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In flooded paddy fields, peak greenhouse gas nitrous oxide (N2O) emission after rewetting the dry soils is widely recognised. However, the relative contribution of biotic and abiotic factors to this emission remains uncertain. In this study, we used the isotope technique (δ18O and δ15NSP) and molecular-based microbial analysis in an anoxic incubation experiment to evaluate the contributions of bacterial, fungal, and chemical denitrification to N2O emissions. We collected eight representative paddy soils across southern China for an incubation experiment. Results show that during the 10-day incubation period, the net N2O emissions were mainly produced by fungal denitrification, which accounted for 58–77% in six of the eight investigated flooded paddy soils. In contrast, bacterial denitrification contributed 6–15% of the net N2O emissions. Moreover, around 11–35% of the total N2O emissions were derived from chemical denitrification in all soil types. Variation partitioning analysis (VPA) and principal component analysis (PCA) demonstrated that initial soil organic carbon (OC) concentrations were the primary regulator of N2O source patterns. Soils with relatively lower OC concentration (7–15 mg g−1) tend to be dominated by fungal denitrification, which accounted for the net N2O production at the end of the incubation period. Overall, these findings highlight the dominance of the fungal denitrification pathway for N2O production in flooded paddy soils, which predominates in soils with relatively lower OC content. This suggests that fungal contribution should be considered when optimizing agricultural management system timing to control N2O emissions in flooded paddy soil ecosystems, and for the relevant establishment of predictive numerical models in the future. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) is a potent greenhouse gas and ozone depletion agent, with a significant natural source from marine oxygen deficient zones (ODZs). Open questions remain, however, about the microbial processes responsible for this N2O production, especially hybrid N2O production when ammonia-oxidizing archaea are present. Using 15N-labeled tracer incubations, we measured the rates of N2O production from ammonium (NH4 +), nitrite (NO2 -), and nitrate (NO3 -) in the Eastern Tropical North Pacific ODZ, as well as the isotopic labeling of the central () and terminal () nitrogen atoms of the N2O molecule. We observed production of both doubly- and singly labeled N2O from each tracer, with the highest rates of labeled N2O production at the same depths as the near-surface N2O concentration maximum. At most stations and depths, the production of 45N2O and 45N2O were statistically indistinguishable, but at a few depths, there were significant differences in the labelling of the two nitrogen atoms in the N2o molecule. Implementing the rates of labeled N2O production in a forward-running model, we found that N2O production from NO3 - dominated at most stations and depths, with rates as high as 1.6±0.2 nM N2O/day. Hybrid N2O production, one of the mechanisms by which ammonia-oxidizing archaea produce N2O, had rates as high as 0.23±0.08 nM N2O/day that peaked in both the near-surface and deep N2O concentration maxima. We inferred from the 45N2O and 45N2O data that hybrid N2O production by ammonia-oxidizing archaea may have a variable site preference that depends on the 15N content of each substrate. We also found that the rates and yields of hybrid N2O production exhibited a clear [O2] inhibition curve, with the hybrid N2O yields as high as 20% at depths where dissolved [O2] was 0 μM but nitrification was still active. Finally, we identified a few incubations with dissolved [O2] up to 20 μM where N2O production from NO3 - was still active. A relatively high O2 tolerance for N2O production via denitrification has implications for the feedbacks between marine deoxygenation and greenhouse gas cycling. [ABSTRACT FROM AUTHOR]

