Consistent responses of surface- and subsurface soil fungal diversity to N enrichment are mediated differently by acidification and plant community in a semi-arid grassland

Soil fungal communities are critical to decomposition, nutrient cycling and the maintenance of plant diversity and abundance. However, despite global increases in reactive nitrogen (N) inputs to terrestrial ecosystems, due to anthropogenic activities, an explicit evaluation of the direct (resource availability) and indirect (acidification and plant community changes) effects of N enrichment on soil fungal communities in grassland ecosystems remains largely unexplored. In this study, we used Illumina sequencing of the ITS1 barcode region to elucidate the responses of soil fungal communities using a 7-year simulated N deposition experiment that spanned a broad range of N addition rates and made a systematic evaluation of the role and relative importance of N availability, plant community and soil acidification as drivers of soil fungal diversity in a semi-arid grassland ecosystem. Our results showed that N enrichment led to significant declines in soil fungal alpha diversity and promoted strong shifts in beta diversity (community composition) in both surface and subsurface soil layers. We found that N addition-induced soil acidification overwhelmed the effects of increased N availability and plant community changes, and played a primary role in shaping the observed changes in fungal alpha and beta diversity in surface soil. Conversely, in the subsurface soil layer, both fungal alpha and beta diversity were primarily controlled by N addition-induced changes in plant community attributes (i.e., aboveground plant productivity and plant community composition). Thus, our work illustrates the consistent responses of surface and subsurface soil fungal diversity (both alpha and beta diversity) to N addition that are mediated by different mechanisms and provides an integrated insight into how N enrichment could alter soil fungal diversity in semi-arid grassland in future scenarios of elevated N deposition.