Long-Term Overfertilization Alters the Temperature Sensitivity of Soil Organic Carbon Decomposition Through Changes in Carbon Pool Composition

Mitigating climate change necessitates a thorough understanding of soil organic carbon (SOC) decomposition and its response to warming. The overuse of synthetic fertilizers can alter SOC composition and affect carbon cycling, potentially changing the temperature sensitivity (Q10) of SOC decomposition. This study evaluated the Q10 of SOC decomposition after long-term (37-year) excessive fertilization in a loess soil. Four treatments were compared: control (no nutrient input, CK); recommended rates of synthetic nitrogen (N) and phosphorus (P) fertilizers (CFr); excessive rates of N and P fertilizers (CFh); and CFh plus organic manure (MCFh). The Q10 of SOC decomposition was investigated via an incubation experiment at temperatures of 15 °C, 25 °C and 35 °C within 63 days. Compared with CFr, long-term CFh and MCFh significantly increased SOC contents by 14% and 67%, and this increase was driven primarily by rises in mineral-associated organic carbon (MOC) of 28% and 62% and particulate organic carbon (POC) of 32% and 79%, respectively, under CFh and MCFh. While CFh and MCFh did not change the SOC composition, they increased the proportions of fine POC (fPOC) to SOC by 10% and 91%, and the ratio of light POC to SOC by 78% and 143%, respectively. Q10 values ranged from 2.18 to 3.00 across all treatments, with a mean of 2.64. Both CFh and MCFh drastically enhanced the Q10 values by 38% and 25% compared with CFr at 15–25 °C. However, MCFh significantly decreased the Q10 value by 31% relative to CFr at 25–35 °C. Partial least squares path modeling showed that soil physicochemical properties and labile carbon fractions differed significantly among treatments, with physical properties regulating labile carbon fractions. Fertilization significantly increased the Q10 value at 15–25 °C owing to increased proportion of labile carbon fractions and decreased labile carbon content. Our results suggest that SOC gains from continuous addition of synthetic fertilizers are vulnerable to loss under warming. However, this loss could be alleviated by incorporation of organic manure. Thus, integration of organic manure into nutrient management practices could be an efficient way to counteract warming-induced SOC decomposition.