Denitrifier abundance has a greater influence on denitrification rates at larger landscape scales but is a lesser driver than environmental variables

Nitrous oxide is a potent greenhouse gas and its production is mediated by the soil microbial processes of denitrification and nitrification. A thorough understanding of denitrification drivers is necessary to accurately predict and manage nitrous oxide emissions. However, studies disagree on the utility of quantifying the denitrifier community to predict denitrification rates. This study examines the influence of nitrite reductase gene (nirK and nirS) abundance on denitrification rates across a topographically sloping region. The study was carried out over different seasons (autumn, winter, spring, summer) and topographic positions (shoulder and backslope) within a field cropped to spring wheat. A footslope cropped to winter wheat was included in certain analyses to introduce additional variation at a broader scale. We measured denitrification enzyme activity (DEA) and basal denitrification (BD), nirK and nirS abundance, bacterial and archaeal ammonia monooxygenase gene (amoA) abundance, and soil environmental and chemical characteristics which included gravimetric water content, pH, electrical conductivity, and concentrations of nitrate (NO3-N), ammonium (NH4-N), total nitrogen, total soluble nitrogen, total carbon and soluble non-purgeable organic carbon (NPOC). Stepwise multivariate regression (SMR) models of DEA and BD were performed using the measured soil characteristics and denitrifier and nitrifier abundance as explanatory factors to determine whether the microbial community size influenced prediction of denitrification activity. Two SMR models were generated: one SMR model was that with the greatest R2 from utilizing 1–3 significant variables (P