Oxygen-depletion by rapid ammonia oxidation regulates kinetics of N2O, NO and N2 production in an ammonium fertilised agricultural soil

Abstract

Oxygen (O2) is a key factor driving the expression of N-cycle-related functional genes and regulating nitrogenous gas production in the soil. However, how and to what extent the associated gene transcription and corresponding gas kinetics interact with the transition of soil O2 status caused by N fertilisation, remains poorly understood. In this context, we conducted a robotized incubation experiment using a He/O2 atmosphere with different amounts of ammonium-based fertiliser (0 (control), 60 (AS60) and 200 mg N kg−1 (NH4)2SO4 (AS200)) applied to an agricultural soil with a strong nitrification potential under three different initial O2 levels (oxic 21%, sub-oxic 3%, and anoxic 0%) over 14 days. Through repeated measurements of N2O, NO, and N2 concentrations and kinetics of mineral N following the decline of O2 concentrations, we found that higher ammonium addition (200 vs. 60 mg N kg−1) caused faster O2 consumption in the headspace and induced up to 238 times higher net accumulation of N2O under initially oxic headspace condition. We speculate that this was due to: 1) the rapid transition of O2 status from oxic to anoxic due to vigorous ammonia oxidation; and 2) the increased N2O/(N2O + NO + N2) ratio of denitrification with higher N addition. The amoA and nosZ gene transcript numbers changed significantly in response to ammonium addition and decreasing O2 concentration, whereas high-throughput sequencing revealed a significant structural alteration of the soil microbiota along with the transition of O2 status. Our results highlight that the vigorousness of oxygen depletion in the soil matrix driven by rapid ammonia oxidation is the proximal factor that regulates gas kinetics in high nitrification-potential soil when O2 diffusion is limited. This implies that practices which reduce hotspots of ammonia oxidation have the potential to mitigate N2O emissions from nitrifier denitrification and denitrification in agricultural soil.