Optimized Fertilization Balances Microbial Carbon and Nitrogen Use Efficiency: Evidence From Microbial Communities

ABSTRACT Optimized fertilization involving reduced mineral N input and organic substitution can alter resource stoichiometry, microbial carbon use efficiency (CUE) and nitrogen use efficiency (NUE). However, how fertilization‐induced resource imbalance reshapes microbial community to regulate CUE and NUE remains unclear. We conducted a field experiment in a Camellia oleifera plantation under conventional mineral nitrogen fertilizer (CK), organic manure (OM), and reduced mineral nitrogen combined with manure (RNM). Microbial CUE and NUE (via stoichiometric modeling), community structures and functions were determined at 30 and 120 days after fertilization. At 30 days, RNM and OM significantly increased NUE and alleviated resource limitations compared with CK. OM improved most microbial network properties relative to CK and RNM ( p < 0.05). At 120 days, C‐limitation disappeared while nutrient‐limitation persisted across all treatments. RNM and OM increased NUE by 1.13‐ and 4.26‐fold but decreased CUE by 7.5% and 61.3%, respectively. RNM showed the most balanced CUE‐NUE trade‐off. Notably, OM exhibited significantly lower microbial network metrics than CK and RNM except for the number of nodes ( p < 0.05), revealing a time‐dependent reversal effect. The laccase‐related microbial metabolic function was significantly associated with resource use efficiency ( p < 0.05), with a stronger association with NUE than CUE. Linear mixed‐effects models identified microbial network properties and resource limitation status were dominant predictors governing the CUE‐NUE balance ( > 0.3). Model explanatory power was substantially improved after fertilization type was included ( > 0.9). Our study provides novel evidence that fertilization‐induced stoichiometric imbalance reshapes microbial networks and further modulates microbial CUE and NUE. These findings highlight that microbial network attributes should be integrated into fertilization optimization strategies to balance soil carbon sequestration and nutrient retention.