The use of urea fertilizers in agriculture is associated with many negative environmental impacts and is a source of ammonia (NH 3) and nitrous oxide (N 2 O) emissions. Such losses from urea fertilizer can be avoided by different mitigation techniques. Three different mitigation principles, urease inhibitor (N-(2-Nitrophenyl) phosphoric triamide, 2-NPT) (UI) alone and urease inhibitor in combination with nitrification inhibitors (N-[3(5)-methyl-1 H-pyrazol-1-yl) methyl] acetamide, MPA) (NI) and closed slit incorporation of urea fertilizer into the soil, were compared on a sandy loam soil at a soil water level of 70 % water-holding capacity. An in vitro microcosm approach with open dynamic incubation chambers was used to monitor NH 3 emissions over two weeks with NH 3 sampling by washing bottles. N 2 O emissions were studied over ten weeks in slow throughflow mesocosms with continuous gas chromatographic (GC) measurements. To get insights into N 2 O production and consumption processes, gas samples were taken after six weeks and N 2 O isotopocules were analyzed by isotope ratio mass spectrometry (IRMS). Slit injection showed the greatest effect on NH 3 emission reduction by 79.6 % (40.6 % by UI, and 46.7 % by UINI) compared to surface applied urea. Minor pollution swapping to N 2 O was observed at the beginning of the trial due to incorporation but not in the cumulative emissions over the entire incubation time. The reduction effect of UINI on N 2 O emissions decreased over time with no cumulative emission reduction at the end of experimentation. N 2 O isotopocules confirmed the high contribution of nitrification to N 2 O production. In contrast and bacterial denitrification, nitrifier denitrification and fungal denitrification were involved on a much lower level and N 2 O reduction to N 2 was not pronounced. All NH 3 mitigation measurements where effective to decrease NH 3 emissions while their effects on N 2 O emission varied over time. Factors as crop N uptake and rainfall would further modify the overall effect on N 2 O emissions and need to be considered for final pollution swapping assessment. Further research on the impact of NI on non-target microbial communities is warranted to elucidate potential environmental consequences and long-term efficacy of inhibitor compounds. • Ammonia volatilization and nitrous oxide emissions from urea fertilizer. • Emission mitigation measures. • Urease inhibitor (N-(2-Nitrophenyl) phosphoric triamide, 2-NPT) Nitrification inhibitor (N-[3(5)-methyl-1 H-pyrazol-1-yl) methyl] acetamide, MPA). • Fertilizer incorporation. • Isotopocules, SP/O map, FRAME model. [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]

Ocean–atmosphere gas exchange in the Arctic Ocean is sensitive to global warming because the decrease of sea‐ice covered area enhances the exchange. Melting sea ice affects the vertical transport of dissolved gases. Few reports of Arctic Ocean studies have described observations of dissolved N2O or temporal variation of sea‐to‐atmosphere N2O flux. Therefore, production mechanisms of this greenhouse gas and the stratospheric ozone depleting gas remain unclear. We measured dissolved N2O and its isotopic signatures in the western Arctic shelf region of the Chukchi Sea and surrounding areas during September in 2014 and 2015. In the Chukchi Sea shelf, the N2O concentration was slightly higher than the value expected under sea–air equilibrium in the surface water. It increased with depth to 23 nmol kg−1 in 2014, although it showed a vertically homogeneous distribution in 2015. Isotopocule ratios of N2O, which include 15N‐site preference in the N2O molecule as well as elemental N and O isotope ratios, indicate that N2O in the Chukchi Sea shelf is a mixture of N2O produced in the bottom water and that of atmospheric origin. The isotopic signature of excess N2O (δ15Nbulk = −3.6‰ to −1.4‰, δ18O = 61.5‰–63.0‰, SP = 38.7‰–49.0‰) suggests that it is produced by archeal or bacterial nitrification and that it is partly reduced by denitrification. Although further quantitative observations are necessary, the results confirmed that the western Arctic Ocean can act as a source of N2O to the atmosphere when sea‐ice cover retreats and the pycnocline is weakened. Key Points: Continental shelf region in the western Arctic (Chukchi Sea) can act as a N2O source to the atmosphere in the summerThe N2O, produced mainly by archeal or bacterial nitrification, is partly reduced by denitrification in bottom water or shelf sedimentsVertical distribution of N2O in the water column is affected strongly by the degree of stratification caused by sea ice melting [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.