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Urease and nitrification inhibitors can reduce ammonia and greenhouse gas emissions from fertilizers and manure but their effectiveness depends on the conditions under which they are used. Consequently, it is essential for the credibility of emission reductions reported in regulatory emission inventories that their effectiveness is assessed under real-world conditions and not just in the laboratory. Here, we specify the criteria we consider necessary before the effects of inhibitors are included in regulatory emission inventories., (© 2024 John Wiley & Sons Ltd.)

Anthropogenic nitrogen (N) loading alters soil ammonia‐oxidizing archaea (AOA) and bacteria (AOB) abundances, likely leading to substantial changes in soil nitrification. However, the factors and mechanisms determining the responses of soil AOA:AOB and nitrification to N loading are still unclear, making it difficult to predict future changes in soil nitrification. Herein, we synthesize 68 field studies around the world to evaluate the impacts of N loading on soil ammonia oxidizers and nitrification. Across a wide range of biotic and abiotic factors, climate is the most important driver of the responses of AOA:AOB to N loading. Climate does not directly affect the N‐stimulation of nitrification, but does so via climate‐related shifts in AOA:AOB. Specifically, climate modulates the responses of AOA:AOB to N loading by affecting soil pH, N‐availability and moisture. AOB play a dominant role in affecting nitrification in dry climates, while the impacts from AOA can exceed AOB in humid climates. Together, these results suggest that climate‐related shifts in soil ammonia‐oxidizing community maintain the N‐stimulation of nitrification, highlighting the importance of microbial community composition in mediating the responses of the soil N cycle to N loading. [ABSTRACT FROM AUTHOR]

In our study at Mt. Kilimanjaro, East Africa, we quantified gross rates of ammonification, nitrification, nitrogen immobilization, and dissimilatory nitrate reduction to ammonium in soils across different land uses, climate zones (savanna, montane forest ecosystems, extensive agroforest homegarden, and intensively managed coffee plantation), and seasons (dry, wet, and transition from dry to wet season) to identify if and to what extent conversion of natural ecosystems to cultivated land has affected key soil microbial nitrogen turnover processes. Overall variation of gross soil nitrogen turnover rates across different ecosystems was more pronounced than seasonal variations, with the highest turnover rates occurring at the transition between dry and wet seasons. Nitrogen production and immobilization rates positively correlated with soil organic carbon and total nitrogen concentrations as well as substrate availability of dissolved organic carbon and nitrogen r > 0.67, P < 0.05), but did not correlate with soil ammonium and nitrate concentrations. Soil nitrogen turnover rates were highest in the montane Ocotea forest (ammonification 29.84, nitrification 12.67, NH immobilization 38.92, NO immobilization 10.74, and DNRA 1.54 µg N g SDW d) and progressively decreased with decreasing annual rainfall and increasing land-use intensity. Using indicators of N retention and characteristics of soil nutrient status, we observed a grouping of faster, but tighter N cycling in the (semi-) natural savanna and Ocotea forest. This contrasted with a more open N cycle in managed systems (the homegarden and coffee plantation) where N was more prone to leaching or gaseous losses due to high nitrate production rates. The partly disturbed (selected logging) lower montane forest ranged between these two groups. [ABSTRACT FROM AUTHOR]

There is an increasing demand for the sustainable management of old-field communities in northern China, which have developed on abandoned cropland on formerly converted natural steppe sites, to regain forage yield, biodiversity, and soil fertility. In thus study we examined how two management options-clipping and nitrogen (N) addition-may affect net >microbial N mineralization (ammonification + nitrification), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and microbial respirations (MR) in grass dominated, herb dominated, and grass-herb mixed patches in an old-field community in northern China.Topsoil (0-10 cm) net N mineralization rate was 177% and 69% higher in mixed grass and herb patches (patch B) as compared to unmixed grass (patch A) or herb (patch C) patches, respectively. Topsoil MBN was significantly different among the three patches with the highest value for soils taken from umixed grass patches. However, patches with mixed grass and herb or herb dominated patches had 12% higher microbial respiration (MR) than unmixed grass patch. Clipping and N addition had no effects on net N mineralization or MBC, but both treatments decreased MBN and MR and increased the ratio between microbial biomass C and microbial biomass N (MBC/MBN) in the growing season. Incubation of soil cores under optimal water and temperature conditions in the laboratory showed that the response of microbial N transformations in soils under different vegetation patches to experimental N addition and clipping was limited by soil water availability. Our results strongly highlight the need to further study the importance of belowground C supply as a control of microbial N cycling processes. It also suggests that during the restoration process of degenerated croplands N cycling rates are stimulated, but that the magnitude of this stimulation is modulated by plant community composition of the old-fields. [ABSTRACT FROM AUTHOR]

