The contribution of nitrogen transformation processes to total N2O emissions from soils used for intensive vegetable cultivation

The rapid expansion of intensively farmed vegetable fields has substantially contributed to the total N₂O emissions from croplands in China. However, to date, the mechanisms underlying this phenomenon have not been completely understood. To quantify the contributions of autotrophic nitrification, heterotrophic nitrification, and denitrification to N₂O production from the intensive vegetable fields and to identify the affecting factors, a ¹⁵N tracing experiment was conducted using five soil samples collected from adjacent fields used for rice-wheat rotation system (WF), or for consecutive vegetable cultivation (VF) for 0.5 (VF1), 6 (VF2), 8 (VF3), and 10 (VF4) years. Soil was incubated under 50% water holding capacity (WHC) at 25°C for 96 h after being labeled with ¹⁵NH₄NO₃ or NH ₄ ¹⁵ NO₃. The average N₂O emission rate was 24.2 ng N?kg^?1 h^?1 in WF soil, but it ranged from 69.6 to 507 ng N?kg^?1 h^?1 in VF soils. Autotrophic nitrification, heterotrophic nitrification and denitrification accounted for 0.3–31.4%, 25.4–54.4% and 22.5–57.7% of the N₂O emissions, respectively. When vegetable soils were moderately acidified (pH, 6.2 to ?≥?5.7), the increased N₂O emissions resulted from the increase of both the gross autotrophic and heterotrophic nitrification rates and the N₂O product ratio of autotrophic nitrification. However, once severe acidification occurred (as in VF4, pH?≤?4.3) and salt stress increased, both autotrophic and heterotrophic nitrification rates were inhibited to levels similar to those of WF soil. The enhanced N₂O product ratios of heterotrophic nitrification (4.84‰), autotrophic nitrification (0.93‰) and denitrification processes were the most important factors explaining high N₂O emission in VF4 soil. Data from this study showed that various soil conditions (e.g., soil salinity and concentration of NO ₃ ^- or NH ₄ ⁺ ) could also significantly affect the sources and rates of N₂O emission.     

The effect of different N substrates on biological N2O production from forest and agricultural light textured soils

N₂O emissions from two slightly alkaline sandy soils, from arable land and a woodland, were determined in a laboratory experiment in which the soils were incubated with different sources of nitrogen, with or without glucose, and with 0, 1 and 100 mL C₂H₂ L^-1. Large differences in the rate of N₂O production were observed between the two soils and between the different N treatments. The arable soil showed very low N₂O emissions derived from reduced forms of N as compared with the N₂O which was produced when the soil was provided with NO ₂ ^- or NO ₃ ^- and a C source, suggesting a very active denitrifier population. In contrast, the woodland soil showed a very low denitrification activity and a much higher N₂O production derived from the oxidation of NH ₄ ⁺ and reduction of NO ₂ ^- by some processes probably mediated by autotrophic or heterotrophic nitrifiers or dissimilatory NO ₂ ^- reducers. In both soils, the highest N₂O emissions were induced by NO ₂ ^- addition. Those emissions were demonstrated to have a biological origin, as no significant N₂O emissions were measured when the soil was autoclaved.     

Effect of ammonium and nitrate application on the NO and N2O emission out of different soils

The effect of nitrate and ammonium application (0, 50, 100 and 150 mg N kg^-1 soil) was studied in an incubation experiment. Four Belgian soils, selected for different soil characteristics, were used. The application of both nitrate and ammonium caused an increase of the NO and N₂O emission. The NO production from nitrate and ammonium was found to be of the same order of magnitude. At low pH the NO production was found to be highest from nitrate, at higher pH values the production was found to be higher from ammonium. This seems to be the result of the negative effect of low pH on nitrification.The ANOVA analysis was carried out to separate the effect of the form of nitrogen, quantily of N applied and soil characteristics. The total production of NO was found to depend for 97% on the soil characteristics and for 3% on the quantity of N added. The total N₂O production depended for 100% on the soil characteristics.Stepwise regression analysis showed that the total NO production was best predicted by a combination of the factors CaCO₃ content and NH₄ ⁺ concentration in the soil. Total N₂O production was best described by a combination of CaCO₃, water soluble carbon (WSC) and sand-content.The N₂O/NO ratio was found to be highly variable, indicating that their productions react differently to changes in conditions, or are partly independent.It may be concluded that to NO and N₂O from soils both nitrification and denitrification may be equally important, their relative importance depending on local conditions such as substrate availability, water content of the soil etc. However, the NO production seems to be more nitrification dependent than the N₂O production. ei]{gnE}{fnMerckx}{edSection editor}     

