Divergent biotic-abiotic mechanisms of soil organic carbon storage between bulk and rhizosphere soils of rice paddies in the Yangtze River Delta.

Rice paddies serve as important reservoirs of soil organic carbon (SOC) and are hotspots for microbial-mediated carbon conversion. Understanding the regulatory mechanisms of SOC in rice paddies is important for carbon sequestration management under global warming. Most previous studies on the influencing factors of paddy SOC have focused on a single habitat, such as bulk soil (BS) or rhizosphere soil (RS). However, the divergence in SOC regulatory mechanisms between BS and RS, the mediating role of rhizoplane soil (RP) microbial communities on SOC in RS, and the interactive effects of multiple influencing factors on habitat-specific SOC remain poorly quantified, particularly at regional scales. Herein we used piecewise structural equation modeling and random forest model to explore the effects of biotic-abiotic factors on SOC between BS and RS of rice paddies in the Yangtze River Delta. Significant differences in SOC, soil physicochemical, and microbial community properties between BS and RS. Soil physical and chemical properties had the greatest effect on SOC, with a standardized total effect of 0.76 vs. 0.53 for BS, and 0.72 vs. 0.94 for RS, respectively. The direct effect of microbial communities on SOC in RS (standardized direct effect, 0.33) was significant and positive, while it was marginal in BS (standardized direct effect, 0.14). Methane microbial communities in RP on SOC in RS were also markedly important. Notably, soil metallic elements had a significant positive effect on SOC in BS with a standardized path coefficient of 0.30 (P < 0.05), but a negative effect on SOC in RS with a path coefficient of -0.18 (P < 0.05). Soil physical properties had a positive indirect effect on SOC in BS with an indirect effect of 0.39 through its effect on soil chemical properties, metallic elements, and microbial community properties, while a positive indirect effect on SOC in RS with an indirect effect of 0.66 mainly through its effect on soil chemical properties. The indirect effects of climatic factors and agricultural management on SOC between BS and RS were also nonnegligible. Our study reveals the complex interactive influence of various categories of biotic-abiotic factors with different functions on paddy SOC between BS and RS at a regional scale, as well as the contribution of microbial communities in RP to SOC in RS. This study improves our understanding of the regulatory mechanisms for SOC storage and offers valuable guidance for paddy soil carbon sequestration management.