Long-term mineral fertilization increases microbial carbon use efficiency and necromass in upland but not in paddy soils

The magnitude and persistence of soil organic carbon (SOC) are governed by its microbial carbon use efficiency (CUE) and microbial necromass (MNC). However, whether long-term mineral fertilization builds microbially derived SOC via shared or system-specific mechanisms in paddy versus upland areas remains unresolved. For this study, two >30 year field experiments involving paddies and uplands were conducted under four fertilization treatments (no fertilizer (CK), balanced mineral nitrogen, phosphorous, and potassium (NPK), manure (M), and NPK combined with manure (NPKM)). The application of NPK and NPKM in the uplands significantly increased CUE by 35.4% and 19.9%, respectively, compared with the CK and M. No significant differences in these parameters were observed in the paddies. The enhancing effects of NPK on the upland CUE were associated with the increased availability of soil phosphorus and sustained carbon-degradation capacities, while the anaerobic conditions of the paddy soils under NPK reduced its functional responses. The CUE was significantly positively associated with the MNC across both land-uses, where the contribution of CUE to MNC was stronger in the paddies (slope = 24.17) than uplands (slope = 11.81). In paddies, the contribution of CUE was stronger for the fungal necromass (slope = 18.87) than bacterial necromass (slope = 5.30). These results demonstrated that mineral fertilization affects microbially derived soil organic carbon through land-use–specific mechanisms, which advocates for fertilization strategies that are tailored to hydrologic and phosphorus constraints.