Processes leading to N2O and NO emissions from two different Chinese soils under different soil moisture contents

Background and Aims

Great attention has been paid to N₂O emissions from paddy soils under summer rice-winter wheat double-crop rotation, while less focus was given to the NO emissions. Besides, neither mechanism is completely understood. Therefore, this study aimed at evaluating the relative importance of nitrification and denitrification to N₂O and NO emissions from the two soils at different soil moisture contents

Methods

N₂O and NO emissions during one winter wheat season were simultaneously measured in situ in two rice-wheat based field plots at two different locations in Jiangsu Province, China. One soil was neutral in pH with silt loam texture (NSL), the other soil alkaline in pH with a clay texture (AC). A ¹⁵?N tracer incubation experiment was conducted in the laboratory to evaluate the relative importance of nitrification and denitrification for N₂O and NO emissions at soil moisture contents of 40 % water holding capacity (WHC), 65 % WHC and 90 % WHC.

Results

Higher N₂O emission rates in the AC soil than in the NSL soil were found both in the field and in the laboratory experiments; however, the differences in N₂O emissions between AC soil and NSL soil were smaller in the field than in the laboratory. In the latter experiment, nitrification was observed to be the more important source of N₂O emissions (>70 %) than denitrification, regardless of the soils and moisture treatments, with the only exception of the AC soil at 90 % WHC, at which the contributions of nitrification and denitrification to N₂O emissions were comparable. The ratios of NO/N₂O also supported the evidence that the nitrification process was the dominant source of N₂O and NO both in situ and in the laboratory. The proportion of nitrified N emitted as N₂O (P _N2O ) in NSL soil were around 0.02 % in all three moisture treatments, however, P _N2O in the AC soil (0.04 % to 0.10 %) tended to decrease with increasing soil moisture content.

Conclusions

Our results suggest that N₂O emission rates obtained from laboratory incubation experiments are not suitable for the estimation of the true amount of N₂O fluxes on a field scale. Besides, the variations of P _N2O with soil property and soil moisture content should be taken into account in model simulations of N₂O emission from soils.     

Nitrogen oxide gas emissions from temperate forest soils receiving long-term nitrogen inputs

From spring 2000 through fall 2001, we measured nitric oxide (NO) and nitrous oxide (N₂O) fluxes in two temperate forest sites in Massachusetts, USA that have been treated since 1988 with different levels of nitrogen (N) to simulate elevated rates of atmospheric N deposition. Plots within a pine stand that were treated with either 50 or 150 kg N ha^?1 yr^?1 above background displayed consistently elevated NO fluxes (100–200 µg N m^?2 h^?1) compared to control plots, while only the higher N treatment plot within a mixed hardwood stand displayed similarly elevated NO fluxes. Annual NO emissions estimated from monthly sampling accounted for 3.0–3.7% of N inputs to the high‐N plots and 8.3% of inputs to the Pine low‐N plot. Nitrous oxide fluxes in the N‐treated plots were generally < 10% of NO fluxes. Net nitrification rates (NRs) and NO production rates measured in the laboratory displayed patterns that were consistent with field NO fluxes. Total N oxide gas flux was positively correlated with contemporaneous measurements of NR and concentration. Acetylene inhibited both nitrification and NO production, indicating that autotrophic nitrification was responsible for the elevated NO production. Soil pH was negatively correlated with N deposition rate. Low levels (3–11 µg N kg^?1) of nitrite () were detected in mineral soils from both sites. Kinetic models describing NO production as a function of the protonated form of (nitrous acid [HNO₂]) adequately described the mineral soil data. The results indicate that atmospheric deposition may generate losses of gaseous NO from forest soils by promoting nitrification, and that the response may vary significantly between forest types under similar climatic regimes. The lowering of pH resulting from nitrification and/or directly from deposition may also play a role by promoting reactions involving HNO₂.     

