Interactions of nematodes and ammonia-oxidizing bacteria mediate nitrification in two contrasting soils

Ammonia oxidation, the first and rate-limiting step of nitrification, is essential for converting ammonium (NH₄⁺) to nitrite (NO₂^-) in soil, and is a key process in nitrogen (N) cycling that supports crop growth in agroecosystems. Previous research has focused on the impacts of ammonia-oxidizing microbes on soil nitrification under agricultural management, but the influence of the interaction between microfauna, particularly nematodes, and ammonia-oxidizing microbes on soil nitrification remains unclear. In this study, we selected four rates of N applied to lime concretion black soil and fluvo-aquic soil and tested the effect of the interplay of nematodes with ammonia-oxidizing archaea (AOA) and bacteria (AOB) on the potential nitrification rate (PNR). The results demonstrated that the application of N to the fluvo-aquic soil led to an increase in the PNR, as well as a significant enhancement in the abundance of copies of the AOA and AOB amoA genes. However, no consistent outcomes were observed in the lime concretion black soil. The application of N increased the relative abundance of bacterivorous nematodes, particularly Chiloplacus, in the fluvo-aquic soil, but it decreased their relative abundance in the lime concretion black soil. A co-occurrence network analysis indicated that the AOB nodes accounted for a higher proportion in the network and had more potential associations with bacterivorous nematodes in the fluvo-aquic soil. The partial least-squares path model suggests that bacterivorous nematodes positively regulated the AOB and further influenced the PNR in the fluvo-aquic soil. These results provide novel insights into our understanding of the processes of soil nitrification, as well as the interactions between soil microorganisms and nematodes.