Microbial respiratory reduction of nitrous oxide (N2O) to dinitrogen (N2) via denitrification plays a key role within the global N-cycle since it is the most important process for converting reactive nitrogen back into inert molecular N2. However, due to methodological constraints, we still lack a comprehensive, quantitative understanding of denitrification rates and controlling factors across various ecosystems. We investigated N2, N2O and NO emissions from irrigated cotton fields within the Aral Sera Basin using the He/O2 atmosphere gas flow soil core technique and an incubation assay. NH4NO3 fertilizer, equivalent to 75 kg ha−1 and irrigation water, adjusting the water holding capacity to 70, 100 and 130% were applied to the incubation vessels to assess its influence on gaseous N emissions. Under soil conditions as they are naturally found after concomitant irrigation and fertilization, denitrification was the dominant process and N2 the main end product of denitrification. The mean ratios of N2/N2O emissions increased with increasing soil moisture content. N2 emissions exceeded N2O emissions by a factor of 5 ± 2 at 70% soil water holding capacity (WHC) and a factor of 55 ± 27 at 130% WHC. The mean ratios of N2O/NO emissions varied between 1.5 ± 0.4 (70% WHC) and 644 ± 108 (130% WHC). The magnitude of N2 emissions for irrigated cotton was estimated to be in the range of 24 ± 9 to 175 ± 65 kg-N ha−1season−1, while emissions of NO were only of minor importance (between 0.1 to 0.7 kg-N ha−1 season−1). The findings demonstrate that for irrigated dryland soils in the Aral Sera Basin, denitrification is a major pathway of N-loss and that substantial amounts of N-fertilizer are lost as N2 to the atmosphere for irrigated dryland soils. [ABSTRACT FROM AUTHOR]