Contributions of Autotrophic and Heterotrophic Nitrifiers to Soil NO and N2O Emissions

Soil emission of gaseous N oxides during nitrification of ammonium represents loss of an available plant nutrient and has an important impact on the chemistry of the atmosphere. We used selective inhibitors and a glucose amendment in a factorial design to determine the relative contributions of autotrophic ammonium oxidizers, autotrophic nitrite oxidizers, and heterotrophic nitrifiers to nitric oxide (NO) and nitrous oxide (N₂O) emissions from aerobically incubated soil following the addition of 160 mg of N as ammonium sulfate kg⁻¹. Without added C, peak NO emissions of 4 μg of N kg⁻¹ h⁻¹ were increased to 15 μg of N kg⁻¹ h⁻¹ by the addition of sodium chlorate, a nitrite oxidation inhibitor, but were reduced to 0.01 μg of N kg⁻¹ h⁻¹ in the presence of nitrapyrin [2-chloro-6-(trichloromethyl)-pyridine], an inhibitor of autotrophic ammonium oxidation. Carbon-amended soils had somewhat higher NO emission rates from these three treatments (6, 18, and 0.1 μg of N kg⁻¹ h⁻¹ after treatment with glucose, sodium chlorate, or nitrapyrin, respectively) until the glucose was exhausted but lower rates during the remainder of the incubation. Nitrous oxide emission levels exhibited trends similar to those observed for NO but were about 20 times lower. Periodic soil chemical analyses showed no increase in the nitrate concentration of soil treated with sodium chlorate until after the period of peak NO and N₂O emissions; the nitrate concentration of soil treated with nitrapyrin remained unchanged throughout the incubation. These results suggest that chemoautotrophic ammonium-oxidizing bacteria are the predominant source of NO and N₂O produced during nitrification in soil.     

Predicting in situ soil N2O emission using NOE algorithm and soil database

This paper presents a new algorithm, Nitrous Oxide Emission (NOE) for simulating the emission of the greenhouse gas N₂O from agricultural soils. N₂O fluxes are calculated as the result of production through denitrification and nitrification and reduction through the last step of denitrification. Actual denitrification and nitrification rates are calculated from biological parameters and soil water‐filled pore space, temperature and mineral nitrogen contents. New suggestions in NOE consisted in introducing (1) biological site‐specific parameters of soil N₂O reduction and (2) reduction of the N₂O produced through nitrification to N₂ through denitrification. This paper includes a database of 64 N₂O fluxes measured on the field scale with corresponding environmental parameters collected from five agricultural situations in France. This database was used to test the validity of this algorithm. Site per site comparison of simulated N₂O fluxes against observed data leads to mixed results. For 80% of the tested points, measured and simulated fluxes are in accordance whereas the others resulted in an important discrepancy. The origin of this discrepancy is discussed. On the other hand, mean annual fluxes measured on each site were strongly correlated to mean simulated annual fluxes. The biological site‐specific parameter of soil N₂O reduction introduced into NOE appeared particularly useful to discriminate the general level of N₂O emissions from site to site. Furthermore, the relevance of NOE was confirmed by comparing measured and simulated N₂O fluxes using some data from the US TRAGNET database. We suggest the use of NOE on a regional scale in order to predict mean annual N₂O emissions.     

Fluxes of nitrous oxide, methane and carbon dioxide during freezing–thawing cycles in an Inner Mongolian steppe

Combined measurements of nitrification activity and N₂O emissions were performed in a lowland and a montane tropical rainforest ecosystem in NE-Australia over a 18 months period from October 2001 until May 2003. At both sites gross nitrification rates, measured by the BaPS technique, showed a strong seasonal pattern with significantly higher rates of gross nitrification during wet season conditions. Nitrification rates at the montane site (1.48?±?0.24–18.75?±?2.38 mg N kg^?1 day^?1) were found to be significantly higher than at the lowland site (1.65?±?0.21–4.54?±?0.27 mg N kg^?1 day^?1). The relationship between soil moisture and gross nitrification rates could be described best by O’Neill functions having a soil moisture optimum of nitrification at app. 65% WFPS. At the lowland site, for which continuous measurements of N₂O emissions were available, nitrification was positively correlated with N₂O emission. Nitrification contributed significantly to N₂O formation during dry season (app.85%) but less (app. 30%) during wet season conditions. In average 0.19‰ of the N metabolized by nitrification was released as N₂O. The N₂O fraction loss for nitrification was positively correlated with changes in soil moisture and varied slightly between 0.15 and 0.22‰. Our results demonstrate that combined N₂O emission and microbial N turnover studies covering prolonged observation periods are needed to clarify and quantify the role of the microbial processes nitrification and denitrification for annual N₂O emissions from soils of terrestrial ecosystems.     

