Contribution of Ammonium-Induced Nitrifier Denitrification to NO in Paddy Fields.

Paddy fields are one of the most important sources of nitrous oxide (NO), but biogeochemical NO production mechanisms in the soil profile remain unclear. Our study used incubation, dual-isotope (N-O) labeling methods, and molecular techniques to elucidate NO production characteristics and mechanisms in the soil profile (0-60 cm) during summer fallow, rice cropping, and winter fallow periods. The results pointed out that biotic processes dominated NO production (72.2-100%) and NO from the tillage layer accounted for 91.0-98.5% of total NO in the soil profile. Heterotrophic denitrification (HD) was the main process generating NO, contributing between 53.4 and 96.6%, the remainder being due to ammonia oxidation pathways, which was further confirmed by metagenomics and quantitative polymerase chain reaction (qPCR) assays. Nitrifier denitrification (ND) was an important NO production source, contributing 0-46.6% of total NO production, which showed similar trends with NO emissions. Among physicochemical and biological factors, ammonium content and the ratio of total organic matter to nitrate were the main driving factors affecting the contribution ratios of the ammonia oxidation pathways and HD pathway, respectively. Moisture content and pH affect -carrying and thus the NO production rate. These findings confirm the importance of ND to NO production and help to elucidate the impact of anthropogenic activities, including tillage, fertilization, and irrigation, on NO production.