Abstract: Reduction of nitrous oxide (N2O) to dinitrogen (N2) by denitrification in soils is of outstanding ecological significance since it is the prevailing natural process converting reactive nitrogen back into inert molecular dinitrogen. Furthermore, the extent to which N2O is reduced to N2 via denitrification is a major regulating factor affecting the magnitude of N2O emission from soils. However, due to methodological problems in the past, extremely little information is available on N2 emission and the N2:N2O emission ratio for soils of terrestrial ecosystems. In this study, we simultaneously determined N2 and N2O emissions from intact soil cores taken from a mountainous beech forest ecosystem. The soil cores were taken from plots with distinct differences in microclimate (warm-dry versus cool-moist) and silvicultural treatment (untreated control versus heavy thinning). Due to different microclimates, the plots showed pronounced differences in pH values (range: 6.3–7.3). N2O emission from the soil cores was generally very low (2.0±0.5–6.3±3.8μgNm−2 h−1 at the warm-dry site and 7.1±3.1–57.4±28.5μgNm−2 h−1 at the cool-moist site), thus confirming results from field measurements. However, N2 emission exceeded N2O emission by a factor of 21±6–220±122 at the investigated plots. This illustrates that the dominant end product of denitrification at our plots and under the given environmental conditions is N2 rather than N2O. N2 emission showed a huge variability (range: 161±64–1070±499μgNm−2 h−1), so that potential effects of microclimate or silvicultural treatment on N2 emission could not be identified with certainty. However, there was a significant effect of microclimate on the magnitude of N2O emission as well as on the mean N2:N2O emission ratio. N2:N2O emission ratios were higher and N2O emissions were lower for soil cores taken from the plots with warm-dry microclimate as compared to soil cores taken from the cool-moist microclimate plots. We hypothesize that the increase in the N2:N2O emission ratio at the warm-dry site was due to higher N2O reductase activity provoked by the higher soil pH value of this site. Overall, the results of this study show that the N2:N2O emission ratio is crucial for understanding the regulation of N2O fluxes of the investigated soil and that reliable estimates of N2 emissions are an indispensable prerequisite for accurately calculating total N gas budgets for the investigated ecosystem and very likely for many other terrestrial upland ecosystems as well. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Gross rates of N mineralization and nitrification, and soil–atmosphere fluxes of N2O, NO and NO2 were measured at differently grazed and ungrazed steppe grassland sites in the Xilin river catchment, Inner Mongolia, P. R. China, during the 2004 and 2005 growing season. The experimental sites were a plot ungrazed since 1979 (UG79), a plot ungrazed since 1999 (UG99), a plot moderately grazed in winter (WG), and an overgrazed plot (OG), all in close vicinity to each other. Gross rates of N mineralization and nitrification determined at in situ soil moisture and soil temperature conditions were in a range of 0.5–4.1 mg N kg−1 soil dry weight day−1. In 2005, gross N turnover rates were significantly higher at the UG79 plot than at the UG99 plot, which in turn had significantly higher gross N turnover rates than the WG and OG plots. The WG and the OG plot were not significantly different in gross ammonification and in gross nitrification rates. Site differences in SOC content, bulk density and texture could explain only less than 15% of the observed site differences in gross N turnover rates. N2O and NO x flux rates were very low during both growing seasons. No significant differences in N trace gas fluxes were found between plots. Mean values of N2O fluxes varied between 0.39 and 1.60 μg N2O-N m−2 h−1, equivalent to 0.03–0.14 kg N2O-N ha−1 y−1, and were considerably lower than previously reported for the same region. NO x flux rates ranged between 0.16 and 0.48 μg NO x -N m−2 h−1, equivalent to 0.01–0.04 kg NO x -N ha−1 y−1, respectively. N2O fluxes were significantly correlated with soil temperature and soil moisture. The correlations, however, explained only less than 20% of the flux variance. [ABSTRACT FROM AUTHOR]

The article offers information on the denitrification models and approaches for terrestrial and aquatic ecosystems. A discussion on the current approaches used for modeling denitrification, as well as the nitrogen (N) cycling processes for both ecosystems are enumerated. The approaches described, creates predictions for microbial dynamics by representing an environment where denitrification is likely to occur. The models used in both ecosystems indicate the importance of denitrification of the N cycle. Further, current models need more development and testing to eliminate the uncertainties associated.

Examines the effects of temperature and moisture on nitrification rates in tropical rainforest soils. Use of barometric process separation technique in determining the effects; Correlation between gross nitrification and increase in soil temperature; Investigation of seasonal changes in gross nitrification.

