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Arable soil continues to be the dominant anthropogenic source of nitrous oxide (N 2 O) emissions owing to application of nitrogen (N) fertilizers and manures across the world. Using laboratory and in situ studies to elucidate the key factors controlling soil N 2 O emissions remains challenging due to the potential importance of multiple complex processes. We examined soil surface N 2 O fluxes in an arable soil, combined with in situ high-frequency measurements of soil matrix oxygen (O 2 ) and N 2 O concentrations, in situ 15 N labeling, and N 2 O 15 N site preference (SP). The in situ O 2 concentration and further microcosm visualized spatiotemporal distribution of O 2 both suggested that O 2 dynamics were the proximal determining factor to matrix N 2 O concentration and fluxes due to quick O 2 depletion after N fertilization. Further SP analysis and in situ 15 N labeling experiment revealed that the main source for N 2 O emissions was bacterial denitrification during the hot-wet summer with lower soil O 2 concentration, while nitrification or fungal denitrification contributed about 50.0% to total emissions during the cold-dry winter with higher soil O 2 concentration. The robust positive correlation between O 2 concentration and SP values underpinned that the O 2 dynamics were the key factor to differentiate the composite processes of N 2 O production in in situ structured soil. Our findings deciphered the complexity of N 2 O production processes in real field conditions, and suggest that O 2 dynamics rather than stimulation of functional gene abundances play a key role in controlling soil N 2 O production processes in undisturbed structure soils. Our results help to develop targeted N 2 O mitigation measures and to improve process models for constraining global N 2 O budget., (© 2023 John Wiley & Sons Ltd.)

Nitrification, a key step in soil nitrogen cycling, is a biologically mediated process crucial to the ecological environment. However, how nitrifiers drive nitrification under different soil properties and climatic factors at large spatial scales is poorly understood. Here, using metagenomic sequencing and network‐based approaches, we identified key nitrifying species of upland agricultural soils in northern China, which spans a wide range of climates and geographic distances. We found that potential nitrification rates (PNRs) varied in different soils and were positively correlated with soil pH (5.42–8.46) and mean annual temperature (MAT) and negatively correlated with the C/N ratio. Network analysis revealed that one module (module 3) was significantly correlated with PNR. In this module, 16 dominant nodes were associated with AOB Nitrosomonas and most nodes were significantly correlated with environmental factors, suggesting that abiotic conditions are important for determining the assembly of these key nitrifiers. Our study advanced the understanding of the key nitrifying populations and their environmental drivers in upland agricultural soil across different soil and climate types. [ABSTRACT FROM AUTHOR]

Arable soil continues to be the dominant anthropogenic source of nitrous oxide (N2O) emissions owing to application of nitrogen (N) fertilizers and manures across the world. Using laboratory and in situ studies to elucidate the key factors controlling soil N2O emissions remains challenging due to the potential importance of multiple complex processes. We examined soil surface N2O fluxes in an arable soil, combined with in situ high‐frequency measurements of soil matrix oxygen (O2) and N2O concentrations, in situ 15N labeling, and N2O 15N site preference (SP). The in situ O2 concentration and further microcosm visualized spatiotemporal distribution of O2 both suggested that O2 dynamics were the proximal determining factor to matrix N2O concentration and fluxes due to quick O2 depletion after N fertilization. Further SP analysis and in situ 15N labeling experiment revealed that the main source for N2O emissions was bacterial denitrification during the hot‐wet summer with lower soil O2 concentration, while nitrification or fungal denitrification contributed about 50.0% to total emissions during the cold‐dry winter with higher soil O2 concentration. The robust positive correlation between O2 concentration and SP values underpinned that the O2 dynamics were the key factor to differentiate the composite processes of N2O production in in situ structured soil. Our findings deciphered the complexity of N2O production processes in real field conditions, and suggest that O2 dynamics rather than stimulation of functional gene abundances play a key role in controlling soil N2O production processes in undisturbed structure soils. Our results help to develop targeted N2O mitigation measures and to improve process models for constraining global N2O budget. [ABSTRACT FROM AUTHOR]

Agricultural soils are a major source of anthropogenic N 2 O, but their N 2 O emission estimates are highly uncertain, mainly due to the complexity of nitrogen (N) processes. Most soil N models include only primary N processes such as nitrification and denitrification, which limits their ability to realistically simulate N transformations in soils and accurately estimate N 2 O emissions from soils. This study introduces a Microbial-Explicit Model incorporating Comprehensive Nitrogen processes (MEMCN) to evaluate and quantify the influences of various N processes on the production of N 2 O as well as NO and N 2 , including nitrification, denitrification, anaerobic ammonium oxidation (ANAMMOX), dissimilatory nitrate reduction to ammonium (DNRA), mineralization, and microbial assimilation (i.e., growth) and death. The MEMCN was evaluated in laboratory experiments using agricultural soils with different levels of N additions under anaerobic and aerobic conditions, and reproduced well the dynamics of NH 4 +, NO 3 −, NO 2 −, NO, N 2 O, and N 2. After nitrification and denitrification, ANAMMOX and assimilation were found to be most important in controlling N transformations in agricultural soils. ANAMMOX directly increased N 2 emissions by 139% at the beginning of the simulations (i.e., 48 h) under anaerobic conditions, while microbial assimilation indirectly reduced NO, N 2 O, and N 2 emissions by 88%, 54%, and 58%, respectively, at the end of the simulations (i.e., 336 h) under aerobic conditions. Correspondingly, the biomass of ANAMMOX bacteria increased significantly at the beginning of the simulations under anaerobic conditions, while the biomass of nitrite oxidizing bacteria increased substantially under aerobic conditions. In contrast, DNRA, mineralization and microbial death had minor effect on soil N transformations, and the biomass of DNRA bacteria and heterotrophs did not change significantly during the simulations. Our study shows that it is necessary to include ANAMMOX and microbial assimilation in soil N models, while explicit simulation of microbial biomass dynamics may only be necessary if microbial biomass pools change significantly. • Develope a Microbial-Explicit Model incorporating Comprehensive N processes. • Quantify contributions of individual N processes to gaseous N production. • ANAMMOX increased N 2 produciton under aerobic and anaerobic condtions. • Assimilation reduced NO, N 2 O, and N 2 emissions under aerobic condtions. • Micoribal biomass should be explicitly expressed as they change substantially. [ABSTRACT FROM AUTHOR]