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Introduction: Hydroponic vegetable cultivation is characterized by high intensity and frequent nitrogen fertilizer application, which is related to greenhouse gas emissions, especially in the form of nitrous oxide (N2O). So far, there is little knowledge about the sources of N2O emissions from hydroponic systems, with the few studies indicating that denitrification could play a major role. Methods: Here, we use evidence from an experiment with tomato plants (Solanum lycopersicum) grown in a hydroponic greenhouse setup to further shed light into the process of N2O production based on the N2O isotopocule method and the 15N tracing approach. Gas samples from the headspace of rock wool substrate were collected prior to and after 15N labeling at two occasions using the closed chamber method and analyzed by gas chromatography and stable isotope ratio mass spectrometry. Results: The isotopocule analyses revealed that either heterotrophic bacterial denitrification (bD) or nitrifier denitrification (nD) was the major source of N2O emissions, when a typical nutrient solution with a low ammonium concentration (1-6 mg L-1) was applied. Furthermore, the isotopic shift in 15N site preference and in δ18O values indicated that approximately 80-90% of the N2O produced were already reduced to N2 by denitrifiers inside the rock wool substrate. Despite higher concentrations of ammonium present during the 15N labeling (30-60 mg L-1), results from the 15N tracing approach showed that N2O mainly originated from bD. Both, 15N label supplied in the form of ammonium and 15N label supplied in the form of nitrate, increased the 15N enrichment of N2O. This pointed to the contribution of other processes than bD. Nitrification activity was indicated by the conversion of small amounts of 15N-labeled ammonium into nitrate. Discussion/Conclusion: Comparing the results from N2O isotopocule analyses and the 15N tracing approach, likely a combination of bD, nD, and coupled nitrification and denitrification (cND) was responsible for the vast part of N2O emissions observed in this study. Overall, our findings help to better understand the processes underlying N2O and N2 emissions from hydroponic tomato cultivation, and thereby facilitate the development of targeted N2O mitigation measures. [ABSTRACT FROM AUTHOR]

Returning crop residues to agricultural fields can accelerate nutrient turnover and increase N2O and NO emissions. Increased microbial respiration may lead to formation of local hotspots with anoxic or microoxic conditions promoting denitrification. To investigate the effect of litter quality on CO2, NO, N2O, and N2 emissions, we conducted a laboratory incubation study in a controlled atmosphere (He/O2, or pure He) with different maize litter types (Zea mays L., young leaves and roots, straw). We applied the N2O isotopocule mapping approach to distinguish between N2O emitting processes and partitioned the CO2 efflux into litter- and soil organic matter (SOM)-derived CO2 based on the natural 13C isotope abundances. Maize litter increased total and SOM derived CO2 emissions leading to a positive priming effect. Although C turnover was high, NO and N2O fluxes were low under oxic conditions as high O2 diffusivity limited denitrification. In the first week, nitrification contributed to NO emissions, which increased with increasing net N mineralization. Isotopocule mapping indicated that bacterial processes dominated N2O formation in litter-amended soil in the beginning of the incubation experiment with a subsequent shift towards fungal denitrification. With onset of anoxic incubation conditions after 47 days, N fluxes strongly increased, and heterotrophic bacterial denitrification became the main source of N2O. The N2O/(N2O+N2) ratio decreased with increasing litter C:N ratio and Corg:NO3− ratio in soil, confirming that the ratio of available C:N is a major control of denitrification product stoichiometry. [ABSTRACT FROM AUTHOR]

