was statistically similar to the N-Serve® 24 treatment (60% reduction) although lower than Agrotain® Plus (78% reduction). The KTS resulted in significantly less unaccounted (total N) loss compared to the commercial inhibitors. If the N₂O emissions reductions observed in this laboratory study are validated in the field, using KTS for this purpose can also provide a fertility benefit and may reduce total chemical inputs into agronomic systems. Future research needs to determine the effectiveness of thiosulfate for improving overall nutrient management while reducing N₂O emissions under field conditions.

Nitrogen nutrition in cotton and control strategies for greenhouse gas emissions: a review

Cotton (Gossypium hirustum L.) is grown globally as a major source of natural fiber. Nitrogen (N) management is cumbersome in cotton production systems; it has more impacts on yield, maturity, and lint quality of a cotton crop than other primary plant nutrient. Application and production of N fertilizers consume large amounts of energy, and excess application can cause environmental concerns, i.e., nitrate in ground water, and the production of nitrous oxide a highly potent greenhouse gas (GHG) to the atmosphere, which is a global concern. Therefore, improving nitrogen use efficiency (NUE) of cotton plant is critical in this context. Slow-release fertilizers (e.g., polymer-coated urea) have the potential to increase cotton yield and reduce environmental pollution due to more efficient use of nutrients. Limited literature is available on the mitigation of GHG emissions for cotton production. Therefore, this review focuses on the role of N fertilization, in cotton growth and GHG emission management strategies, and will assess, justify, and organize the researchable priorities. Nitrate and ammonium nitrogen are essential nutrients for successful crop production. Ammonia (NH₃) is a central intermediate in plant N metabolism. NH₃ is assimilated in cotton by the mediation of glutamine synthetase, glutamine (z-) oxoglutarate amino-transferase enzyme systems in two steps: the first step requires adenosine triphosphate (ATP) to add NH₃ to glutamate to form glutamine (Gln), and the second step transfers the NH₃ from glutamine (Gln) to α-ketoglutarate to form two glutamates. Once NH₃ has been incorporated into glutamate, it can be transferred to other carbon skeletons by various transaminases to form additional amino acids. The glutamate and glutamine formed can rapidly be used for the synthesis of low-molecular-weight organic N compounds (LMWONCs) such as amides, amino acids, ureides, amines, and peptides that are further synthesized into high-molecular-weight organic N compounds (HMWONCs) such as proteins and nucleic acids.

Quantifying nitrous oxide production pathways in wastewater treatment systems using isotope technology - A critical review

Nitrous oxide (N₂O) is an important greenhouse gas and an ozone-depleting substance which can be emitted from wastewater treatment systems (WWTS) causing significant environmental impacts. Understanding the N₂O production pathways and their contribution to total emissions is the key to effective mitigation. Isotope technology is a promising method that has been applied to WWTS for quantifying the N₂O production pathways. Within the scope of WWTS, this article reviews the current status of different isotope approaches, including both natural abundance and labelled isotope approaches, to N₂O production pathways quantification. It identifies the limitations and potential problems with these approaches, as well as improvement opportunities. We conclude that, while the capabilities of isotope technology have been largely recognized, the quantification of N₂O production pathways with isotope technology in WWTS require further improvement, particularly in relation to its accuracy and reliability.

Pathways and Controls of N2O Production in Nitritation-Anammox Biomass

Nitrous oxide (N₂O) is an unwanted byproduct during biological nitrogen removal processes in wastewater. To establish strategies for N₂O mitigation, a better understanding of production mechanisms and their controls is required. A novel stable isotope labeling approach using ¹⁵N and ¹⁸O was applied to investigate pathways and controls of N₂O production by biomass taken from a full-scale nitritation-anammox reactor. The experiments showed that heterotrophic denitrification was a negligible source of N₂O under oxic conditions (≥0.2 mg O₂ L^-1). Both hydroxylamine oxidation and nitrifier denitrification contributed substantially to N₂O accumulation across a wide range of conditions with varying concentrations of O₂, NH₄⁺, and NO₂^-. The O₂ concentration exerted the strongest control on net N₂O production with both production pathways stimulated by low O₂, independent of NO₂^- concentrations. The stimulation of N₂O production from hydroxylamine oxidation at low O₂ was unexpected and suggests that more than one enzymatic pathway may be involved in this process. N₂O production by hydroxylamine oxidation was further stimulated by NH₄⁺, whereas nitrifier denitrification at low O₂ levels was stimulated by NO₂^- at levels as low as 0.2 mM. Our study shows that ¹⁵N and ¹⁸O isotope labeling is a useful approach for direct quantification of N₂O production pathways applicable to diverse environments.

