Transient nitrite accumulation explains the variation of N2O emissions to N fertilization in upland agricultural soils

Nitrogen (N) fertilization in agricultural soils is the largest anthropogenic source of nitrous oxide (N2O) emissions, but the underlying mechanism of the varied response of N2O emissions to N fertilizer is unclear. We investigated the response of N2O emissions to ammonium-based fertilizers and nitrification inhibitors in upland agricultural soils across northern China. Large variations in N2O emissions in different upland soils in response to N fertilization were found, whereas the variations were well correlated with soil nitrite (NO2−) dynamics (P < 0.01). A higher pH soil is generally correlated with a higher potential nitrification and denitrification rate, a higher AOB/AOA gene abundance ratio, higher NO2− content, and higher N2O emission. The efficacy of 3,4-dimethylpyrazole phosphate (DMPP) for inhibiting net nitrification increased with increasing soil pH. Further investigation showed that rapid ammonia oxidation drives oxygen (O2) depletion and NO2− accumulation in the soil matrix, which stimulates N2O emission and inhibits N2 emission under anoxic conditions. Collectively, our results showed that alkaline soils tend to have rapid ammonia oxidation potential, leading to transient NO2− accumulation and O2 depletion in the soil matrix after large ammonium addition. This might trigger significant N2O emissions from denitrification and DNRA, which explains the spatial heterogeneity of N2O emissions in response to N fertilization in various upland soils.