High concentrations of ammonium (NH 4 + ) in soil have been shown to inhibit nitrification, and fertilizer injection as conducted during CULTAN (controlled uptake long-term ammonium nutrition) management might thus have the potential to reduce N 2 O emission from arable soil. We conducted an incubation experiment with different NH 4 + concentrations in soil that resembled concentrations as expected at and around injection spots (5000, 2250, 1000, 450, 0 mg NH 4 + -N kg −1 soil) directly after fertilization and after dilution due to plant uptake or precipitation. N 2 O emission was measured in dynamic soil mesocosms over a period of 21 days. Acetylene inhibition and 15 N tracer approaches were used to calculate the relative contribution of nitrification and denitrification to N 2 O emission. An isotopomer approach was applied to gain further insight into N 2 O producing processes. We expected lower contribution of nitrification-derived N 2 O to total N 2 O emission and a higher N 2 O/NO 3 − ratio from nitrification with increasing NH 4 + levels. Nitrification indeed declined with increasing NH 4 + level, and no nitrification occurred in the 5000 mg NH 4 + -N kg −1 soil treatment. A pool dilution approach showed that gross nitrification in 450 mg NH 4 + -N kg −1 soil (nitrification rate: 4.96 mg NO 3 − -N kg soil d −1 ) was by a factor of 2.6 and 6 higher than in 1000 and 2250 mg NH 4 + -N kg −1 soil treatments. In the 5000 mg NH 4 + -N kg −1 soil treatment, gross nitrification occurred at very small rates (0.1 mg NO 3 − -N kg soil d −1 ). Similarly, N 2 O emission declined with increasing NH 4 + level. The N 2 O yield of nitrification was between 0.07 and 0.15% of NO 3 − production, but was not affected by increasing NH 4 + level. Nitrification was the dominant source of N 2 O throughout the incubation at all NH 4 + levels, and there was no significant change in the relative contribution of nitrification and denitrification with NH 4 + level or time. This finding indicates that denitrification derived N 2 O emissions were similarly reduced at high NH 4 + levels. Applying the non-equilibrium technique to our 15 N tracer data revealed heterogeneous distribution of denitrification in soil, with at least two distinct NO x − (NO 3 − + NO 2 − ) pools and spatial separation of NO x − formation and consumption. The isotopomer approach provided reasonable results in comparison with the acetylene inhibition and 15 N tracer approaches and indicated substantial contribution of nitrifier denitrification and/or coupled nitrification-denitrification (10–40%) to total N 2 O production. [ABSTRACT FROM AUTHOR]

Agricultural soils have a great share on global nitrous oxide (N 2 O) emissions. The method of nitrogen fertilization is a manageable control parameter of N 2 O production in soil. Controlled uptake long-term ammonia nutrition (CULTAN) intends to aliment field growing crops mainly with ammonium instead of nitrate, aiming at a better N use efficiency and less N leaching by placing ammonium-based N fertilizer in highly concentrated depots in the soil. In this two years field study, we analyzed N 2 O flux rates and dynamics of mineral N in soils after injection of ammonium sulfate solution (CULTAN) and conventional surface application of the same fertilizer type (ammonium sulfate at a rate of 130 kg N ha −1 ) to winter wheat at two sites with different soil texture. Using 15 N-ammonium as a tracer, we additionally measured fertilizer-derived emissions and fertilizer N uptake at one CULTAN plot. Grain yields were higher after CULTAN fertilization than after surface application of N fertilizer; significantly so in one year at each site. Neither N uptake nor N use efficiency were consistently different between fertilization methods. Nitrate accumulation in CULTAN treated plots occurred after fertilizer injection, showing that the concentrated NH 4 + depots did not sufficiently inhibit nitrification. Total annual N 2 O emission ranged from 0.29 to 1.9 kg N ha −1 yr −1 , with higher emissions from fertilized than unfertilized plots, but no significant difference between fertilizer application methods. N 2 O emission was higher at the loam than the sandy loam site, with twice as high annual emission at the loam site (1.2 ± 0.5 kg N ha −1 yr −1 ) compared to the sandy loam site (0.6 ± 0.2 kg N ha −1 yr −1 ) after CULTAN fertilization. Temporal N 2 O emission dynamics were influenced by weather conditions (i.e., thawing of soil) and irrigation and could partly be explained by changes in soil moisture and soil mineral N. With only 1–17% of total annual fluxes at the 15 N CULTAN plot, fertilizer-derived N 2 O emissions were small, highlighting the dominance of soil N for N 2 O emission. In terms of N 2 O emission, CULTAN fertilization did thus not proof beneficial over surface application of the same fertilizer. [ABSTRACT FROM AUTHOR]