Quantifying N2O emissions and production pathways from fresh waste during the initial stage of disposal to a landfill

Intensive nitrous oxide (N₂O) emissions usually occur at the working face of landfills. However, the specific amounts and contributions of the multiple pathways to N₂O emissions are poorly understood. N₂O emissions and the mutual conversions of N-species in both open and sealed simulated landfill reactors filled with fresh refuse were examined during a 100-h incubation period, and N₂O sources were calculated using ¹⁵N isotope labelling. N₂O peak fluxes were above 70μgNkg^-1 waste h^-1 for both treatments. The sealed incubation reactors became a N₂O sink when N₂O in the ambient environment was sufficient. The total amount of N₂O emissions under sealed conditions was 2.15±0.56mgNkg^-1 waste, which was higher than that under open conditions (1.91±0.34mgNkg^-1 waste). The NO₂^- peak appeared prior to the peak in N₂O flux. The degree and duration of total nitrogen reduction in open incubations were larger and longer than those of sealed incubations and could possibly be due to oxygen supplementation. Denitrification (DF) was a major source of N₂O generation during these incubations. The contribution of the DF pathway decreased from 89.2% to 61.3% during the open incubations. The effects of nitrification (NF) and nitrification-coupled denitrification (NCD) increased during the increasing phase and the decreasing phase of N₂O flux, contributing 24.1-37.4% and 31.7-34.4% of total N₂O emissions, respectively. In sealed treatments, the DF pathway accounted for more than 90% of the total N₂O emission during the entire incubation.

Linkage between N2O emission and functional gene abundance in an intensively managed calcareous fluvo-aquic soil

The linkage between N₂O emissions and the abundance of nitrifier and denitrifier genes is unclear in the intensively managed calcareous fluvo-aquic soils of the North China Plain. We investigated the abundance of bacterial amoA for nitrification and narG, nirS, nirK, and nosZ for denitrification by in situ soil sampling to determine how the abundance of these genes changes instantly during N fertilization events and is related to high N₂O emission peaks. We also investigated how long-term incorporated straw and/or manure affect(s) the abundance of these genes based on a seven-year field experiment. The overall results demonstrate that the long-term application of urea-based fertilizer and/or manure significantly enhanced the number of bacterial amoA gene copies leading to high N₂O emission peaks after N fertilizer applications. These peaks contributed greatly to the annual N₂O emissions in the crop rotation. A significant correlation between annual N₂O emissions and narG, nirS, and nirK gene numbers indicates that the abundance of these genes is related to N₂O emission under conditions for denitrification, thus partly contributing to the annual N₂O emissions. These findings will help to draw up appropriate measures for mitigation of N₂O emissions in this ‘hotspot’ region.

Stoichiometric ratio of dissolved organic carbon to nitrate regulates nitrous oxide emission from the biochar-amended soils

Biochar has the potential to mitigate nitrous oxide (N₂O) emissions from soils. However, the mechanisms responsible for N₂O emission in biochar-amended soils are yet to be elucidated. In this study, an incubation experiment was carried out to investigate the effects of seven biochars (eucalyptus, softwood mixture, mallee, jarrah, peanut shell, green waste and radiata pine) on the stoichiometric shifts of dissolved organic carbon (DOC), nitrate (NO₃^--N) and N₂O emission in two contrasting soils (Ferrosol with 5.3% total C, 0.46% total N; Tenosol with 0.4% total C, 0.01% total N). All biochar treatments were found to significantly reduce N₂O emission in Tenosol by 61-72%. However, in Ferrosol, biochars' impacts on N₂O emission were variable, with only peanut shell, green waste and radiata pine bicohars significantly reducing N₂O emission by 17-23%. A decrease in NO₃^- availability in most biochar-amended treatments also was observed in both soils compared with the control. The N₂O fluxes in Ferrosol were mainly regulated by the shifts in the availability and stoichiometry of DOC and NO₃^- induced by the biochar amendments. The DOC derived from biochars increased DOC:NO₃^- ratio in Ferrosol at the beginning of the experiment, but these effects disappeared 7days after incubation. Overall, the N₂O fluxes were C-limited due to the presence of high concentrations of NO₃^- in Ferrosol. However, in Tenosol, the relationship between stoichiometry of DOC:NO₃^- and N₂O fluxes was much weaker than Ferrosol and N₂O fluxes mainly limited by the concentration of NO₃^-. This study demonstrated that the mechanisms responsible for biochar effects on soil N₂O fluxes are considered to be soil and biochar specific.