Nitrogen loss from grassland on peat soils through nitrous oxide production

Nitrous oxide (N₂O) in soils is produced through nitrification and denitrification. The N₂O produced is considered as a nitrogen (N) loss because it will most likely escape from the soil to the atmosphere as N₂O or N₂. Aim of the study was to quantify N₂O production in grassland on peat soils in relation to N input and to determine the relative contribution of nitrification and denitrification to N₂O production. Measurements were carried out on a weekly basis in 2 grasslands on peat soil (Peat I and Peat II) for 2 years (1993 and 1994) using intact soil core incubations. In additional experiments distinction between N₂O from nitrification and denitrification was made by use of the gaseous nitrification inhibitor methyl fluoride (CH₃F).Nitrous oxide production over the 2 year period was on average 34 kg N ha^-1 yr^-1 for mown treatments that received no N fertiliser and 44 kg N ha^-1 yr^-1 for mown and N fertilised treatments. Grazing by dairy cattle on Peat I caused additional N₂O production to reach 81 kg N ha^-1 yr^-1. The sub soil (20–40 cm) contributed 25 to 40% of the total N₂O production in the 0–40 cm layer. The N₂O production:denitrification ratio was on average about 1 in the top soil and 2 in the sub soil indicating that N₂O production through nitrification was important. Experiments showed that when ratios were larger than l, nitrification was the major source of N₂O. In conclusion, N₂O production is a significant N loss mechanism in grassland on peat soil with nitrification as an important N₂O producing process.     

Nitrous oxide emissions from agricultural fields: Assessment, measurement and mitigation

In this paper we discuss three topics concerning N₂O emissions from agricultural systems. First, we present an appraisal of N₂O emissions from agricultural soils (Assessment). Secondly, we discuss some recent efforts to improve N₂O flux estimates in agricultural fields (Measurement), and finally, we relate recent studies which use nitrification inhibitors to decrease N₂O emissions from N-fertilized fields (Mitigation).To assess the global emission of N₂O from agricultural soils, the total flux should represent N₂O from all possible sources; native soil N, N from recent atmospheric deposition, past years fertilization, N from crop residues, N₂O from subsurface aquifers below the study area, and current N fertilization. Of these N sources only synthetic fertilizer and animal manures and the area of fields cropped with legumes have sufficient global data to estimate their input for N₂O production. The assessment of direct and indirect N₂O emissions we present was made by multiplying the amount of fertilizer N applied to agricultural lands by 2% and the area of land cropped to legumes by 4 kg N₂O-N ha^-1. No regard to method of N application, type of N, crop, climate or soil was given in these calculations, because the data are not available to include these variables in large scale assessments. Improved assessments should include these variables and should be used to drive process models for field, area, region and global scales.Several N₂O flux measurement techniques have been used in recent field studies which utilize small and ultralarge chambers and micrometeorological along with new analytical techniques to measure N₂O fluxes. These studies reveal that it is not the measurement technique that is providing much of the uncertainty in N₂O flux values found in the literature but rather the diverse combinations of physical and biological factors which control gas fluxes. A careful comparison of published literature narrows the range of observed fluxes as noted in the section on assessment. An array of careful field studies which compare a series of crops, fertilizer sources, and management techniques in controlled parallel experiments throughout the calendar year are needed to improve flux estimates and decrease uncertainty in prediction capability.There are a variety of management techniques which should conserve N and decrease the amount of N application needed to grow crops and to limit N₂O emissions. Using nitrification inhibitors is an option for decreasing fertilizer N use and additionally directly mitigating N₂O emissions. Case studies are presented which demonstrate the potential for using nitrification inhibitors to limit N₂O emissions from agricultural soils. Inhibitors may be selected for climatic conditions and type of cropping system as well as the type of nitrogen (solid mineral N, mineral N in solution, or organic waste materials) and applied with the fertilizers.     