RATIONALE An enhanced analytical approach for analyzing gaseous products from 15N-enriched pools has been developed. This technique can be used to quantify nitrous oxide (N2O) and dinitrogen (N2) fluxes from denitrification. It can also help in distinguishing different N2- and N2O-forming processes, such as denitrification, nitrification, anaerobic ammonium oxidation or co-denitrification. METHODS The measurement instrumentation was based on a commercially available automatic preparation system allowing collection and separation of gaseous samples. The sample transfer paths, valves, liquid nitrogen traps, gas chromatography column and open split of the original system were modified. A reduction oven (Cu) was added in order to eliminate oxygen and measure N2O-N as N2. Gases leaving the separation system entered an isotope ratio mass spectrometer where masses 28N2, 29N2 and 30N2 were measured. RESULTS The enhanced technique enabled rapid simultaneous measurement of stable isotope ratios 29N2/28N2 and 30N2/28N2 originating from dinitrogen alone (N2) and from the sum of the denitrification products (N2+N2O) as well as the determination of 15N enrichment in N2O. The 15N fraction in the N pool undergoing N2 and N2O production (15 XN) and the contribution of N2 and N2O originating from this pool ( d) were determined with satisfactory accuracy of better than 3.3% and 2.9%, respectively. CONCLUSIONS The precision and accuracy of this method were comparable with or better than previously reported for similar measurements. The proposed method allows for the analysis of all quantities within one run, thus reducing the measurement and sample preparation time as well as increasing the reliability of the results. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Soils represent the major source of the atmospheric greenhouse gas nitrous oxide (N2O) and there is a need to better constrain the total global flux and the relative contribution of the microbial source processes. The aim of our study was to determine variability and control of the isotopic fingerprint of N2O fluxes following NH4 +-fertilization and dominated by nitrification. We conducted a microcosm study with three arable soils fertilized with 0–140mgNH4 +–Nkg−1. Fractions of N2O derived from nitrification and denitrification were determined in parallel experiments using the 15N tracer and acetylene inhibition techniques or by comparison with unfertilized treatments. Soils were incubated for 3–10 days at low moisture (30–55% water-filled pore space) in order to establish conditions favoring nitrification. Dual isotope and isotopomer ratios of emitted N2O were determined by mass spectrometric analysis of δ 18O, average δ 15N (δ 15Nbulk) and 15N site preference (SP=difference in δ 15N between the central and peripheral N positions of the asymmetric N2O molecule). N2O originated mainly from nitrification (>80%) in all treatments and the proportion of NH4 + nitrified that was lost as N2O ranged between 0.07 and 0.45%. δ 18O and SP of N2O fluxes ranged from 15 to 28.4‰ and from 13.9 to 29.8‰, respectively. These ranges overlapped with isotopic signatures of N2O from denitrification reported previously. There was a negative correlation between SP and δ 18O which is opposite to reported trends in N2O from denitrification. Variation of average 15N signatures of N2O (δ 15Nbulk) did not supply process information, apparently because a strong shift in precursor signatures masked process-specific effects on δ 15Nbulk. Maximum SP of total N2O fluxes and of nitrification fluxes was close to reported SP of N2O from NH4 + or NH2OH conversion by autotrophic nitrifiers, suggesting that SP close to 30‰ is typical for autotrophic nitrification in soils following NH4 +-fertilization. The results suggest that the δ 18O/SP fingerprint of N2O might be used as a new indicator of the dominant source process of N2O fluxes in soils. [Copyright &y& Elsevier]

Abstract: Soils represent the major source of the atmospheric greenhouse gas nitrous oxide (N2O) and there is a need to better constrain the total global flux and the relative contribution of the microbial source processes. The aim of our study was to evaluate isotopomer analysis of N2O (intramolecular distribution of 15N) as well as conventional nitrogen and oxygen isotope ratios (i) as a tool to identify N2O production processes in soils and (ii) to constrain the isotopic fingerprint of soil-derived N2O. We conducted a microcosm study with arable loess soil fertilized with 20mgNkg−1 of 15NO3 −-labeled or non-labeled ammonium nitrate. Soils were incubated for 16d at varying moisture (55%, 75% and 85% water-filled pore space (WFPS)) in order to establish different levels of nitrification and denitrification. Dual isotope and