Biochar has been reported to reduce emission of nitrous oxide (N 2 O) from soils, but the mechanisms responsible remain fragmentary. For example, it is unclear how biochar effects on N 2 O emissions are mediated through biochar effects on soil gross N turnover rates. Hence, we conducted an incubation study with three contrasting agricultural soils from Kenya (an Acrisol cultivated for 10-years (Acrisol10); an Acrisol cultivated for over 100-years (Acrisol100); a Ferralsol cultivated for over 100 years (Ferralsol)). The soils were amended with biochar at either 2% or 4% w/w. The 15N pool dilution technique was used to quantify gross N mineralization and nitrification and microbial consumption of extractable N over a 20-day incubation period at 25 °C and 70% water holding capacity of the soil, accompanied by N 2 O emissions measurements. Direct measurements of N 2 emissions were conducted using the helium gas flow soil core method. N 2 O emissions varied across soils with higher emissions in Acrisols than in Ferralsols. Addition of 2% biochar reduced N 2 O emissions in all soils by 53 to 78% with no significant further reduction induced by addition at 4%. Biochar effects on soil nitrate concentrations were highly variable across soils, ranging from a reduction, no effect and an increase. Biochar addition stimulated gross N mineralization in Acrisol-10 and Acrisol-100 soils at both addition rates with no effect observed for the Ferralsol. In contrast, gross nitrification was stimulated in only one soil but only at a 4% application rate. Also, biochar effects on increased NH 4 + immobilization and NO 3 −consumption strongly varied across the three investigated soils. The variable and bidirectional biochar effects on gross N turnover in conjunction with the unambiguous and consistent reduction of N 2 O emissions suggested that the inhibiting effect of biochar on soil N 2 O emission seemed to be decoupled from gross microbial N turnover processes. With biochar application, N 2 emissions were about an order of magnitude higher for Acrisol-10 soils compared to Acrisol-100 and Ferralsol-100 soils. Our N 2 O and N 2 flux data thus support an explanation of direct promotion of gross N 2 O reduction by biochar rather than effects on soil extractable N dynamics. Effects of biochar on soil extractable N and gross N turnover, however, might be highly variable across different soils as found here for three typical agricultural soils of Kenya. Unlabelled Image • Biochar application consistently reduced N 2 O emissions from all soils. • Biochar effects on gross N turnover were highly variable across soils. • Biochar effects on N 2 O were decoupled from effects on mineral N availability. • Kenyan Acrisols showed higher N 2 O emissions than Ferralsols. [ABSTRACT FROM AUTHOR]

Nitrification is a central component of the terrestrial nitrogen (N) cycle, but the contribution of autotrophic and heterotrophic nitrification to total gross nitrification remains poorly understood. To clarify their relative importance in neutral and moderate acid soils, an incubation experiment was conducted with 15 N-ammonium isotopic pool dilution techniques and combined with acetylene (C 2 H 2 , 10 Pa) as a specific inhibitor of autotrophic nitrification and sodium chlorate (NaClO 3 ) as a potential inhibitor of heterotrophic nitrification. Additionally, CO 2 , N 2 O and CH 4 fluxes were measured to identify potential side-effects of inhibitors on soil respiration and CH 4 fluxes. The presence of C 2 H 2 completely eliminated gross nitrification in all investigated soil samples. The addition of NaClO 3 affected neither gross nitrification nor gross ammonification in soils of both investigated grassland sites. This provided strong evidence that heterotrophic nitrification was not an important process in the investigated grassland soils. Acetylene but not NaClO 3 decreased net CH 4 uptake, likely due to homology of the enzymes ammonia monooxygenase. Overall, the present study shows a dominant role of autotrophic nitrification in gross nitrate production for both neutral and slightly acid soils and illustrates the potential of acetylene as an inhibitor of gross autotrophic nitrification. [ABSTRACT FROM AUTHOR]

Abstract: Tropical savanna ecosystems are a major contributor to global CO2, CH4 and N2O greenhouse gas exchange. Savanna fire events represent large, discrete C emissions but the importance of ongoing soil-atmosphere gas exchange is less well understood. Seasonal rainfall and fire events are likely to impact upon savanna soil microbial processes involved in N2O and CH4 exchange. We measured soil CO2, CH4 and N2O fluxes in savanna woodland (Eucalyptus tetrodonta/Eucalyptus miniata trees above sorghum grass) at Howard Springs, Australia over a 16 month period from October 2007 to January 2009 using manual chambers and a field-based gas chromatograph connected to automated chambers. The effect of fire on soil gas exchange was investigated through two controlled burns and protected unburnt areas. Fire is a frequent natural and management action in these savanna (every 1–2 years). There was no seasonal change and no fire effect upon soil N2O exchange. Soil N2O fluxes were very low, generally between −1.0 and 1.0μg Nm−2 h−1, and often below the minimum detection limit. There was an increase in soil NH4 + in the months after the 2008 fire event, but no change in soil NO3 −. There was considerable nitrification in the early wet season but minimal nitrification at all other times. Savanna soil was generally a net CH4 sink that equated to between −2.0 and −1.6kg CH4 ha−1 y−1 with no clear seasonal pattern in response to changing soil moisture conditions. Irrigation in the dry season significantly reduced soil gas diffusion and as a consequence soil CH4 uptake. There were short periods of soil CH4 emission, up to 20μg Cm−2 h−1, likely to have been caused by termite activity in, or beneath, automated chambers. Soil CO2 fluxes showed a strong bimodal seasonal pattern, increasing fivefold from the dry into the wet season. Soil moisture showed a weak relationship with soil CH4 fluxes, but a much stronger relationship with soil CO2 fluxes, explaining up to 70% of the variation in unburnt treatments. Australian savanna soils are a small N2O source, and possibly even a sink. Annual soil CH4 flux measurements suggest that the 1.9million km2 of Australian savanna soils may provide a C sink of between −7.7 and −9.4 Tg CO2-e per year. This sink estimate would offset potentially 10% of Australian transport related CO2-e emissions. This CH4 sink estimate does not include concurrent CH4 emissions from termite mounds or ephemeral wetlands in Australian savannas. [Copyright &y& Elsevier]

