Soil biota associated with soil N cycling under multiple anthropogenic stressors in grasslands

Soil nitrogen (N) processes and statuses are two essential components for soil N cycling in terrestrial ecosystems. Although studies have estimated the effects of multiple anthropogenic stressors (e.g., nutrient enrichment and soil acidification) on soil N cycling, how multiple trophic levels of soil biota regulate the responses of soil N processes and statuses under multiple anthropogenic stressors has not yet been studied. We established an N and phosphorus (P) enrichment and acid addition experiment in a semi-arid grassland on the Mongolian Plateau. After five years, our results showed that soil acidification strongly inhibited soil N processes (via decreasing N mineralization rate, potential nitrification rate, and potential denitrification rate) and soil N statuses (via decreasing dissolved organic N, NO3−-N, and microbial biomass N). Acid addition-induced declines in soil N processes or N statuses were mainly associated with the inhibition of the abundance/richness of soil N-related bacteria and nematodes. Compared with soil acidification, N or P enrichment had relatively minor effects on soil N processes or N statuses, except for the N enrichment-induced decline in the potential nitrification rate and increase in soil NH4+-N and P enrichment-induced decline in soil NO3−-N. Furthermore, we found that the responses of soil N processes or statuses to nutrient enrichment were different between non-acidified soils and acidified soils. Overall, our results evidenced that soil biota strongly mediated the responses of soil N processes and statuses to nutrient enrichment and acid addition in semi-arid grasslands, and soil acidification weakened the associations between soil biota and soil N cycling. These findings indicate that we should concentrate more on the importance of soil biota in maintaining ecosystem N functions and services against a background of global change.