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Denitrification plays a critical role in regulating ecosystem nutrient availability and anthropogenic reactive nitrogen (N) production. Its importance has inspired an increasing number of studies, yet it remains the most poorly constrained term in terrestrial ecosystem N budgets. We censused the peer-reviewed soil denitrification literature (1975-2015) to identify opportunities for future studies to advance our understanding despite the inherent challenges in studying the process. We found that only one-third of studies reported estimates of both nitrous oxide (N 2 O) and dinitrogen (N 2 ) production fluxes, often the dominant end products of denitrification, while the majority of studies reported only net N 2 O fluxes or denitrification potential. Of the 236 studies that measured complete denitrification to N 2 , 49% used the acetylene inhibition method, 84% were conducted in the laboratory, 81% were performed on surface soils (0-20 cm depth), 75% were located in North America and Europe, and 78% performed treatment manipulations, mostly of N, carbon, or water. To improve understanding of soil denitrification, we recommend broadening access to technologies for new methodologies to measure soil N 2 production rates, conducting more studies in the tropics and on subsoils, performing standardized experiments on unmanipulated soils, and using more precise terminology to refer to measured process rates (e.g., net N 2 O flux or denitrification potential). To overcome the greater challenges in studying soil denitrification, we envision coordinated research efforts based on standard reporting of metadata for all soil denitrification studies, standard protocols for studies contributing to a Global Denitrification Research Network, and a global consortium of denitrification researchers to facilitate sharing ideas, resources, and to provide mentorship for researchers new to the field., (© 2019 by the Ecological Society of America.)

Invasive plants can increase soil nitrogen (N) pools and accelerate soil N cycling rates, but their effect on gross N cycling and nitrous oxide (N2 O) emissions has rarely been studied. We hypothesized that perennial pepperweed (Lepidium latifolium) invasion would increase rates of N cycling and gaseous N loss, thereby depleting ecosystem N and causing a negative feedback on invasion. We measured a suite of gross N cycling rates and net N2 O fluxes in invaded and uninvaded areas of an annual grassland in the Sacramento-San Joaquin River Delta region of northern California. During the growing season, pepperweed-invaded soils had lower microbial biomass N, gross N mineralization, dissimilatory nitrate reduction to ammonium (DNRA), and denitrification-derived net N2 O fluxes (P

Rates of nitrogen transformations support quantitative descriptions and predictive understanding of the complex nitrogen cycle, but measuring these rates is expensive and not readily available to researchers. Here, we compiled a dataset of gross nitrogen transformation rates (GNTR) of mineralization, nitrification, ammonium immobilization, nitrate immobilization, and dissimilatory nitrate reduction to ammonium in terrestrial ecosystems. Data were extracted from 331 studies published from 1984–2022, covering 581 sites. Globally, 1552 observations were appended with standardized soil, vegetation, and climate data (49 variables in total) potentially contributing to the observed variations of GNTR. We used machine learning-based data imputation to fill in partially missing GNTR, which improved statistical relationships between theoretically correlated processes. The dataset is currently the most comprehensive overview of terrestrial ecosystem GNTR and serves as a global synthesis of the extent and variability of GNTR across a wide range of environmental conditions. Future research can utilize the dataset to identify measurement gaps with respect to climate, soil, and ecosystem types, delineate GNTR for certain ecoregions, and help validate process-based models. [ABSTRACT FROM AUTHOR]

