Soil organic carbon stability and exogenous nitrogen fertilizer influence the priming effect of paddy soil under long-term exposure to elevated atmospheric CO 2

Soil organic carbon (SOC) stability and dynamics are greatly influenced by long-term elevated atmospheric CO [CO ]. The priming effect (PE) is vital in SOC stability and dynamics, but its role in paddy soil under long-term elevated [CO ] remains unclear. To examine how SOC stability changed in paddy soil after long-term elevated atmospheric CO enrichment, the PE was quantified through a C-glucose-induced experiment with different N levels for topsoil (0-20 cm) from paddy free-air CO enrichment (FACE) platform. Compared with the ambient CO concentration ([CO ]), 10 years of elevated [CO ] (500 µmol·mol ) significantly increased SOC and TN content by 18.4% and 19.0%, respectively, while the C/N ratio was not changed. The labile C fractions including dissolved organic carbon (DOC) and readily oxidizable organic carbon (ROC), but excluding microbial biomass C (MBC), accumulated faster than SOC in paddy soil, which implied the reduced SOC stability for long-term elevated [CO ] enrichment. With the decline of SOC stability, the exogenously induced cumulative specific PE (PE per gram of SOC) remarkably increased by 41.1-72.7% for elevated [CO ] fumigation. The cumulative PE, especially the cumulative specific PE, was found significantly linearly correlated with the ROC content or ROC/SOC ratio (labile SOC pool). Furthermore, the application of nitrogen fertilizer slowed down the PE under elevated [CO ] condition. Our results showed that long-term elevated [CO ] enrichment reduced SOC stability and, together with exogenous nitrogen fertilizer, regulated the PE in paddy soil.