Several tropical grasses, particularly Urochloa humidicola (Rendle) Schweick.) (Syn. Brachiaria humidicola), have been associated with low soil nitrate content and reduced nitrogen (N) losses from pasture systems. Previous studies have detected that root exudates of certain Urochloa genotypes can inhibit ammonium oxidation in in vitro bioassay with Nitrosomonas and reduce net nitrification in soils, a phenomenon termed biological nitrification (BNI). However, net nitrification rates reflect the result of several N transformation processes that co-occur and together determine changes in NO 3 − concentrations in soils. Thus, to better understand the mechanisms underlying BNI, gross rates of nitrification, ammonification and inorganic N immobilization need to be assessed. In this study we used, for the first time, stable isotopes techniques to disentangle the different gross N transformation processes that determine net nitrification rates in soil of Urochloa genotypes differing in BNI. Intact soil cores were collected from two experimental sites in Colombia under different Urochloa genotypes, classified as low-BNI and high-BNI based on net nitrification rates determined in previous studies. Soil cores were labeled with (15NH 4) 2 SO 4 and K15NO 3 to quantify gross N ammonification and nitrification, respectively and calculate the immobilization rates. Our results do not confirm lower gross nitrification rates under the high-BNI genotypes, despite reduced potential net nitrification rates and lower abundance of ammonia oxidising organisms, hence not supporting the widely accepted mechanism of BNI. Instead, the low potential nitrification rates could be explained by increased inorganic N immobilization under high-BNI genotypes, both in NH 4 +-N and NO 3 −-N form. The lack of differences might be explained by a strong N-limitation in our experiment, unlike previous BNI studies when large amounts of fertilizers were applied. Our results suggest that a combination of different mechanisms, particularly stimulation of N immobilization may be responsible for the BNI capacity observed as low NO 3 − soil content and reduced N losses. Image 1 • We evaluated gross N transformations in Urochloa genotypes differing in BNI capacity. • Gross nitrification was unaffected by BNI despite the contrasting net nitrification. • High N immobilization could explain low NO 3 −environment in high-BNI genotypes. • Low N environments under high-BNI are explained by a mix of N conservative process. [ABSTRACT FROM AUTHOR]

