Refining the nitrogen saturation hypothesis by accounting for microbial roles in nitrogen and phosphorus cycling

Human activities have increased nitrogen (N) and phosphorus (P) deposition, disrupting microbial activity and altering N-P cycling. Understanding how nutrient limitations and additions affect soil microbes is critical for predicting ecosystem succession and mitigating greenhouse gas emissions. Leveraging long-term N-P addition experiments in a subtropical forest, we developed an enhanced Microbial-ENzyme Decomposition (MEND) model by incorporating an enzyme-mediated P module. Following rigorous calibration and validation with multi-source data, we found that N-P addition has antagonistic effects on main fluxes, with P application mitigating N stimulation of fluxes and partially reducing N₂O emissions. On this basis, we refined the nitrogen saturation hypothesis (NSH) for subtropical ecosystems by attributing divergent nitrification patterns to ammonia inhibition, and we expanded the hypothesis to encompass denitrification and N fixation. By integrating microbiome data, we demonstrated the intrinsic effects of N addition on N cycle through differential expression of genes due to community change, while P addition can counteract effects of N increase by alleviating microbial P limitations. Additionally, we highlight the significance of microbial-enzyme activities feedback in regulating P cycle to maintain ecological balance. Integrating microbially-enabled C-N-P model with diverse experimental data, particularly microbiome information, enhances interpretability and reveals ecosystem mechanisms beyond direct experimental observation.