Neglecting diurnal variations leads to uncertainties in terrestrial nitrous oxide emissions

Nitrous oxide (N₂O) is an important greenhouse gas produced in soil and aquatic ecosystems. Its warming potential is 296 times higher than that of CO₂. Most N₂O emission measurements made so far are limited in temporal and spatial resolution causing uncertainties in the global N₂O budget. Recent advances in laser spectroscopic techniques provide an excellent tool for area-integrated, direct and continuous field measurements of N₂O fluxes using the eddy covariance method. By employing this technique on an agricultural site with four laser-based analysers, we show here that N₂O exchange exhibits contrasting diurnal behaviour depending upon soil nitrogen availability. When soil N was high due to fertilizer application, N₂O emissions were higher during daytime than during the night. However, when soil N became limited, emissions were higher during the night than during the day. These reverse diurnal patterns supported by isotopic analyses may indicate a dominant role of plants on microbial processes associated with N₂O exchange. This study highlights the potential of new technologies in improving estimates of global N₂O sources.

Diversity, structure, and size of N(2)O-producing microbial communities in soils--what matters for their functioning?

Nitrous oxide (N(2)O) is mainly generated via nitrification and denitrification processes in soils and subsequently emitted into the atmosphere where it causes well-known radiative effects. How nitrification and denitrification are affected by proximal and distal controls has been studied extensively in the past. The importance of the underlying microbial communities, however, has been acknowledged only recently. Particularly, the application of molecular methods to study nitrifiers and denitrifiers directly in their habitats enabled addressing how environmental factors influence the diversity, community composition, and size of these functional groups in soils and whether this is of relevance for their functioning and N(2)O production. In this review, we summarize the current knowledge on community-function interrelationships. Aerobic nitrification (ammonia oxidation) and anaerobic denitrification are clearly under different controls. While N(2)O is an obligatory intermediate in denitrification, its production during ammonia oxidation depends on whether nitrite, the end product, is further reduced. Moreover, individual strains vary strongly in their responses to environmental cues, and so does N(2)O production. We therefore conclude that size and structure of both functional groups are relevant with regard to production and emission of N(2)O from soils. Diversity affects on function, however, are much more difficult to assess, as it is not resolved as yet how individual nitrification or denitrification genotypes are related to N(2)O production. More research is needed for further insights into the relation of microbial communities to ecosystem functions, for instance, how the actively nitrifying or denitrifying part of the community may be related to N(2)O emission.

Denitrification kinetics in biomass- and biochar-amended soils of different landscape positions

Knowledge of how biochar impacts soil denitrification kinetics as well as the mechanisms of interactions is essential in order to better predict the nitrous oxide (N2O) mitigation capacity of biochar additions. This study had multiple experiments in which the effect of three biochar materials produced from corn stover (Zea mays L.), ponderosa pine wood residue (Pinus ponderosa Douglas ex Lawson and C. Lawson), switchgrass (Panicum virgatum L.), and their corresponding biomass materials (corn stover, ponderosa pine wood residue, and switchgrass) on cumulative N2O emissions and total denitrification in soils from two different landscape positions (crest and footslope) were studied under varying water-filled pore space (40, 70, and 90% WFPS). Cumulative N2O emissions were reduced by 30 to 70% in both crest and footslope soils. The effect of biochars and biomass treatments on cumulative N2O emissions and total denitrification were only observed at ≥40% WFPS. The denitrification enzyme activity (DEA) kinetic parameters, K s (half-saturation constant), and V max (maximum DEA rate) were both significantly reduced by biochar treatments, with reductions of 70-80% in footslope soil and 80-90 % in the crest soil. The activation energy (E a) and enthalpy of activation of DEA (ΔH) were both increased with biochar application. The trends in DEA rate constants (K s and V max) were correlated by the trends of thermodynamic parameters (activation energy E a and enthalpy of activation ΔH) for denitrifying enzyme activity (DEA). The rate constant V max/K s evaluated the capacity of biochars to mitigate the denitrification process. Denitrifying enzyme kinetic parameters can be useful in evaluating the ability of biochars to mitigate N2O gas losses from soil.