Soil dinitrogen (N 2) emissions are a key nitrogen loss pathway of terrestrial ecosystems. However, the quantification of field N 2 emissions from terrestrial ecosystems remains challenging, as sensitive field methods for measuring N 2 fluxes are lacking. Here, we report a new approach to quantify field N 2 emissions by (i) parameterizing the molar ratio of nitrous oxide (N 2 O) to N 2 O plus N 2 emissions (R N 2 O) in the laboratory and (ii) measuring field N 2 O emissions and soil factors. Soil samples were taken from a maize field and incubated in the laboratory under simulated field conditions. Soil N 2 and N 2 O emissions were determined using the gas-flow-soil-core method. The measurements revealed that the R N 2 O values were significantly higher (0.06–0.67) following urea fertilization and soil rewetting compared to those periods with no fertilization (0.03–0.08) (P < 0.01). A multivariate, nonlinear parameterization of R N 2 O against four easily measured soil factors (ammonia and nitrate concentrations, temperature, and moisture) (n = 20, r 2 = 0.92, P < 0.001) was developed. The seasonal N 2 emissions at the field scale were calculated by combining the laboratory-measured R N 2 O with the field-measured N 2 O emissions and the soil factors. Based on this approach, the cumulative emissions of N 2 and N 2 +N 2 O for the maize season were 7.2 ± 2.8 and 9.6 ± 2.1 (standard error) kg N ha−1, respectively. Using a fixed R N 2 O , i.e., disregarding the temporal and spatial variability of R N 2 O , resulted in approximately 50%–70% lower estimates. Our study shows that a combination of field N 2 O and soil factors measurements and laboratory parameterization of R N 2 O allows field N 2 emissions from croplands to be constrained. With additional measurements, including other soil properties, the development of a generalized parameterization of R N 2 O may become feasible. This approach would allow for a better understanding of gaseous N losses from agricultural ecosystems. Image 1 • A method was developed to quantify dynamical field N 2 fluxes from upland soils. • This method combined N 2 O/(N 2 O + N 2) ratios with field-measured N 2 O fluxes. • N 2 O/(N 2 O + N 2) ratio was parameterized as a function of multiple soil factors. • The obtained multivariate function was validated by independent literature data. [ABSTRACT FROM AUTHOR]

In order to investigate the effect of climate change on the functional role of microbial community, we studied links between microbial community structure and the net ammonification and nitrification rates in a space for time climate change experiment. Abundance of bacteria and fungi as well as the induced climate change effects varied with seasons. The highest PLFA concentrations were found in frozen soils of the climate change treatment in which organic carbon (C) and nitrogen (N) substrates accumulated. It indicated that frozen soil can be the hot moment for soil microbial community. We found that climate change significantly increased total PLFA concentrations while decrease F:B ratio in July. It suggested that climate change could increase growth of bacteria more than that of fungi, and thus changed the soil microbial community structure. On the other hand, net N turnover itself was largely negative (i.e. illustrating net immobilization and the high N retention capacity). Since the ability of net rates of N turnover to predict plant N availability is relying on the (invalid) assumption that plants poorly compete for mineral N against soil microbes, this might be related to the strong plant-soil-microbe carbon-nitrogen interactions in the investigated soil, and unfortunately climate change would enhance the interaction. Therefore, net N mineralization has its limitation to deeply study the microbial N turnover in N biogeochemistry cycles. • Climate change stimulated microbial growth performance in early winter. • Climate change would increase growth of bacteria more than fungi. • Negative net N mineralization rates illustrated immobilization and higher N retention capacity. [ABSTRACT FROM AUTHOR]