The relative importance of autotrophic and heterotrophic nitrification in a conifer forest soil as measured by 15N tracer and pool dilution techniques

The importance of heterotrophic nitrification was studied in soil from a mixed-conifer forest. Three sites in the forest were sampled: a clear cut area, a young stand and a mature stand. In the mature stand, the mineral soil (0–10 cm) and the organic layer were sampled separately. Gross rates of N mineralization and nitrification were measured by¹⁵NH ₄ ⁺ and¹⁵NO ₃ ^– isotopic pool dilution, respectively. The rates of autotrophic and heterotrophic nitrification were distinguished by use of acetylene as a specific inhibitor of autotrophic nitrification. In samples supplemented with¹⁵NH ₄ ⁺ and treated with acetylene, no¹⁵NO ₃ ^– was detectable showing that the acetylene treatment effectively blocked the autotrophic nitrification, and that NH ₄ ⁺ was not a substrate for heterotrophic nitrification. In the clear cut area, autotrophic nitrification was the most important NO ₃ ^– generating process with total nitrification (45 ug N kg^–1h^–1) accounting for about one-third of gross N mineralization (140 ug N kg^–1 h^–1). In the young and mature forested sites, gross nitrification rates were largely unaffected by acetylene treatment indicating that heterotrophic nitrification dominated the NO ₃ ^– generating process in these areas. In the mature forest mineral and organic soil, nitrification (heterotrophic) was equal to only about 5% of gross mineralization (gross mineralization rates of 90 ug N kg^–1 h^–1 mineral; 550 ug N kg^–1 h^–1 organic). The gross nitrification rate decreased from the clear cut area to the young forest area to the mineral soil of the mature forest (45; 17; 4.5 ug kg^–1 h^–1 respectively). The¹⁵N isotope pool dilution method, combined with acetylene as an inhibitor of autotrophic nitrification provided an effective technique for assessing the importance of heterotrophic nitrification in the N-cycle of this mixed-conifer ecosystem.     

Factors promoting emissions of nitrous oxide and nitric oxide from denitrifying sequencing batch reactors operated with methanol and ethanol as electron donors

The emissions of nitrous oxide (N₂O) and nitric oxide (NO) from biological nitrogen removal (BNR) operations via nitrification and denitrification is gaining increased prominence. While many factors relevant to the operation of denitrifying reactors can influence N₂O and NO emissions from them, the role of different organic carbon sources on these emissions has not been systematically addressed or interpreted. The overall goal of this study was to evaluate the impact of three factors, organic carbon limitation, nitrite concentrations, and dissolved oxygen concentrations on gaseous N₂O and NO emissions from two sequencing batch reactors (SBRs), operated, respectively, with methanol and ethanol as electron donors. During undisturbed ultimate‐state operation, emissions of both N₂O and NO from either reactor were minimal and in the range of <0.2% of influent nitrate‐N load. Subsequently, the two reactors were challenged with transient organic carbon limitation and nitrite pulses, both of which had little impact on N₂O or NO emissions for either electron donor. In contrast, transient exposure to oxygen led to increased production of N₂O (up to 7.1% of influent nitrate‐N load) from ethanol grown cultures, owing to their higher kinetics and potentially lower susceptibility to oxygen inhibition. A similar increase in N₂O production was not observed from methanol grown cultures. These results suggest that for dissolved oxygen, but not for carbon limitation or nitrite exposure, N₂O emission from heterotrophic denitrification reactors can vary as a function of the electron donor used. Biotechnol. Bioeng. 2010; 106: 390–398. © 2010 Wiley Periodicals, Inc.     

Dominance of bacterial ammonium oxidizers and fungal denitrifiers in the complex nitrogen cycle pathways related to nitrous oxide emission