Linking N2O emissions from biochar-amended soil to the structure and function of the N-cycling microbial community

Nitrous oxide (N2O) contributes 8% to global greenhouse gas emissions. Agricultural sources represent about 60% of anthropogenic N2O emissions. Most agricultural N2O emissions are due to increased fertilizer application. A considerable fraction of nitrogen fertilizers are converted to N2O by microbiological processes (that is, nitrification and denitrification). Soil amended with biochar (charcoal created by pyrolysis of biomass) has been demonstrated to increase crop yield, improve soil quality and affect greenhouse gas emissions, for example, reduce N2O emissions. Despite several studies on variations in the general microbial community structure due to soil biochar amendment, hitherto the specific role of the nitrogen cycling microbial community in mitigating soil N2O emissions has not been subject of systematic investigation. We performed a microcosm study with a water-saturated soil amended with different amounts (0%, 2% and 10% (w/w)) of high-temperature biochar. By quantifying the abundance and activity of functional marker genes of microbial nitrogen fixation (nifH), nitrification (amoA) and denitrification (nirK, nirS and nosZ) using quantitative PCR we found that biochar addition enhanced microbial nitrous oxide reduction and increased the abundance of microorganisms capable of N2-fixation. Soil biochar amendment increased the relative gene and transcript copy numbers of the nosZ-encoded bacterial N2O reductase, suggesting a mechanistic link to the observed reduction in N2O emissions. Our findings contribute to a better understanding of the impact of biochar on the nitrogen cycling microbial community and the consequences of soil biochar amendment for microbial nitrogen transformation processes and N2O emissions from soil.

Fungal oxygen exchange between denitrification intermediates and water

RATIONALE

Fungi can contribute greatly to N2O production from denitrification. Therefore, it is important to quantify the isotopic signature of fungal N2O. The isotopic composition of N2O can be used to identify and analyze the processes of N2O production and N2O reduction. In contrast to bacteria, information about the oxygen exchange between denitrification intermediates and water during fungal denitrification is lacking, impeding the explanatory power of stable isotope methods.

METHODS

Six fungal species were anaerobically incubated with the electron acceptors nitrate or nitrite and (18)O-labeled water to determine the oxygen exchange between denitrification intermediates and water. After seven days of incubation, gas samples were analyzed for N2O isotopologues by isotope ratio mass spectrometry.

RESULTS

All the fungal species produced N2O. N2O production was greater when nitrite was the sole electron acceptor (129 to 6558 nmol N2O g dw(-1)  h(-1)) than when nitrate was the electron acceptor (6 to 47 nmol N2O g dw(-1)  h(-1)). Oxygen exchange was complete with nitrate as electron acceptor in one of five fungi and with nitrite in two of six fungi. Oxygen exchange of the other fungi varied (41 to 89% with nitrite and 11 to 61% with nitrate).

CONCLUSIONS

This is the first report on oxygen exchange with water during fungal denitrification. The exchange appears to be within the range previously reported for bacterial denitrification. This adds to the difficulty of differentiating N2O producing processes based on the origin of N2O-O. However, the large oxygen exchange repeatedly observed for bacteria and now also fungi could lead to less variability in the δ(18)O values of N2O from soils, which could facilitate the assessment of the extent of N2O reduction.

Nitrous oxide emissions from soils: how well do we understand the processes and their controls?