isotopomer ratios of emitted N2O were determined by mass spectrometric analysis of δ 18O, average δ 15N (δ 15Nbulk) and 15N site preference (SP=difference in δ 15N between the central and peripheral N-positions of the asymmetric N2O molecule). Total rates and N2O emission of denitrification and nitrification were determined by 15N analysis of headspace gases and soil extracts of the 15NO3 − treatment. N2O emission and denitrification increased with moisture whereas gross nitrification was almost constant. In the 55% WFPS treatment, more than half of the N2O flux was derived from nitrification, whereas denitrification was the dominant N2O source in the 75% WFPS and 85% WFPS treatments. Moisture conditions were reflected by the isotopic signatures since highly significant differences were observed for average δ 15Nbulk, SP and δ 18O. Experiment means of the 75% WFPS and 85% WFPS treatments gave negative δ 15Nbulk (−18.0‰ and −34.8‰, respectively) and positive SP (8.6‰ and 15.3‰, respectively), which we explained by the fractionation during N2O production and partial reduction to N2. In the 55% WFPS treatment, mean SP was relatively low (1.9‰), which suggests that nitrification produced N2O with low or negative SP. The observed influence of process condition on isotopomer signatures suggests that the isotopomer approach might be suitable for identifying N2O source processes. However, more research is needed to determine the impact from process rates and microbial community structure. Isotopomer signatures were within the range reported from previous soil studies which supports the assumption that SP of soil-derived N2O is lower than SP of tropospheric N2O. [Copyright &y& Elsevier]

Soil N2O fluxes originate from a multiple of mostly microbial processes where the currently known include production by nitrification (including hydroxylamine oxidation and nitrifier denitrification), fungal and bacterial denitrification, co-denitrification, DNRA as well as N2O reduction to N2 by bacterial denitrification. Better knowledge on their significance and control is needed to better predict gaseous N fluxes from soil. In recent years, stable isotope signatures of N2O such as δ18O, average δ15N (δ15Nbulk) and 15N site preference (SP = difference in δ15N between the central and peripheral N positions of the asymmetric N2O molecule) have been used to characterize N2O turnover processes including N2O production and reduction by microbial denitrification. While it is generally accepted that different microbial processes of N2O production are associated with specific isotope effects leading to characteristic "endmember" values of N2O produced, there is also consensus that a clear distinction and identification of processes contributing to N2O fluxes is hampered by several factors including the impact of N2O reduction and its (variable) isotope effect, variability of endmember values as well as isotopic values of N2O precursors and their spatial variability. This leads to substantial uncertainty in identification and quantification of different N2O processes, unless some of these factors can be estimated or excluded, which we will illustrate by Monte-Carlo modelling. Therefore, in order to obtain useful information from N2O isotopocules, it is necessary to constrain as many unknowns as possible. We will show examples how this can be done. Moreover, we will illustrate why δ15Nbulk is currently a poor indicator for source processes due to the difficulty to determine δ15N of the precursors of N2O. Finally, we show a comparison of N2O reduction in the field determined by the isotopocule approach and by the 15N gas flux method as independent reference method. We conclude that the isotopocule approach is principally a powerful tool to identify N2O processes that are difficult or impossible to determine otherwise, but to obtain meaningful results its prerequisites and limitations must be taken into account. [ABSTRACT FROM AUTHOR]

