Ammonia oxidation as the engine to induce denitrification to produce N2O in alkaline agricultural soils

Ammonia oxidation contributes to global N₂O emissions both by direct production and by fueling denitrification (provision of nitrite or nitrate to denitrifiers). Urea or ammonia-based fertilizers account for approximately 70% of annual nitrogen fertilizer inputs globally. This means that the subsequent coupling processes driven by ammonia oxidation are of great significance to N₂O emissions. However, for this important source of N₂O production, direct evidence at the process- and microorganism- level is highly lacking, leading to that the systematic mechanism of the coupling processes and microbial activities in response to environmental changes (especially O₂ changes) remains unknown. Here, we investigated ammonia oxidation, other soil N transformation processes, N₂O emissions and related environmental changes (e.g., O₂ consumption) and microbial activities in a calcareous upland soil in Northern China, which has a strong ammonia oxidizing rate and is a global N₂O hotspot, by combining field observations, microcosm, molecular and isotope techniques. The results showed that the soil has a far (5-30 times) higher nitrification potential and gross nitrification rate than other cropland soils in the world. The strong ammonia oxidation led to rapid O₂ consumption and NO₂^- accumulation in soil matrix, causing strong emission peaks of N₂O, which was in line with the N₂O yield from denitrifiers. The N₂O ¹⁵N site preference and ¹⁵N labeling data further revealed a major role of nitrification-induced denitrification in high N₂O emissions. A higher AOB/AOA gene abundance ratio correlated the higher nitrification potential, higher NO₂^- transition and higher N₂O emissions. A coupling expression of nitrifying and denitrifying genes (amoA, narG, nirS, nirK, nosZ) and a significant structural alteration of the soil microbiota along with the O₂ consumption driven by ammonia oxidation linked to higher N₂O emissions. All the evidence points to ammonia oxidation-linked denitrification being the major process generating N₂O. In consequence, reducing N surplus, slowing down nitrification rate using nitrification inhibitors, and avoiding high ammonium microzones using slow-release or organic fertilizers are main measures to reduce N₂O emissions per unit of urea or NH₄⁺-based N input in intensively managed alkaline soils globally.