Both AOA and AOB contribute to nitrification and show linear correlation with nitrate leaching in purple soils with a...

Ammonia oxidizers play an important role in nitrification that forms nitrate, the main form of leaching nitrogen (N). However, little is known about how ammonia oxidizers bridge long-term N fertilization levels and soil nitrate leaching. We conducted a field experiment in purple soil, investigating the interactions among soil physico-chemical parameters, ammonia-oxidizing microbial communities, and N leaching under 0, 90, 180, 270, and 360 kg N ha^-1 yr^-1 fertilization levels. We found that soil inorganic N leaching increased exponentially with increasing N application rate. N fertilization enhanced the abundances of the amoA gene in ammonia-oxidizing archaea (AOA) and bacteria (AOB), while partial least squares regression analysis revealed that AOA and AOB abundances were correlated with pH and soil organic carbon (SOC). Compared with no N fertilization, N application reduced AOA alpha diversity and increased AOB alpha diversity. AOA alpha diversity was associated with pH and bulk density, whereas soil SOC and inorganic N content were more important in predicting changes in AOB alpha diversity. A linear relationship was established between soil NO₃^--N leaching, the potential nitrification rate (PNR), and the abundances of AOA and AOB. The association of soil NO₃^--N leaching and PNR with both AOA and AOB abundances were further corroborated by Mantel test, random forest regression, and partial least squares path modelling. Furthermore, alterations in the AOB alpha diversity, soil pH and NH₄⁺-N content also contribute to the increasing soil NO₃^--N leaching along the N application rate. Our results suggest that AOA, which previous studies have found to be active only under low N conditions, can also contribute to nitrification and support soil NO₃^--N leaching at a wide range of N gradients. Overall, this finding advances the current understanding of the relationship between soil N leaching and microbial functional properties to some extent.