The absence of N2 flux measurements in liquid manure-amended soils has resulted in a poor understanding of the effect of manure application on gaseous N losses. The aim of this study was to quantify N2, N2O, CO2, N2O reduction to N2, depth distribution of moisture, water-extractable organic C, NO3−, NH4+, pH, and diffusivity in a laboratory incubation experiment with an arable silt-loam soil. To quantify N processes and gaseous fluxes, 15N tracing was applied. An artificial livestock slurry-mixture was added to the soil in various treatments (control, surface or injected application; slurry-application rate: 42.9 kg N ha− 1; soil water content of either 40% or 60% water-filled pore space (WFPS)). The soil was incubated for 10 days. The depth distribution of the control parameters was measured twice during the experiment on days 5 and 10. The average increase in N2 and N2O fluxes from denitrification was about 900% in slurry-amended soils. The highest N2 and N2O fluxes from denitrification were measured in the slurry injection, 60% WFPS treatment (7.83 ± 3.50 and 11.22 ± 7.60 mg N m− 2 d− 1, respectively). The hypothesis that injected slurry at a higher water content enhances denitrification was confirmed. This study provides important insights into the formation, spatial and temporal variation of the manure-soil hotspot and its impact on the denitrification process. The results will form part of a dataset to develop, improve and test manure application submodules of biogeochemical models and will help to understand in detail the effect of hotspots on N-cycling in manure-treated soils. [ABSTRACT FROM AUTHOR]

The application of reactive nitrogen (N) in the form of synthetic/organic fertilizers plays a central role in supporting a larger human population, but also contributes to global warming through the emission of nitrous oxide (N 2 O). The use of nitrification inhibitors (NIs) has repeatedly been shown to minimize N 2 O emissions; however, their effectiveness in reducing N 2 O emissions varies greatly under different environmental conditions. A better understanding of how and to what extent NIs can mitigate fertilizer-related soil-borne N 2 O emissions under a range of different conditions is required. In the present study, we carried out a soil incubation experiment in a fully automated continuous-flow incubation system under conditions favoring either nitrification- or denitrification-derived N 2 O emissions. Additionally, the abundance of AOB amoA , and AOA amoA genes was quantified and N 2 O isotopic signatures were analyzed. We mixed a common NI (PIADIN ® ) with mineral fertilizer (ammonium sulfate) and examined the N 2 O mitigation potential of the NI in a fertilized sandy soil (low denitrification potential) and a sandy soil mixed with wheat straw (high denitrification potential) at 70% water holding capacity (WHC). In non-NI treatments, the addition of straw led to a drastic increase of CO 2 and N 2 O emissions compared to the non-straw-amended soils, suggesting stimulated microbial activity and higher denitrification rate. The NI reduced N 2 O emissions in the straw-amended treatment by 41%, whereas in the treatment without straw this was only 17%. With the combination of N 2 O isotopic signatures and functional gene abundances, fungal denitrification was considered to be the major process contributing to the higher N 2 O fluxes specifically in straw-amended soils. Overall, our study indicated that NI can be used as an effective method for mitigating N 2 O emissions in cropland specifically when the denitrification potential is high, e.g. in moist N-fertilized and straw-amended soils. [ABSTRACT FROM AUTHOR]

Grassland renewal and grassland conversion to arable land are common agricultural practices on intensively used grassland sites, especially in north-western Europe. However, grassland ploughing can cause a flush of soil organic nitrogen (N) mineralisation due to soil disturbance during tillage and decomposition of stubble and roots from the old grass sward. This is known to result in enhanced nitrous oxide (N 2 O) emissions, but information about the underlying microbial processes, especially the role of N 2 O reduction to N 2 via denitrification, is scarce. Therefore we applied the 15 N gas flux method in situ to grassland recently ploughed under for maize cropping, renewed grassland and permanent grassland on a Histic Gleysol and a Plaggic Anthrosol which differed in organic matter content and drainage. We used needle injection of 15 N-labelled KNO 3 − at three different depths in the soil to achieve homogeneous label distribution. Fluxes of N 2 O and N 2 , mineral N concentration and 15 N enrichment of these were measured for 44 days after label addition. Overall, no differences in N 2 O and N 2 emissions were found between grassland conversion/renewal and permanent grassland. N 2 emissions increased up to 9115 g N ha −1 day −1 on a single sampling day following grassland conversion to maize cropping on the Histic Gleysol, leading to a great contribution of denitrification when N 2 O/(N 2 O + N 2 ) ratio was low. However, heterogeneity of 15 N label distribution proved to be a major difficulty in the water-saturated Histic Gleysol and caused potential uncertainty in identification of various production pathways. Lower gaseous losses and higher nitrification potential were detected in the Plaggic Anthrosol, indicating a higher threat of possible leaching of excess mineral N following grassland conversion/renewal. [ABSTRACT FROM AUTHOR]