Although it is well established that soils are the dominating source for atmospheric nitrous oxide (N₂O), we are still struggling to fully understand the complexity of the underlying microbial production and consumption processes and the links to biotic (e.g. inter- and intraspecies competition, food webs, plant–microbe interaction) and abiotic (e.g. soil climate, physics and chemistry) factors. Recent work shows that a better understanding of the composition and diversity of the microbial community across a variety of soils in different climates and under different land use, as well as plant–microbe interactions in the rhizosphere, may provide a key to better understand the variability of N₂O fluxes at the soil–atmosphere interface. Moreover, recent insights into the regulation of the reduction of N₂O to dinitrogen (N₂) have increased our understanding of N₂O exchange. This improved process understanding, building on the increased use of isotope tracing techniques and metagenomics, needs to go along with improvements in measurement techniques for N₂O (and N₂) emission in order to obtain robust field and laboratory datasets for different ecosystem types. Advances in both fields are currently used to improve process descriptions in biogeochemical models, which may eventually be used not only to test our current process understanding from the microsite to the field level, but also used as tools for up-scaling emissions to landscapes and regions and to explore feedbacks of soil N₂O emissions to changes in environmental conditions, land management and land use.

Ammonia oxidation pathways and nitrifier denitrification are significant sources of N2O and NO under low oxygen availability

The continuous increase of nitrous oxide (N2O) abundance in the atmosphere is a global concern. Multiple pathways of N2O production occur in soil, but their significance and dependence on oxygen (O2) availability and nitrogen (N) fertilizer source are poorly understood. We examined N2O and nitric oxide (NO) production under 21%, 3%, 1%, 0.5%, and 0% (vol/vol) O2 concentrations following urea or ammonium sulfate [(NH4)2SO4] additions in loam, clay loam, and sandy loam soils that also contained ample nitrate. The contribution of the ammonia (NH3) oxidation pathways (nitrifier nitrification, nitrifier denitrification, and nitrification-coupled denitrification) and heterotrophic denitrification (HD) to N2O production was determined in 36-h incubations in microcosms by (15)N-(18)O isotope and NH3 oxidation inhibition (by 0.01% acetylene) methods. Nitrous oxide and NO production via NH3 oxidation pathways increased as O2 concentrations decreased from 21% to 0.5%. At low (0.5% and 3%) O2 concentrations, nitrifier denitrification contributed between 34% and 66%, and HD between 34% and 50% of total N2O production. Heterotrophic denitrification was responsible for all N2O production at 0% O2. Nitrifier denitrification was the main source of N2O production from ammonical fertilizer under low O2 concentrations with urea producing more N2O than (NH4)2SO4 additions. These findings challenge established thought attributing N2O emissions from soils with high water content to HD due to presumably low O2 availability. Our results imply that management practices that increase soil aeration, e.g., reducing compaction and enhancing soil structure, together with careful selection of fertilizer sources and/or nitrification inhibitors, could decrease N2O production in agricultural soils.

Analysis of the coexisting pathways for NO and N2O formation in Chernozem using the (15)N-tracer SimKIM-Advanced model

The nitrogen (N) cycle consists of a variety of microbial processes. These processes often occur simultaneously in soils, but respond differently to local environmental conditions due to process-specific biochemical restrictions (e.g. oxygen levels). Hence, soil nitrogen cycling (e.g. soil N gas production through nitrification and denitrification) is individually affected through these processes, resulting in the complex and highly dynamic behaviour of total soil N turnover. The development and application of methods that facilitate the quantification of individual contributions of coexisting processes is a fundamental prerequisite for (i) understanding the dynamics of soil N turnover and (ii) implementing these processes in ecosystem models. To explain the unexpected results of the triplet tracer experiment (TTE) of Russow et al. (Role of nitrite and nitric oxide in the processes of nitrification and denitrification in soil: results from (15)N tracer experiments. Soil Biol Biochem. 2009;41:785-795) the existing SimKIM model was extended to the SimKIM-Advanced model through the addition of three separate nitrite subpools associated with ammonia oxidation, oxidation of organic nitrogen (Norg), and denitrification, respectively. For the TTE, individual treatments with (15)N ammonium, (15)N nitrate, and (15)N nitrite were conducted under oxic, hypoxic, and anoxic conditions, respectively, to clarify the role of nitric oxide as a denitrification intermediate during N2O formation. Using a split nitrite pool, this analysis model explains the observed differences in the (15)N enrichments in nitric oxide (NO) and nitrous oxide (N2O) which occurred in dependence on different oxygen concentrations. The change from oxic over hypoxic to anoxic conditions only marginally increased the NO and N2O release rates (1.3-fold). The analysis using the model revealed that, under oxic and hypoxic conditions, Norg-based N2O production was the dominant pathway, contributing to 90 and 50 % of the total soil N2O release. Under anoxic conditions, denitrification was the dominant process for soil N2O release. The relative contribution of Norg to the total soil NO release was small. Ammonia oxidation served as the major pathway of soil NO release under oxic and hypoxic conditions, while denitrification was dominant under anoxic conditions. The model parameters for soil with moderate soil organic matter (SOM) content were not scalable to an additional data set for soil with higher SOM content, indicating a strong influence of SOM content on microbial N turnover. Thus, parameter estimation had to be re-calculated for these conditions, highlighting the necessity of individual soil-dependent parameter estimations.

