Nitrite regulated by soil pH and organic carbon drives explosive N2O emissions in upland agricultural soils

Nitrite (NO 2 − ) has been recognized as a pivotal intermediate in ammonia oxidation by upland agricultural soils, particularly in alkaline soils, yet its role in nitrous oxide (N 2 O) production remains unclear. To elucidate the mechanism of N 2 O production from NO 2 − and ammonia pathways, we conducted a microcosm incubation experiment with eight upland agricultural soils from China, with varying pH and soil organic carbon (SOC). N 2 O emissions in acid soils following NO 2 − additions were 13 to 15 times higher than those following ammonium (NH 4 + ) additions, likely due to abiotic chemical reactions producing N 2 O. In alkaline soils, NH 4 + additions stimulated higher N 2 O emissions associated with greater NO 2 − accumulation ( R 2 =0.67, P <0.001), which was regulated by soil pH and SOC. Interestingly, direct NO 2 − addition resulted in lower N 2 O emissions and yields in strongly alkaline soils, indicating that strong ammonia oxidation predominates over NO 2 − -driven pathways for N 2 O production. CO 2 emissions increased following NH 4 + application except in the strongly acidic soil, likely due to calcium carbonate dissolution in the alkaline soils during the nitrification. Collectively, our findings emphasize that edaphic variables are key drivers in N 2 O emissions, while NO 2 − serves as the proximal factor for explosive N 2 O emissions in both acid and alkaline soils. Preventing transient peaks in NO 2 − production by regulating soil pH and SOC may offer a potential measure for mitigating N 2 O emissions in agricultural soils.