Assessing effects of organic fertilizer applications on N2O emissions is of great interest because they can cause higher N2O emissions compared to inorganic fertilizers for a given amount of added nitrogen (N). But there are also reports about enhanced N2O reduction to climate-neutral elemental N2 after application of organic manures to soils. Factors controlling the N2O/(N2O + N2) product ratio of denitrification are interrelated, and also the ratio is difficult to study because of limitations in N2 flux measurements. In this study, we investigated N2O and N2 emissions from soil treated with organic fertilizers with different C/N ratios. An N2O isotopomer approach combined with conventional N2O and N2 flux measurements was employed to study underlying microbial pathways. A grassland soil was amended with anaerobic digestate (AD) from food waste digestion (low C/N ratio) or cattle slurry (CS; high C/N ratio), respectively, adjusted to 90% WFPS, and incubated for 52 days under helium-oxygen atmosphere (10% O2) using a soil incubation system capable of automated N2O, N2, and CO2 measurements. N2O isotopomer signatures, i.e. the d18O and SP values (site preference between 15N at the central and the peripheral position in the N2O molecule), were determined by Isotope Ratio Mass Spectrometry and used to model and subsequently estimate the contribution of bacterial denitrification and autotrophic nitrification to N2O production. For this approach the direct determination of emitted N2 is essential to take isotope effects during N2O reduction to N2 into account by correcting the measured isotope signatures for isotope effects during N2O reduction using previously determined fractionation factor ranges. The addition of both organic fertilizers to soil drastically increased the rate of gaseous N emissions (N2O + N2), probably due to the effects of concurrent presence of nitrate and labile C on the denitrification rate. In the initial phase of the experiment (day 1 to ~15), gaseous N emissions were dominated by N2 fluxes in soils amended with organic manures; meanwhile, N2O emissions were lower compared to untreated Control soils, but increased after 15-20 days relative to the initial fluxes, especially with CS. Extremely low N2O, but high N2 emissions in the initial phase suggest that reduction of N2O to N2 via denitrification was triggered when the soil was amended with organic fertilizers. In contrast in the untreated Control, N2O release was highest during the initial phase. Total N2O release from AD treated soil was similar to Control, while N2O from CS treated soil was considerably higher, indicating that denitrification was triggered more by the high labile carbon content in CS, while the cumulative N2O/(N2O + N2) product ratio and thus N2O reduction were similar with both organic fertilizers. The results of the N2O source partitioning based on the isotopomer data suggest that about 8-25% (AD) and 33-43% (CS) of the cumulated N2O emission was due to nitrification in organically amended soil, while in the untreated Control nitrification accounted for about 5-16%. The remaining N2O production was attributed mainly to denitrification, while the poor model fit for other source pathways like fungal denitrification suggested their contribution to be of minor importance. The observed rather distinct phases with predominance first of denitrification and later of nitrification may help developing mitigation measures by addressing N2O source processes individually with appropriate management options. The observation of relatively large shares of nitrification-derived N2O is surprising, but may possibly be related to the low soil pH and will require further investigation. The determination of N2 production is essential for this isotopomer-based source partitioning approach, but so far only applicable under laboratory conditions. The results of this study indicate that the combination of N2O d18O and SP values is very useful in obtaining more robust source estimates as compared to using SP values alone. [ABSTRACT FROM AUTHOR]

