AOB Nitrosospira cluster 3a.2 (D11) dominates N2O emissions in fertilised agricultural soils.

Ammonia-oxidation process directly contribute to soil nitrous oxide (N 2O) emissions in agricultural soils. However, taxonomy of the key nitrifiers (within ammonia oxidising bacteria (AOB), archaea (AOA) and complete ammonia oxidisers (comammox Nitrospira)) responsible for substantial N 2O emissions in agricultural soils is unknown, as is their regulation by soil biotic and abiotic factors. In this study, cumulative N 2O emissions, nitrification rates, abundance and community structure of nitrifiers were investigated in 16 agricultural soils from major crop production regions of China using microcosm experiments with amended nitrogen (N) supplemented or not with a nitrification inhibitor (nitrapyrin). Key nitrifier groups involved in N 2O emissions were identified by comparative analyses of the different treatments, combining sequencing and random forest analyses. Soil cumulative N 2O emissions significantly increased with soil pH in all agricultural soils. However, they decreased with soil organic carbon (SOC) in alkaline soils. Nitrapyrin significantly inhibited soil cumulative N 2O emissions and AOB growth, with a significant inhibition of the AOB Nitrosospira cluster 3a.2 (D11) abundance. One Nitrosospira multiformis-like OTU phylotype (OTU34), which was classified within the AOB Nitrosospira cluster 3a.2 (D11), had the greatest importance on cumulative N 2O emissions and its growth significantly depended on soil pH and SOC contents, with higher growth at high pH and low SOC conditions. Collectively, our results demonstrate that alkaline soils with low SOC contents have high N 2O emissions, which were mainly driven by AOB Nitrosospira cluster 3a.2 (D11). Nitrapyrin can efficiently reduce nitrification-related N 2O emissions by inhibiting the activity of AOB Nitrosospira cluster 3a.2 (D11). This study advances our understanding of key nitrifiers responsible for high N 2O emissions in agricultural soils and their controlling factors, and provides vital knowledge for N 2O emission mitigation in agricultural ecosystems.