Nitric oxide and nitrous oxide turnover in natural and engineered microbial communities: biological pathways, chemical reactions, and novel technologies

Nitrous oxide (N(2)O) is an environmentally important atmospheric trace gas because it is an effective greenhouse gas and it leads to ozone depletion through photo-chemical nitric oxide (NO) production in the stratosphere. Mitigating its steady increase in atmospheric concentration requires an understanding of the mechanisms that lead to its formation in natural and engineered microbial communities. N(2)O is formed biologically from the oxidation of hydroxylamine (NH(2)OH) or the reduction of nitrite (NO(-) (2)) to NO and further to N(2)O. Our review of the biological pathways for N(2)O production shows that apparently all organisms and pathways known to be involved in the catabolic branch of microbial N-cycle have the potential to catalyze the reduction of NO(-) (2) to NO and the further reduction of NO to N(2)O, while N(2)O formation from NH(2)OH is only performed by ammonia oxidizing bacteria (AOB). In addition to biological pathways, we review important chemical reactions that can lead to NO and N(2)O formation due to the reactivity of NO(-) (2), NH(2)OH, and nitroxyl (HNO). Moreover, biological N(2)O formation is highly dynamic in response to N-imbalance imposed on a system. Thus, understanding NO formation and capturing the dynamics of NO and N(2)O build-up are key to understand mechanisms of N(2)O release. Here, we discuss novel technologies that allow experiments on NO and N(2)O formation at high temporal resolution, namely NO and N(2)O microelectrodes and the dynamic analysis of the isotopic signature of N(2)O with quantum cascade laser absorption spectroscopy (QCLAS). In addition, we introduce other techniques that use the isotopic composition of N(2)O to distinguish production pathways and findings that were made with emerging molecular techniques in complex environments. Finally, we discuss how a combination of the presented tools might help to address important open questions on pathways and controls of nitrogen flow through complex microbial communities that eventually lead to N(2)O build-up.

A review of ammonia-oxidizing bacteria and archaea in Chinese soils

Ammonia (NH(3)) oxidation, the first and rate-limiting step of nitrification, is a key step in the global Nitrogen (N) cycle. Major advances have been made in recent years in our knowledge and understanding of the microbial communities involved in ammonia oxidation in a wide range of habitats, including Chinese agricultural soils. In this mini-review, we focus our attention on the distribution and community diversity of ammonia-oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) in Chinese soils with variable soil properties and soil management practices. The niche differentiation of AOB and AOA in contrasting soils have been functionally demonstrated using DNA-SIP (stable isotope probing) methods, which have shown that AOA dominate nitrification processes in acidic soils, while AOB dominated in neutral, alkaline and N-rich soils. Finally, we discuss the composition and activity of ammonia oxidizers in paddy soils, as well as the mitigation of the greenhouse gas nitrous oxide (N(2)O) emissions and nitrate leaching via inhibition of nitrification by both AOB and AOA.

