Linkages of stoichiometric imbalances to soil microbial respiration with increasing nitrogen addition: Evidence from a long-term grassland experiment

Rapidly increasing atmospheric nitrogen (N) deposition has substantially altered resource availability and the stoichiometry of microbial biomass in terrestrial ecosystems. However, variations of microbial biomass stoichiometry are not paralleled by changes in the stoichiometry of available resources, resulting in stoichiometric imbalances that constrain microbial growth and nutrient cycling and thus affect carbon (C) cycling. How soil microbes cope with stoichiometric imbalances and the impacts of their responses on microbial-mediated C cycling still remain a puzzle. To help address this puzzle, we performed an eight-year field manipulative experiment with six N addition levels in a semiarid grassland in northern China. We measured soil available nutrients, nutrients within microbial biomass, and the potential activity of ecoenzymes related to microbial nutrient acquisition. Our results showed that resource stoichiometric imbalances, including C:N, C:P, and N:P, responded non-linearly to N addition. Specifically, stochiometric imbalances increased up to intermediate doses and then decreased. These nonlinear responses implied that increasing N addition enhanced microbial C limitation rather than P limitation. Data on microbial adaptive responses to resource stoichiometric imbalances revealed that, under C limitation, soil microbial communities regulated their ecoenzyme production and threshold element ratios (TER) to maintain stoichiometric homeostasis, supporting the consumer-driven nutrient recycling theory (CNR). Using piecewise structural equation modeling (SEM), we found that the N-induced reduction of soil microbial respiration was directly linked to increasing TER but was indirectly linked to soil enzyme stoichiometry and microbial biomass stoichiometry. These results suggest that coordinated regulation of microbial biomass stoichiometry and soil enzyme stoichiometry lead to a higher C use efficiency (CUE) and a lower nutrient use efficiency, further lowering microbial respiration. These results highlight the importance of stoichiometric imbalance in regulating microbial respiration and may help project how stoichiometric changes induced by global N deposition control terrestrial C and nutrient flows.