Installation of subsurface drainage systems has profoundly altered the nitrogen cycle in agricultural regions across the globe, facilitating substantial loss of nitrate (NO3−) to surface water systems. Lack of understanding of the sources and processes controlling NO3− loss from tile‐drained agroecosystems hinders the development of management strategies aimed at reducing this loss. The natural abundance nitrogen and oxygen isotopes of NO3− provide a valuable tool for differentiating nitrogen sources and tracking the biogeochemical transformations acting on NO3−. This study combined multi‐years of tile drainage measurements with NO3− isotopic analysis to examine NO3− source and transport mechanisms in a tile‐drained corn‐soybean field. The tile drainage NO3− isotope data were supplemented by characterization of the nitrogen isotopic composition of potential NO3− sources (fertilizer, soil nitrogen, and crop biomass) in the field and the oxygen isotopic composition of NO3− produced by nitrification in soil incubations. The results show that NO3− isotopes in tile drainage were highly responsive to tile discharge variation and fertilizer input. After accounting for isotopic fractionations during nitrification and denitrification, the isotopic signature of tile drainage NO3− was temporally stable and similar to those of fertilizer and soybean residue during unfertilized periods. This temporal invariance in NO3− isotopic signature indicates a nitrogen legacy effect, possibly resulting from N recycling at the soil microsite scale and a large water storage for NO3− mixing. Collectively, these results demonstrate how combining field NO3− isotope data with knowledge of isotopic fractionations can reveal mechanisms controlling NO3− cycling and transport under complex field conditions. Plain Language Summary: Installation of subsurface tile pipes in many poorly drained agricultural lands has facilitated a substantial loss of nitrate (NO3−) to surface water systems. However, the nitrogen sources and related processes controlling NO3− export from tile‐drained agricultural systems remain unclear. This study employed stable isotope techniques to investigate how NO3− is biologically produced and hydrologically transported in a tile‐drained field. Stable isotopes are chemical variants of the same element and have long been used as a tracer of nitrogen cycling in environmental systems. By combining field measurements of NO3− isotopes in tile drainage with a detailed understanding of how these isotopes are altered by microbial reactions, we estimated the original isotope ratios of NO3− and compared these ratios to those of potential nitrogen sources in the field. The results show that the original isotope ratios of NO3− were similar to those of ammonia fertilizer and soybean biomass nitrogen and did not vary over time when there was no fertilizer input to the system. These findings indicate the presence of a large NO3− pool in the soil and a time lag between the moments when the source nitrogen was introduced into the system and when the NO3− was exported via tile drainage. Key Points: The oxygen isotopic composition of nitrate produced by soil nitrification varied with the degree of soil nitrite accumulationThe dual isotopes of nitrate in tile drainage exhibited coupled variations and were highly responsive to variations in tile dischargeCombining field nitrate isotope data with the isotopic systematics of nitrification reveals a legacy effect controlling nitrate dynamics [ABSTRACT FROM AUTHOR]

Sea level rise will change inundation regimes in salt marshes, altering redox dynamics that control nitrification - a potential source of the potent greenhouse gas, nitrous oxide (N2 O) - and denitrification, a major nitrogen (N) loss pathway in coastal ecosystems and both a source and sink of N2 O. Measurements of net N2 O fluxes alone yield little insight into the different effects of redox conditions on N2 O production and consumption. We used in situ measurements of gross N2 O fluxes across a salt marsh elevation gradient to determine how soil N2 O emissions in coastal ecosystems may respond to future sea level rise. Soil redox declined as marsh elevation decreased, with lower soil nitrate and higher ferrous iron in the low marsh compared to the mid and high marshes (P

Salt marshes currently play an important role as filters for upslope nitrogen (N) inputs. This could change in the future with sea level rise, warming and eutrophication, which are expected to favor monocultures over diverse plant communities. We explored patterns in gross N cycling, dissimilatory nitrate (NO3-) reduction to ammonium (NH4+) (DNRA), and denitrification in a salt marsh soil under two typical redox conditions (aerobic and anaerobic), and in soils under plant communities manipulated to simulate potential future composition (forb and graminoid monocultures). Natural salt marsh soils exhibited high potential gross N mineralization rates. Averaging 50.4 6 5.7 lg N g-1 d-1 under aerobic conditions; rates declined to 23.6 6 3.4 lg N g-1 d-1 under an N2 headspace. Microbial NH4+ uptake and gross nitrification together accounted for only 14 % of gross N mineralization. Nitrogen retention via DNRA and microbial uptake greatly exceeded N losses via denitrification. Gross nitrification rates were greater in the forb and graminoid monocultures than in the control. This effect may be mediated by the lower plant biomass in the monocultures than in the control, which may have reduced competition between plants and nitrifiers for NH4+. Soil NO3- concentrations and net nitrous oxide (N2O) fluxes were greatest for the forb monoculture, likely due to higher soil oxygen (O2) concentrations in these plots. Our results suggest that salt marsh soils with a diverse plant community have high potential rates of N mineralization and microbial N retention, and the establishment of forb monocultures could lead to greater ecosystem N losses.