Organic compounds and mineral nitrogen (N) usually increase nitrous oxide (N₂O) emissions. Vinasse, a by‐product of bio‐ethanol production that is rich in carbon, nitrogen, and potassium, is recycled in sugarcane fields as a bio‐fertilizer. Vinasse can contribute significantly to N₂O emissions when applied with N in sugarcane plantations, a common practice. However, the biological processes involved in N₂O emissions under this management practice are unknown. This study investigated the roles of nitrification and denitrification in N₂O emissions from straw‐covered soils amended with different vinasses (CV: concentrated and V: nonconcentrated) before or at the same time as mineral fertilizers at different time points of the sugarcane cycle in two seasons. N₂O emissions were evaluated for 90 days, the period that occurs most of the N₂O emission from fertilizers; the microbial genes encoding enzymes involved in N₂O production (archaeal and bacterial amoA, fungal and bacterial nirK, and bacterial nirS and nosZ), total bacteria, and total fungi were quantified by real‐time PCR. The application of CV and V in conjunction with mineral N resulted in higher N₂O emissions than the application of N fertilizer alone. The strategy of vinasse application 30 days before mineral N reduced N₂O emissions by 65% for CV, but not for V. Independent of rainy or dry season, the microbial processes were nitrification by ammonia‐oxidizing bacteria (AOB) and archaea and denitrification by bacteria and fungi. The contributions of each process differed and depended on soil moisture, soil pH, and N sources. We concluded that amoA‐AOB was the most important gene related to N₂O emissions, which indicates that nitrification by AOB is the main microbial‐driven process linked to N₂O emissions in tropical soil. Interestingly, fungal nirK was also significantly correlated with N₂O emissions, suggesting that denitrification by fungi contributes to N₂O emission in soils receiving straw and vinasse application.     

Pulse Increase of Soil N2O Emission in Response to N Addition in a Temperate Forest on Mt Changbai,Northeast China

Nitrogen (N) deposition has increased significantly globally since the industrial revolution. Previous studies on the response of gaseous emissions to N deposition have shown controversial results, pointing to the system-specific effect of N addition. Here we conducted an N addition experiment in a temperate natural forest in northeastern China to test how potential changes in N deposition alter soil N₂O emission and its sources from nitrification and denitrification. Soil N₂O emission was measured using closed chamber method and a separate incubation experiment using acetylene inhibition method was carried out to determine denitrification fluxes and the contribution of nitrification and denitrification to N₂O emissions between Jul. and Oct. 2012. An NH₄NO₃ addition of 50 kg N/ha/yr significantly increased N₂O and N₂ emissions, but their “pulse emission” induced by N addition only lasted for two weeks. Mean nitrification-derived N₂O to denitrification-derived N₂O ratio was 0.56 in control plots, indicating higher contribution of denitrification to N₂O emissions in the study area, and this ratio was not influenced by N addition. The N₂O to (N₂+N₂O) ratio was 0.41–0.55 in control plots and was reduced by N addition at one sampling time point. Based on this short term experiment, we propose that N₂O and denitrification rate might increase with increasing N deposition at least by the same fold in the future, which would deteriorate global warming problems.     

Nitrous oxide emissions from inland waters: Are IPCC estimates too high?

Nitrous oxide (N₂O) emissions from inland waters remain a major source of uncertainty in global greenhouse gas budgets. N₂O emissions are typically estimated using emission factors (EFs), defined as the proportion of the terrestrial nitrogen (N) load to a water body that is emitted as N₂O to the atmosphere. The Intergovernmental Panel on Climate Change (IPCC) has proposed EFs of 0.25% and 0.75%, though studies have suggested that both these values are either too high or too low. In this work, we develop a mechanistic modeling approach to explicitly predict N₂O production and emissions via nitrification and denitrification in rivers, reservoirs and estuaries. In particular, we introduce a water residence time dependence, which kinetically limits the extent of denitrification and nitrification in water bodies. We revise existing spatially explicit estimates of N loads to inland waters to predict both lumped watershed and half‐degree grid cell emissions and EFs worldwide, as well as the proportions of these emissions that originate from denitrification and nitrification. We estimate global inland water N₂O emissions of 10.6–19.8 Gmol N year^?1 (148–277 Gg N year^?1), with reservoirs producing most N₂O per unit area. Our results indicate that IPCC EFs are likely overestimated by up to an order of magnitude, and that achieving the magnitude of the IPCC's EFs is kinetically improbable in most river systems. Denitrification represents the major pathway of N₂O production in river systems, whereas nitrification dominates production in reservoirs and estuaries.     

