Decoupled Carbon and Nitrogen Cycling Across Soil Particle-Size Fractions in Apple Orchards of the Jiaodong Peninsula, China

The coupled mechanisms governing carbon–nitrogen turnover across soil particle-size fractions remain unclear. This study investigated soil organic carbon (SOC) and total nitrogen (TN) dynamics across five particle-size fractions in Cambisols under conventional and organic orchard management. Results showed that particle size dominated δ13C variation (Partial η2 = 0.36) while management practice regulated δ15N variation (Partial η2 = 0.38), revealing a fundamental decoupling of C and N cycling within the particle-size matrix. The progressive linear increase in δ13C with declining particle size signaled a transition of SOC from net accumulation to net decomposition, accompanied by directional carbon translocation from coarse sand to clay fractions. A universal critical soil pH threshold ~5.3 was identified, where inter-particle-size carbon flow reached its maximum while microbial decomposition was minimized. Organic management reduced the intensity of plant-derived carbon translocation between particle-size fractions, yet substantially enhanced microbial anabolism, leading to drastically elevated stocks of microbial necromass carbon (MNC) in both particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) pools. Notably, the relative proportional distribution of POC and MAOC remained unchanged across the two management practices, which was intrinsically constrained by the inherent textural properties of the studied Cambisols. Counterintuitively, progressive soil acidification concurrently increased SOC lability and overall carbon stabilization, a paradox that directly demonstrated decoupling between chemical oxidizability and physical protection during particle-size carbon translocation. These findings confirmed incomplete carbon–nitrogen coupling within soil particle-size fractions, and demonstrated that SOC stabilization was co-regulated by organo-mineral interactions and microbial processing, whereas nitrogen dynamics were primarily modulated by exogenous management-derived inputs. This work provided novel insights for optimizing agricultural management strategies to synergistically enhance soil fertility and long-term carbon sequestration.