Evolution of Oxidizable Soil Organic Carbon Fractions and Impacts on Rice Yield Under Fertility Gradients in a Typical Double-Cropping System

In double-cropping rice regions, soil organic carbon (SOC) levels are crucial for crop yield, and soil fertility drives rice productivity. This study clarified the driving patterns of oxidizable soil organic carbon fractions on rice yields across fertility gradients to guide fertility management. Soil properties and yields were monitored across eight representative double-rice counties in Hunan Province. Total organic carbon was partitioned into four pools (Fractions I–IV) using a modified Walkley–Black acid-gradient oxidation method, and multiple linear regression (MLR) was used for seasonal modeling. High-productivity fields achieved an annual yield of 16.69 t·ha−1 (65% higher than low-productivity fields), consistent with SOC enrichment; the combined proportion of the Cfrac1 and Cfrac2 decreased significantly from 45.6% in HPPS to 36.8% in LPPS. MLR models (R2 = 0.593–0.759) showed that high-productivity early rice was driven by the synergy of Cfrac1 with nitrogen/phosphorus, while late rice was driven by Cfrac1. Conversely, Cfrac2 was the universal driver in low-to-medium productivity fields. Total nitrogen negatively correlated with yield across all productivity levels, highlighting nitrogen inefficiency. Future management must transition to carbon-based nitrogen promotion, optimizing active carbon proportions to balance seasonal and fertility-specific nutrient turnover.