Residue incorporation depth is a controlling factor of earthworm‐induced nitrous oxide emissions

Earthworms can increase nitrous oxide (N₂O) emissions, particularly in no‐tillage systems where earthworms are abundant. Here, we study the effect of residue incorporation depth on earthworm‐induced N₂O emissions. We hypothesized that cumulative N₂O emissions decrease with residue incorporation depth, because (i) increased water filled pore space (WFPS) in deeper soil layers leads to higher denitrification rates as well as more complete denitrification; and (ii) the longer upward diffusion path increases N₂O reduction to N₂. Two 84‐day laboratory mesocosm experiments were conducted. First, we manually incorporated maize (Zea mays L.) residue at different soil depths (incorporation experiment). Second, ¹³C‐enriched maize residue was applied to the soil surface and anecic species Lumbricus terrestris (L.) and epigeic species Lumbricus rubellus (Hoffmeister) were confined to different soil depths (earthworm experiment). Residue incorporation depth affected cumulative N₂O emissions in both experiments (P < 0.001). In the incorporation experiment, N₂O emissions decreased from 4.91 mg N₂O–N kg^?1 soil (surface application) to 2.71 mg N₂O–N kg^?1 soil (40–50 cm incorporation). In the earthworm experiment, N₂O emissions from L. terrestris decreased from 3.87 mg N₂O–N kg^?1 soil (confined to 0–10 cm) to 2.01 mg N₂O–N kg^?1 soil (confined to 0–30 cm). Both experimental setups resulted in dissimilar WFPS profiles that affected N₂O dynamics. We also found significant differences in residue C recovery in soil organic matter between L. terrestris (28–41%) and L. rubellus (56%). We conclude that (i) N₂O emissions decrease with residue incorporation depth, although this effect was complicated by dissimilar WFPS profiles; and (ii) larger residue C incorporation by L. rubellus than L. terrestris indicates that earthworm species differ in their C stabilization potential. Our findings underline the importance of studying earthworm diversity in the context of greenhouse gas emissions from agro‐ecosystems.     

Mechanism leading to N<Subscript>2</Subscript>O production in wastewater treating biofilm systems

Wastewater treatment plants are known to be important point sources for nitrous oxide (N₂O) in the anthropogenic N cycle. Biofilm based treatment systems have gained increasing popularity in the treatment of wastewater, but the mechanisms and controls of N₂O formation are not fully understood. Here, we review functional groups of microorganism involved in nitrogen (N) transformations during wastewater treatment, with emphasis on potential mechanism of N₂O production in biofilms. Biofilms used in wastewater treatment typically harbour aerobic and anaerobic zones, mediating close interactions between different groups of N transforming organisms. Current models of mass transfer and biomass interactions in biofilms are discussed to illustrate the complex regulation of N₂O production. Ammonia oxidizing bacteria (AOB) are the prime source for N₂O in aerobic zones, while heterotrophic denitrifiers dominate N₂O production in anoxic zones. Nitrosative stress ensuing from accumulation of NO₂ ^? during partial nitrification or denitrification seems to be one of the most critical factors for enhanced N₂O formation. In AOB, N₂O production is coupled to nitrifier denitrification triggered by nitrosative stress, low O₂ tension or low pH. Chemical N₂O production from AOB intermediates (NH₂OH, HNO, NO) released during high NH₃ turnover seems to be limited to surface-near AOB clusters, since diffusive mass transport resistance for O₂ slows down NH₃ oxidation rates in deeper biofilm layers. The proportion of N₂O among gaseous intermediates (NO, N₂O, N₂) in heterotrophic denitrification increases when NO or nitrous acid (HNO₂) accumulates because of increasing NO₂ ^?, or when transient oxygen intrusion impairs complete denitrification. Limited electron donor availability due to mass transport limitation of organic substrates into anoxic biofilm zones is another important factor supporting high N₂O/N₂ ratios in heterotrophic denitrifiers. Biofilms accommodating Anammox bacteria release less N₂O, because Anammox bacteria have no known N₂O producing metabolism and reduce NO₂ ^? to N₂, thereby lowering nitrosative stress to AOB and heterotrophs.     