Nitrification regulates the potential for nitrogen (N) loss from ecosystems because it converts ammonium to nitrate, which is susceptible to leaching and gaseous emissions. Crops can suppress the microbes that perform nitrification by exuding nitrification‐inhibiting compounds from their roots and taking up available ammonium. However, the effect of nitrification suppression on agroecosystem N losses remains poorly characterized, and a lack of temporal synchrony between nitrification, N losses, and nitrification suppression by plants could limit the effect of nitrification suppression. We used the DayCent‐CABBI model to evaluate the effectiveness of the suppression of nitrification by sorghum to reduce N2O emissions and nitrate leaching in an energy sorghum/soybean rotation at the Energy Farm in Urbana‐Champaign, IL. We simulated nitrification suppression at the measured levels (MeasNS) and at the maximum measured level applied to the entire growing season (MaxNS), and we also explored ways to better utilize nitrification suppression by altering the timing of urea–ammonium–nitrate fertilizer applications. Model experiments showed that most nitrification occurred immediately after fertilizer was applied, whereas nitrification suppression began to ramp up more than a month after planting. On an annual basis, MeasNS experiments showed a 1%–2% reduction in annual N2O emissions relative to no nitrification suppression (NoNS), and MaxNS experiments showed a 4%–9% reduction in annual N2O emissions relative to NoNS. Both nitrification suppression levels showed <1% reduction in nitrate leaching. Altering the timing of fertilizer applications to better synchronize nitrification suppression with high soil ammonium levels had mixed effects on annual N2O emissions and nitrate leaching and sometimes resulted in increased N losses. The timing of simulated N2O emissions shifted with the timing of fertilization, and N2O emissions from denitrification increased when N2O emissions from nitrification decreased. Increasing N retention during the non‐growing season may be more effective than growing‐season nitrification suppression for reducing annual N losses in the rainfed Midwest. [ABSTRACT FROM AUTHOR]

Biological nitrification inhibition (BNI) and plant–microbe competition for ammonium (NH4+) by sorghum (Sorghum bicolor (L.) Moench) have the potential to suppress nitrification, reducing nitrate (NO3−) and nitrous oxide (N2O) production for more sustainable bioenergy feedstock production. However, it is unknown how variability in environmental factors, field management, and plant growth affect the suppression of nitrification. We conducted a field trial with four genotypes of energy sorghum and four fertilization rates in central Illinois, USA, and measured soil N pools, potential nitrification and denitrification rates, and microbial community composition in bulk and rhizosphere soils to assess nitrification suppression throughout the 2018 and 2019 growing seasons. Concentrations of NO3− and NH4+ were very low in rhizosphere soil regardless of fertilization level, suggesting strong N demand by plants and microbes. Potential nitrification was lower in the rhizosphere soil than bulk soil, and this suppression was strongest mid‐season ~2 months after planting in both years (20% suppression in 2018 and 58% in 2019). Since precipitation was lower during the mid‐growing season of 2019 compared to 2018, we speculate that hydrophilic BNI root exudates accumulated in the rhizosphere and suppressed nitrification more than in 2018 when soil moisture was higher. Unfertilized plots had greater nitrification suppression than fertilized plots during the mid‐season in 2018, but otherwise nitrification suppression was insensitive to fertilizer treatment. Potential denitrification was stimulated in the rhizosphere compared to bulk soil in both study years, suggesting that heterotrophic activity was stimulated by plant carbon inputs, possibly further suppressing slower‐growing chemoautotrophic nitrifying microbes. Overall, we found inter‐ and intra‐annual variation in nitrification suppression in the rhizosphere of field‐grown biomass sorghum, suggesting that plant phenology and environmental conditions should be considered when devising strategies to improve the nitrogen sustainability of this annual bioenergy crop. [ABSTRACT FROM AUTHOR]

