Ammonia-oxidizing bacteria-driven autotrophic nitrification dominated nitrous oxide production in calcareous soil under long term plastic film mulching

Fertilized cropland soils are a major source of anthropogenic N2O emissions in terrestrial ecosystems. However, the main N2O production pathways and microbes underlying N2O production in calcareous cropland soil under long-term plastic film mulching and nitrogen (N) management have not yet been clarified. Here, we used 15N tracing combined with quantitative polymerase chain reaction to evaluate the relative contributions of different N2O production pathways (i.e., autotrophic nitrification, heterotrophic nitrification, and denitrification) to N2O production in a calcareous maize field soil; we also used specific inhibitors (acetylene and 1-octyne) to determine the functional importance of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to N2O production. Autotrophic nitrification accounted for 47–63% and 76–100% of total N2O emissions in unfertilized (F0) and N-fertilized soils, respectively. AOB accounted for 62% and 91–98% of total N2O production in F0 and N-fertilized soils, respectively. N2O production was significantly lower in soils following long-term film mulching (F225) compared with the bare land soil (N225), and this was due to the reduced abundance and activity of AOB and N2O derived from autotrophic nitrification. Autotrophic nitrification-derived N2O emissions were 2.2-fold, 3.6-fold, and 6.3-fold higher in the F225 treatment (appropriate N level), the treatment in which the level of N was the same as that applied by local farmers (F380), and the manure addition treatment (F225+M) compared with F0, respectively, indicating that autotrophic nitrification was the main driver of increases in N2O emissions. These findings provide the first direct evidence showing that AOB-driven autotrophic nitrification is the dominant N2O production pathway in the plastic film mulched calcareous soils under different agricultural management practices. Our results also show that autotrophic nitrification is depressed by long-term mulching and promoted by N fertilization. These findings have implications for the development of targeted N2O mitigation strategies.