Long-term ammonium nitrate addition strengthens soil microbial cross-trophic interactions in a Tibetan alpine steppe

Global nitrogen (N) enrichment is modifying microbial interactions, which can be represented by network complexity. While a number of studies have explored how N addition influences the microbial intra-trophic network, its effects on the inter-trophic network have rarely been investigated. Here, we examined the effects of 8 years of multilevel N additions (i.e., 0, 1, 2, 4, 8, 16, 24 and 32 g N m^-2 year^-1) on inter-trophic interactions of soil microbial communities (i.e., protist-fungi, protist-prokaryote and fungi-prokaryote) in a Tibetan alpine steppe. Generally, there was a first increasing and then saturated trend of the complexity of inter-trophic networks along the N-addition gradient, which contrasts with the simplified or minimal response of intra-trophic network complexity reported previously. The intensified cross-trophic interactions were mainly explained by increased plant and litter biomass, which indicates that the N-induced increases in carbon supplies may have alleviated microbial energy limitations and thus resulted in more active metabolic processes, consequently stimulating various biotic interactions (e.g., predation, competition, and commensalism). Further, the enhanced inter-trophic network relationships were found to be associated with increased soil carbon and N mineralization processes. Overall, these findings highlight the importance of microbial cross-trophic interactions and indicate that they should be considered in predictions of ecosystem functioning under global N enrichment.