Many biotic and abiotic processes contribute to nitrous oxide (N2O) production in the biosphere, but N2O consumption in the environment has heretofore been attributed primarily to canonical denitrifying microorganisms. The nosZ genes encoding the N2O reductase enzyme, NosZ, responsible for N2O reduction to dinitrogen are now known to include two distinct groups: the well‐studied Clade I which denitrifiers typically possess, and the novel Clade II possessed by diverse groups of microorganisms, most of which are non‐denitrifiers. Clade II N2O reducers could play an important, previously unrecognized role in controlling N2O emissions for several reasons, including: (1) the consumption of N2O produced by processes other than denitrification, (2) hypothesized non‐respiratory functions of NosZ as an electron sink or for N2O detoxification, (3) possible differing enzyme kinetics of Clade II NosZ compared to Clade I NosZ, and (4) greater nosZ gene abundance for Clade II compared to Clade I in soils of many ecosystems. Despite the potential ecological significance of Clade II NosZ, a census of 800 peer‐reviewed original research articles discussing nosZ and published from 2013 to 2019 showed that the percentage of articles evaluating or mentioning Clade II nosZ increased from 5% in 2013 to only 22% in 2019. The census revealed that the slowly spreading awareness of Clade II nosZ may result in part from disciplinary silos, with the percentage of nosZ articles mentioning Clade II nosZ ranging from 0% in Agriculture and Agronomy journals to 32% in Multidisciplinary Sciences journals. In addition, inconsistent nomenclature for Clade I nosZ and Clade II nosZ, with 17 different terminologies used in the literature, may have created confusion about the two distinct groups of N2O reducers. We provide recommendations to accelerate advances in understanding the role of the diversity of N2O reducers in regulating soil N2O emissions. [ABSTRACT FROM AUTHOR]

Soil nitrous oxide (N2O) emissions are highly variable in space and time, making it difficult to estimate ecosystem level fluxes of this potent greenhouse gas. While topographic depressions are often evoked as permanent N2O hot spots and rain events are well‐known triggers of N2O hot moments, soil N2O emissions are still poorly predicted. Thus, the objective of this study was to determine how to best use topography and rain events as variables to predict soil N2O emissions at the field scale. We measured soil N2O emissions 11 times over the course of one growing season from 65 locations within an agricultural field exhibiting microtopography. We found that the topographic indices best predicting soil N2O emissions varied by date, with soil properties as consistently poor predictors. Large rain events (>30 mm) led to an N2O hot moment only in the early summer and not in the cool spring or later in the summer when crops were at peak growth and likely had high evapotranspiration rates. In a laboratory experiment, we demonstrated that low heterotrophic respiration rates at cold temperatures slowly depleted soil dissolved O2, thus suppressing denitrification over the 2–3 day timescale typical of field ponding. Our findings show that topographic depressions do not consistently act as N2O hot spots and that rainfall does not consistently trigger N2O hot moments. We assert that the spatiotemporal variation in soil N2O emissions is not always characterized by predictable hot spots or hot moments and that controls on this variation change depending on environmental conditions. Plain Language Summary: Soils are the primary source of nitrous oxide (N2O), a greenhouse gas that contributes to global warming. Nitrous oxide is produced under oxygen‐depleted conditions that can occur when soils are saturated with water. Large rain events are therefore thought to lead to short periods of high soil N2O emissions. Landscape topography can shape spatial patterns in soil moisture and other soil properties that may regulate spatial variation in soil N2O emissions. The goals of this study were to determine which topographic factors best predict spatial variation in soil N2O emissions and under what conditions, and to determine when large rain events may not lead to bursts of soil N2O emissions. We found that throughout the growing season, all topographic indices were poor predictors of soil N2O emissions in an agricultural field. A large early summer rain event triggered a pulse of soil N2O emissions, but large rain events did not trigger similar pulses in the cool spring months or later in the summer when the crops were at peak growth with high soil water demand. This has important implications for predicting how soil N2O emissions will respond to expected future changes in temperature and rainfall, feeding back on climate change. Key Points: Topographic depressions did not consistently act as nitrous oxide hot spots, with controls on fluxes varying over the growing seasonCold spring temperatures limit soil respiration that depletes dissolved oxygen to induce nitrous oxide production by denitrificationLarge summer rainfall events may not stimulate soil nitrous oxide emissions, even from topographic depressions, when soil moisture is low [ABSTRACT FROM AUTHOR]