Abstract: Nitrous oxide (N2O) is one of the major greenhouse gases emitted from soils, where it is mainly produced by nitrification and denitrification. It is well known that rates of N2O release from soils are mainly determined by the availability of substrates and oxygen, but N2O source apportioning, highly needed to advance N2O mitigation strategies, still remains challenging. In this study, using an automated soil incubation system, the N2O site preference, i.e. the intramolecular 15N distribution, was analyzed to evaluate the progression in N2O source processes following organic soil amendment. Biogas fermentation residue (BGR; originating from food waste fermentation) was applied to repacked grassland soil cores and compared to ammonium sulfate (AS) application, both at rates equivalent to 160kgNH4 +–Nha−1, and to unamended soil (control). The soil cores were incubated in a helium–oxygen atmosphere with 20kPa O2 for 43 days at 80% water-filled pore space. 43-day cumulative N2O emissions were highest with BGR treated soil accounting for about 1.68kgN2O–Nha−1 while application of AS caused much lower fluxes of c. 0.23kgN2O–Nha−1. Also, after BGR application, carbon dioxide (CO2) fluxes showed a pronounced initial peak with steep decline until day 21 whereas with ammonium addition they remained at the background level. N2O dual isotope and isotopomer analysis of gas samples collected from BGR treated soil indicated bacterial denitrification to be the main N2O generating process during the first three weeks when high CO2 fluxes signified high carbon availability. In contrast, in the second half after all added labile carbon substrates had been consumed, nitrification, i.e. the generation of N2O via oxidation of hydroxylamine, gained in importance reaching roughly the same N2O production rate compared to bacterial denitrification as indicated by N2O SP. Overall in this study, bacterial denitrification seemed to be the main N2O forming process after application of biogas residues and fluxes were mainly driven by available organic carbon. [Copyright &y& Elsevier]

The migration of geogenic gases in continental areas with geothermal activity and active faults is an important process releasing greenhouse gases (GHG) to the lower troposphere. In this respect, caves in hypogenic environments are natural laboratories to study the compositional evolution of deep-endogenous fluids through the Critical Zone. Vapour Cave (Alhama, Murcia, Spain) is a hypogenic cave formed by the upwelling of hydrothermal CO 2 -rich fluids. Anomalous concentrations of N 2 O and NO 2 were registered in the cave's subterranean atmosphere, averaging ten and five times the typical atmospheric backgrounds, respectively. We characterised the thermal conditions, gaseous compositions, sediments, and microbial communities at different depths in the cave. We did so to understand the relation between N-cycling microbial groups and the production and transformation of nitrogenous gases, as well as their coupled evolution with CO 2 and CH 4 during their migration through the Critical Zone to the lower troposphere. Our results showed an evident vertical stratification of selected microbial groups (Archaea and Bacteria) depending on the environmental parameters, including O 2 , temperature, and GHG concentration. Both the N 2 O isotope ratios and the predicted ecological functions of bacterial and archaeal communities suggest that N 2 O and NO 2 emissions mainly depend on the nitrification by ammonia-oxidising microorganisms. Denitrification and abiotic reactions of the reactive intermediates NH 2 OH, NO, and NO 2 − are also plausible according to the results of the phylogenetic analyses of the microbial communities. Nitrite-dependent anaerobic methane oxidation by denitrifying methanotrophs of the NC10 phylum was also identified as a post-genetic process during migration of this gas to the surface. To the best of our knowledge, our report provides, for the first time, evidence of a niche densely populated by Micrarchaeia , which represents more than 50% of the total archaeal abundance. This raises many questions on the metabolic behaviour of this and other archaeal phyla. Unlabelled Image • Geothermal and active faults areas are important sources of GHG to lower troposphere. • Hypogenic caves are natural laboratories to study GHG migration through critical zone. • Microbial groups involved on the production of nitrogenous gases were investigated. • Archaea and Bacteria control GHG fluxes during migration through the Critical Zone. • Nitrogen-cycle microbial groups are determinants in the GHG emissions. [ABSTRACT FROM AUTHOR]