Soil nitrogen (N) transformations play a vital role in maintaining grassland productivity and influence nitrous oxide (N 2 O) emissions. To evaluate annual dynamics of soil gross N turnover and its effects on N 2 O emissions in typical grasslands, we conducted year-round measurements of soil gross N turnover rates, inorganic N pool sizes and N 2 O fluxes in an alpine shrub meadow of the Qinghai–Tibet Plateau. Gross ammonium (NH 4 +) and nitrate (NO 3 −) immobilization rates were calculated by the "isotope dilution method" (i.e., based on the consumption rates) and the "reformed difference method" (i.e., based on the differences between gross and net turnover rates). The "reformed difference method" avoids additional measurements of net N turnover rates in unlabeled soils compared to the traditional "difference method", and provides more reliable estimates of NH 4 + immobilization in soils with low ambient NH 4 + pool sizes than the "isotope dilution method". The annual gross rates of mineralization, nitrification, NH 4 + and NO 3 − immobilization amounted to 606 ± 43, 236 ± 27, 389 ± 24 and 82 ± 19 mg N kg−1 dry soil yr−1, respectively, in a topsoil of 10 cm. The soil gross N turnover rates during freezing and freeze–thaw periods contributed considerably to the annual totals (22–44%). Thus, the freezing and freeze–thaw periods were not dormant seasons for microbial N transformations, even under a frigid alpine climate. The annual N 2 O emission was 0.20 ± 0.03 kg N ha−1 yr−1, 35% of which originated from a short-lived emission pulse during the freeze–thaw period (52 d). The promotion of gross mineralization increased soil NH 4 + pool sizes, whereas the increase in gross nitrification did not enhance soil NO 3 − pool sizes during the freeze–thaw period. Coupled nitrification–denitrification dominated NO 3 − consumption and freeze–thaw related N 2 O production and hence, the low NO 3 − pool sizes were not indicative of the potential of pulsed N 2 O emissions. Year-round measurements with intensive observations during both the non-freezing and freeze–thaw periods are indispensable to fully understand soil N cycling and its response to climate change in alpine ecosystems. • Annual gross N turnover and N 2 O emissions were measured in an alpine meadow soil. • An alternative method was used to estimate gross microbial ammonium immobilization. • Microbial N turnover was persistent in frozen soils despite frigid alpine climate. • Coupled nitrification–denitrification dominated freeze–thaw induced N 2 O emissions. • Year-round measurements are indispensable to fully understand soil N cycling. [ABSTRACT FROM AUTHOR]

Increased emissions from agricultural soils are the most important anthropogenic contribution to the elevated atmospheric abundance of N2O, which is a potent greenhouse gas and the most important contributor to stratospheric ozone destruction. In soils, microorganisms predominately produce N2O by nitrification and denitrification. Biogeochemical models simulate these processes and are increasingly used to assess N2O mitigation strategies. However, the lack of field methods allowing the direct partitioning of N2O to source processes entails a large uncertainty with regard to their relative contribution.The intramolecular distribution of 15N in the N2O molecule, also known as site preference, provides unique information to disentangle the relative contribution of nitrification and denitrification. On-line analysis of the changes in intramolecular N2O isotopic composition in ambient air has been achieved recently, but this approach alone i) does not allow an explicit spatial allocation of the isotopic signature and ii) is limited to events with a strong increase in N2O concentrations, which may obscure short term variations of the dominant N2O producing processes, e.g. due to day-time temperature or radiation changes. To address these shortcomings, we combined laser spectroscopy based on-line analysis of the intramolecular N2O isotopic composition [1] with the biogeochemical model LandcapeDNDC and SIMONE, a stable isotope model for nutrient cycles [2]. N2O emissions were spatially allocated by coupling the laser spectrometer to automated static flux chambers, which also enabled higher sensitivities for periods with low N2O fluxes and higher temporal resolution than previous studies [3, 4]. This approach allowed us assessing the relative contributions of nitrification and denitrification during a field campaign between August and December in 2017 on a grassland site close to Beromünster, Central Switzerland.Both N2O isotope measurements and model results indicated continuous predominance of denitrification and hardly any temporal variability in N2O SP. This finding was independent of the water content close to the soil surface, suggesting that N2O production in the subsoil under high water filled pore space conditions outweighed the production of N2O by nitrification closer to the surface. The modelled isotopic composition was offset by 1.9 ‰ with respect to the observed values. This indicates that the model parametrization reflects the dominant N2O production pathway but slightly overestimates the contribution of denitrification. To our knowledge, this is the first observation-based application and validation of a biochemical soil model using N2O isotopes. The application of this stable isotope based model validation approach at other sites and the comparison with other models will help to identify potential shortcomings and improve our capability to support N2O mitigation strategies using real-time measurements of stable N2O isotopes and corresponding soil chemistry and emission models.References1. Ibraim, E., et al., Isotopes in Environmental and Health Studies, 2018. 54(1): p. 1-15.2. Denk, T.R.A., et al., Soil Biology & Biochemistry, 2017. 105: p. 121-137.3. Ibraim, E., et al., Biogeosciences Discuss., 2018. 2018: p. 1-27.4. Wolf, B., et al., Biogeosciences, 2015. 12(8): p. 2517-2531. [ABSTRACT FROM AUTHOR]