Dinitrogen (N2) production from denitrification and anaerobic ammonium oxidation represents a loss of reactive nitrogen (N) from terrestrial and aquatic ecosystems to the atmosphere. The large 15N additions required to detect 15N2 production against the high atmospheric background precludes the use of the 15N tracer technique in natural terrestrial ecosystems. We present an isotope ratio mass spectrometry technique that dramatically improves the precision of 15N2 measurements. The approach uses gas chromatography to remove oxygen and gas purification techniques to remove water vapor and trace gases that can interfere with 15N2 analysis. The analytical precision for manual gas sample injection was 0.018‰ δ15N; this translates to a minimum detectable N2 flux of 0.12 ng-N g−1 h−1 over a 24 h incubation using our experimental parameters. We measured denitrification-derived N2 production rates of 0.67 ± 0.04 ng N g−1 dry soil h−1 following the addition of 0.1 μg 98 atom % 15N-NO3− g−1 dry soil to anaerobically-incubated soils; rates were significantly higher with the addition of 1.0 μg 98 atom % 15N-NO3− g−1 dry soil (p < 0.001, n = 5), averaging 1.3 ± 0.03 ng N g−1 dry soil h−1. The N2:N2O ratio at 24 h after 15N addition was 48 ± 12 and 5.4 ± 1.9 for the 0.1 μg 15N g−1 and 1.0 μg 15N g−1 treatments, respectively. The decrease in the N2:N2O with the addition of only 1 μg 15N g−1 underscores the need to use very low rates of 15N addition to accurately characterize denitrification dynamics. This analytical advance will allow us to better estimate the N2:N2O ratio of denitrification, constrain ecosystem N budgets, and explore the mechanisms of and controls on N2 production. [ABSTRACT FROM AUTHOR]

Soils are both a major source and sink of nitrous oxide ( N2 O), but the proportion of soil N2 O production released to the atmosphere (termed the N2 O yield) is poorly constrained due to the difficulty in measuring gross N2 O production. The quantification of gross N2 O fluxes would greatly improve our ability to predict N2 O dynamics across the soil-atmosphere interface. We report a new approach, the 15 N2 O pool dilution technique, to measure rates of gross N2 O production and consumption under laboratory and field conditions. In the laboratory, gross N2 O production and consumption compared well between the 15 N2 O pool dilution and acetylene inhibition methods whereas the 15 NO3− tracer method measured significantly higher rates. In the field, N2 O emissions were not significantly affected by increasing chamber headspace concentrations up to 100 ppb 15 N2 O. The pool dilution model estimates of 14 N2 O and 15 N2 O concentrations as well as net N2 O fluxes fit observed data very well, suggesting that the technique yielded robust estimates of gross N2 O production. Estimated gross N2 O consumption rates were underestimated relative to rates calculated as the difference between gross and net N2 O production rates, possibly due to heterogeneous and/or inadequate tracer diffusion to deeper layers in the soil profile. Gross N2 O production rates were high, averaging 8.4 ± 3.2 mg N m−2 day−1, and were most strongly correlated to mineral nitrogen concentrations and denitrifying enzyme activity ( R2 = 0.73). Gross N2 O production rates varied spatially, with the highest rates in soils with the best drainage and the highest mineral N availability. Estimated and calculated N2 O consumption rates constrained the average N2 O yield from 0.70 to 0.84. Our results demonstrate that the 15 N2 O pool dilution technique can provide well-constrained estimates of N2 O yields and field rates of gross N2 O production correlated to soil characteristics, improving our understanding of terrestrial N2 O dynamics. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to an article related to nitrogen production in soil, published in a previous issue.

