Responses of soil microbial functional genes to global changes are indirectly influenced by aboveground plant biomass variation

•Precipitation increment enhanced most of the C and N cycling gene abundances.•Increased precipitation would accelerate biogeocycling in semi-arid grassland.•N deposition stimulates the recalcitrant organic C decomposition genes.•Soil recalcitrant C pool would not accumulate under long-term N deposition.•Plant biomass indirectly influenced gene abundance through enhancing soil DOC.

Global nitrogen (N) deposition and precipitation change are two important factors influencing the diversity and function of terrestrial ecosystems. While considerable efforts have been devoted to investigate the responses of aboveground plant communities to altered precipitation regimes and N enrichment, the variations of belowground soil microbial communities are not well understood, particularly at the functional gene structure level. Based on a 9-year field experiment established in a typical steppe in Inner Mongolia, China, we examined the impacts of projected N deposition and precipitation increment on soil microbial functional gene composition, and assessed the soil/plant factors associated with the observed impacts. The overall functional gene composition significantly shifted in response to precipitation increment, N deposition and their combinations (all ADONIS P < 0.05), and such changes were primarily correlated with soil pH, microbial biomass, and microbial respiration. Water supply increased the abundances of both carbon (C) and N cycling genes, suggesting that the projected precipitation increment could accelerate nutrient cycling in this semi-arid region. N effects were mainly observed on the genes involved in vanillin/lignin degradations, implying that the recalcitrant C would not accumulate in soil under future scenarios of higher N deposition. Structural equation modeling (SEM) analysis revealed that soil dissolved organic carbon (DOC) was a key factor directly determining the abundance of C degradation and N cycling genes, and aboveground plant biomass indirectly influenced gene abundance through enhancing DOC. The present work provides important insights on the microbial functional feedbacks to projected global change in this semi-arid grassland ecosystem, and the mechanisms governing C and N cycles at the regional scale.