Significant role of microbial nitrogen use efficiency in regulating long-term soil carbon and nitrogen stoichiometry

Microbes play a major role in soil biogeochemistry, yet it remains unclear how microbial physiology, particularly microbial nitrogen (N) use efficiency (NUE), regulates the soil C/N ratio to maintain long-term stoichiometric stability. Here, we refined the definition and quantification of microbial NUE in the latest C-N coupled Microbial-ENzyme Decomposition (MEND) model and validated it against field experimental data from subtropical broadleaf and pine forests. Our results show that microbial anabolism, reflected in high C use efficiency (CUE) and NUE, contributes to the stabilization of the soil C/N ratio. Long-term simulations, based on a calibration strategy combining experimental calibration with independent post-experiment evaluation, revealed insignificant difference in microbial CUE between broadleaf and pine forests, but a notable higher NUE in the broadleaf forest (0.64) than that in the pine forest (0.38). Soil C/N ratio decreased as CUE and NUE increased, with a stronger association for NUE. Additionally, following a 33% and 21% increase in litterfall C/N ratios in the broadleaf and pine forests, microbial NUE would need to rise by 10.5% and 11.0%, respectively, to maintain the soil C/N ratio, while microbial CUE was held constant. These findings highlight microbial NUE as a key control on soil C/N stability in subtropical forests, with implications for forest management under global change as litter quality shifts.