Biochar application is a promising approach to decrease nitrous oxide (N 2 O) emissions from agricultural soils, yet little is known about how biochar affects N 2 O-producing and consuming microbial processes under changing field conditions. We conducted a field study to assess if growing season patterns in soil N 2 O emissions were correlated with the underlying microbial processes of nitrification and denitrification. We measured soil N 2 O emissions, potential nitrification and denitrification rates, and the abundance of key soil nitrogen (N)-cycling functional genes in an intensive maize production field trial in Illinois, USA that included the following four treatments: Control (unamended), Biochar (100 Mg ha −1 ), Nitrogen (269 kg N ha −1 as Urea Ammonium Nitrate fertilizer), and Biochar + Nitrogen (100 Mg ha −1 and 269 kg N ha −1 , respectively). Biochar increased potential nitrification rates when soil ammonium concentrations were high following fertilizer application, thus enhancing N 2 O emissions in the Biochar + Nitrogen treatment early in the season which were likely nitrification-associated. However, over the full growing season, biochar application reduced cumulative N 2 O emissions in Biochar + Nitrogen plots to levels similar to the unamended Control. This could be attributed to biochar suppression of potential denitrification throughout the growing season. The treatments amended with biochar avoided large pulses of N 2 O emissions following intense rain events in the mid-season, while also sustaining lower N 2 O emissions in the late-season. Our study demonstrates that biochar can have dynamic effects on soil N 2 O emissions and the underlying microbial processes that depend on changing edaphic conditions, such as soil inorganic nitrogen availability and moisture, over the growing season. [ABSTRACT FROM AUTHOR]

High spatiotemporal variability in soil nitrous oxide (N 2 O) fluxes challenges quantification and prediction of emissions to evaluate the climate change mitigation outcomes of sustainable agricultural practices. Triggers for large, short-lived N 2 O emission pulses, such as rainfall and fertilization, alter soil oxygen (O 2) and nitrate (NO 3 –) availability to favor N 2 O production via denitrification. However, the organic C (OC) needed to fuel denitrification may exhibit subfield variation that constrains the potential for high denitrification rates to occur, leading to spatial variation in N 2 O hot moments. We tested the hypothesis that the particulate organic matter (POM) fraction of soil organic matter (SOM) controls subfield variation in denitrification potential by regulating availability of dissolved organic C (DOC), the form of OC used by denitrifiers. Among 38 soil samples collected across a maize field in central Illinois, USA, we found that potential denitrification rate was best predicted by POM C concentration (R2 = 0.35). Using multiple linear regression analysis that included other soil properties as explanatory variables, we found that POM C fraction of bulk soil (mg POM C/g SOC) was the most important predictor based on regression coefficient size (P < 0.01). Our results, which provide support for our hypothesis, suggest that consideration of the link between C and N cycling may be a key to predicting spatiotemporal variation in soil N 2 O emissions when denitrification is the dominant N 2 O source process. [ABSTRACT FROM AUTHOR]