N2O emission from a partial nitrification-anammox process and identification of a key biological process of N2O emission from anammox granules

Emission of nitrous oxide (N(2)O) during biological wastewater treatment is of growing concern. The emission of N(2)O from a lab-scale two-reactor partial nitrification (PN)-anammox reactor was therefore determined in this study. The average emission of N(2)O from the PN and anammox process was 4.0±1.5% (9.6±3.2% of the removed nitrogen) and 0.1±0.07% (0.14±0.09% of the removed nitrogen) of the incoming nitrogen load, respectively. Thus, a larger part (97.5%) of N(2)O was emitted from the PN reactor. The total amount of N(2)O emission from the PN reactor was correlated to nitrite (NO(2)(-)) concentration in the PN effluent rather than DO concentration. In addition, further studies were performed to indentify a key biological process that is responsible for N(2)O emission from the anammox process (i.e., granules). In order to characterize N(2)O emission from the anammox granules, the in situ N(2)O production rate was determined by using microelectrodes for the first time, which was related to the spatial organization of microbial community of the granule as determined by fluorescence in situ hybridization (FISH). Microelectrode measurement revealed that the active N(2)O production zone was located in the inner part of the anammox granule, whereas the active ammonium consumption zone was located above the N(2)O production zone. Anammox bacteria were present throughout the granule, whereas ammonium-oxidizing bacteria (AOB) were restricted to only the granule surface. In addition, addition of penicillin G that inhibits most of the heterotrophic denitrifiers and AOB completely inhibited N(2)O production in batch experiments. Based on these results obtained, denitrification by putative heterotrophic denitrifiers present in the inner part of the granule was considered the most probable cause of N(2)O emission from the anammox reactor (i.e., granules).

Nitrogen cycling in rice paddy environments: past achievements and future challenges

Nitrogen is generally the most limiting nutrient for rice production. In rice paddy soils, various biochemical processes can occur regarding N cycling, including nitrification, denitrification, and nitrogen fixation. Since its discovery in the 1930s, the nitrification-denitrification process has been extensively studied in Japan. It may cause N loss from rice paddy soils, while it can also reduce environmental pollutions such as nitrate leaching and emission of nitrous oxide (N(2)O). In this review article, we first summarize the early and important findings regarding nitrification-denitrification in rice paddy soils, and then update recent findings regarding key players in denitrification and N(2)O reduction. In addition, we also discuss the potential occurrence of other newly found reactions in the N cycle, such as archaeal ammonia oxidization, fungal denitrification, anaerobic methane oxidation coupled with denitrification, and anaerobic ammonium oxidation.

Towards food, feed and energy crops mitigating climate change

Agriculture is an important source of anthropogenic emissions of the greenhouse gases (GHG), methane (CH(4)) and nitrous oxide (N(2)O), and crops can affect the microbial processes controlling these emissions in many ways. Here, we summarize the current knowledge of plant-microbe interactions in relation to the CH(4) and N(2)O budgets and show how this is promoting new generations of crop cultivars that have the potential to mitigate GHG emissions for future agricultural use. The possibility of breeding low GHG-emitting cultivars is a paradigm shift towards sustainable agriculture that balances climate change and food and bioenergy security.

Nitrous Oxide Emissions from Ephemeral Wetland Soils are Correlated with Microbial Community Composition

Nitrous oxide (N₂O) is a greenhouse gas with a global warming potential far exceeding that of CO₂. Soil N₂O emissions are a product of two microbially mediated processes: nitrification and denitrification. Understanding the effects of landscape on microbial communities, and the subsequent influences of microbial abundance and composition on the processes of nitrification and denitrification are key to predicting future N₂O emissions. The objective of this study was to examine microbial abundance and community composition in relation to N₂O associated with nitrification and denitrification processes over the course of a growing season in soils from cultivated and uncultivated wetlands. The denitrifying enzyme assay and N15O3− pool dilution methods were used to compare the rates of denitrification and nitrification and their associated N₂O emissions. Functional gene composition was measured with restriction fragment length polymorphism profiles and abundance was measured with quantitative polymerase chain reaction. The change in denitrifier nitrous oxide reductase gene (nosZ) abundance and community composition was a good predictor of net soil N₂O emission. However, neither ammonia oxidizing bacteria ammonia monooxygenase (bacterial amoA) gene abundance nor composition predicted nitrification-associated-N₂O emissions. Alternative strategies might be necessary if bacterial amoA are to be used as predictive in situ indicators of nitrification rate and nitrification-associated-N₂O emission.