Table 1_Synergistic effects of 13-year warming and nitrogen fertilization accelerating soil carbon destabilization in North China Plain farmland.docx

Soil carbon stabilization is critical for mitigating carbon decomposition under global change scenarios. However, the long-term combined effects of warming and nitrogen fertilization on the stabilization of the soil carbon pool in farmland remain poorly understood. Through a 13-year field experiment conducted in the North China Plain, we investigated the response of soil carbon pool components under four treatments: Ambient control (CK), warming (W1.5 °C), nitrogen fertilization (N240), and combined warming with nitrogen fertilization (WN). A three-pool carbon decomposition model (active, slow, and passive carbon pools) was employed to characterize carbon stabilization. Soils with straw addition were incubated indoors to quantify priming effects (PEs), complemented by microbial biomass carbon (MBC) measurements and δ13C isotopic tracing. Our results revealed that the WN treatment reduced passive carbon by 26%, exceeding the effects of individual treatments (10–18% reduction under warming and 6–8% under nitrogen fertilization). However, the slow carbon pool size increased by 41% under the WN treatment, surpassing the effects of individual treatments (18% under warming and 7–9% under nitrogen fertilization). These findings suggest that the combined treatment accelerated the decomposition of passive carbon pools and contributed to the accumulation of slow carbon pools, relative to single warming or nitrogen fertilization treatments. In soils amended with straw and incubated for 120 days, the WN treatment exhibited 42% higher MBC and 3.9‰ more depleted δ13C-MBC values compared to single-factor treatments and the CK. The WN treatment significantly reduced the cumulative PE by 3%, whereas single treatments increased it by 2–4%. Structural equation modeling identified soil total nitrogen (TN) and passive carbon pool size as the primary regulators of these contrasting effects. Our findings suggest that the long-term combination of warming and nitrogen fertilization could exacerbate soil carbon destabilization in farmland. Microbial metabolic shifts potentially serve as an important regulatory mechanism influencing agricultural soil carbon stocks under future climate warming.