Tree species richness and functional composition drive soil nitrification through ammonia-oxidizing archaea in subtropical forests

Understanding the links between plant diversity and functional microbiota is crucial for unraveling the mechanisms by which plant communities regulate ecosystem functions. However, the underlying mechanism of how tree diversity influences soil nitrification remains elusive in forest ecosystems. Here we investigated the impacts of tree species richness and their functional composition (including functional identity and functional trait diversity) on the dynamics of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB) and arbuscular mycorrhizal (AM) fungi using a tree diversity experiment established with 32 native tree species in a subtropical forest biome. Our study revealed that tree species richness had a positive concave-down relationship with the abundance of AOA. Furthermore, the community-weighted mean of leaf nitrogen and specific root length were significant positive factors for the abundances of AOA and AOB, respectively. By contrast, functional trait diversity of specific root length was negatively associated with the biomass of AM fungi. Regression analysis showed that the abundance of AOA was positively correlated with soil net nitrification rates, while the biomass of AM fungi was negatively correlated with soil net nitrification rates. The structural equation model showed that the positive effect of leaf nitrogen on nitrification was driven by the increase in AOA. By contrast, the functional trait diversity of specific root length indirectly increased soil nitrification via a negative impact on the biomass of AM fungi. Taken together, our findings demonstrate that both tree species richness and tree functional composition promoted soil nitrification by increasing the AOA abundance, and emphasize the key role of the interplay between tree diversity and functional microbiota in predicting ecosystem functions.