Nitrogen addition impacts on soil microbial stoichiometry are driven by changes in plant resource stoichiometry not by the composition of main microbial groups in an alpine meadow

Increased nitrogen (N) inputs and subsequent effects on soil microbial stoichiometry have strong influences on organic matter decomposition and nutrient cycling. The effects of N addition on soil microbial stoichiometry are well documented, but we know little about the mechanisms linking between N addition and soil microbial stoichiometry. We examined how the effects of N addition cascade through soil properties (pH, available N (AN), available phosphorus (AP), dissolved organic carbon (DOC), DOC/AN, AN/AP and DOC/AP ratios), plant community composition (grasses, sedges, forbs, and legumes), plant resource stoichiometry (community-level leaf and root C/N/P ratios), and the composition of main microbial groups (phospholipid fatty acids profile) to influence microbial stoichiometry using a 5-year N fertilization field experiment in a Tibetan alpine meadow. We found that N addition changed soil microbial C/N and N/P ratios, but not microbial C/P ratios, indicating plasticity in microbial stoichiometry to increasing N deposition. Moreover, the changes in microbial stoichiometry were driven by N not C and P concentrations. Structural equation modeling revealed that N addition predominantly controlled soil microbial C/N and N/P ratios through plant leaf and root stoichiometry, but not the composition of plant community and main microbial groups. Our findings suggest that N addition-induced changes in plant resource stoichiometry are the core drivers of soil microbial stoichiometry responses to N deposition.