Isotopic signatures of N2O produced by ammonia-oxidizing archaea from soils

N₂O gas is involved in global warming and ozone depletion. The major sources of N₂O are soil microbial processes. Anthropogenic inputs into the nitrogen cycle have exacerbated these microbial processes, including nitrification. Ammonia-oxidizing archaea (AOA) are major members of the pool of soil ammonia-oxidizing microorganisms. This study investigated the isotopic signatures of N₂O produced by soil AOA and associated N₂O production processes. All five AOA strains (I.1a, I.1a-associated and I.1b clades of Thaumarchaeota) from soil produced N₂O and their yields were comparable to those of ammonia-oxidizing bacteria (AOB). The levels of site preference (SP), δ¹⁵N^bulk and δ¹⁸O -N₂O of soil AOA strains were 13–30%, −13 to −35% and 22–36%, respectively, and strains MY1–3 and other soil AOA strains had distinct isotopic signatures. A ¹⁵N-NH₄⁺-labeling experiment indicated that N₂O originated from two different production pathways (that is, ammonia oxidation and nitrifier denitrification), which suggests that the isotopic signatures of N₂O from AOA may be attributable to the relative contributions of these two processes. The highest N₂O production yield and lowest site preference of acidophilic strain CS may be related to enhanced nitrifier denitrification for detoxifying nitrite. Previously, it was not possible to detect N₂O from soil AOA because of similarities between its isotopic signatures and those from AOB. Given the predominance of AOA over AOB in most soils, a significant proportion of the total N₂O emissions from soil nitrification may be attributable to AOA.     

Denitrification and N2O emission from urine-affected grassland soil

Denitrification and N₂O emission rates were measured following two applications of artificial urine (40 g urine-N m^–2) to a perennial rye-grass sward on sandy soil. To distinguish between N₂O emission from denitrification or nitrification, urine was also applied with a nitrification inhibitor (dicyandiamide, DCD). During a 14 day period following each application, the soil was frequently sampled, and incubated with and without acetylene to measure denitrification and N₂O emission rates, respectively.Urine application significantly increased denitrification and N₂O emission rates up to 14 days after application, with rates amounting to 0.9 and 0.6 g N m^–2 day^–1 (9 and 6 kg N ha^–1 day^–1), respectively. When DCD was added to the urine, N₂O emission rates were significantly lower from 3 to 7 days after urine application onwards. Denitrification was the main source of N₂O immediately following each urine application. 14 days after the first application, when soil water contents dropped to 15% (v/v) N₂O mainly derived from nitrification.Total denitrification losses during the 14 day periods were 7 g N m^–2, or 18% of the urine-N applied. Total N₂O emission losses were 6.5 and 3 g N m^–2, or 16% and 8% of the urine-N applied for the two periods. The minimum estimations of denitrification and N₂O emission losses from urine-affected soil were 45 to 55 kg N ha^–1 year^–1, and 20 to 50 kg N ha^–1 year^–1, respectively.     

Deciphering the oxygen isotope composition of nitrous oxide produced by nitrification

The ability to use δ¹⁸O values of nitrous oxide (N₂O) to apportion environmental emissions is currently hindered by a poor understanding of the controls on δ¹⁸O–N₂O from nitrification (hydroxylamine oxidation to N₂O and nitrite reduction to N₂O). In this study fertilized agricultural soils and unfertilized temperate forest soils were aerobically incubated with different ¹⁸O/¹⁶O waters, and conceptual and mathematical models were developed to systematically explain the δ¹⁸O–N₂O formed by nitrification. Modeling exercises used a set of defined input parameters to emulate the measured soil δ¹⁸O–N₂O data (Monte Carlo approach). The Monte Carlo simulations implied that abiotic oxygen (O) exchange between nitrite (NO₂^?) and H₂O is important in all soils, but that biological, enzyme‐controlled O‐exchange does not occur during the reduction of NO₂^? to N₂O (nitrifier‐denitrification). Similarly, the results of the model simulations indicated that N₂O consumption is not characteristic of aerobic N₂O formation. The results of this study and a synthesis of the published literature data indicate that δ¹⁸O–N₂O formed in aerobic environments is constrained between +13‰ and +35‰ relative to Vienna Standard Mean Ocean Water (VSMOW). N₂O formed via hydroxylamine oxidation and nitrifier‐denitrification cannot be separated using δ¹⁸O unless ¹⁸O tracers are employed. The natural range of nitrifier δ¹⁸O–N₂O is discussed and explained in terms of our conceptual model, and the major and minor controls that define aerobically produced δ¹⁸O–N₂O are identified. Despite the highly complex nature of δ¹⁸O–N₂O produced by nitrification this δ¹⁸O range is narrow. As a result, in many situations δ¹⁸O values may be used in conjunction with δ¹⁵N–N₂O data to apportion nitrifier‐ and denitrifier‐derived N₂O. However, when biological O‐exchange during denitrification is high and N₂O consumption is low, there may be too much overlap in δ¹⁸O values to distinguish N₂O formed by these pathways.