Influence of Long-Term Application of Different Organic Amendments on Labile Soil Organic Carbon and Yield in a Vertisol

Soil fertility drives crop productivity, with integrated organic-chemical fertilization effectively boosts soil organic carbon (SOC), nutrient availability, and microbial activity, especially in low-fertility soils like Vertisol in the Huaibei Plain. However, the role of labile organic carbon (LOC) fractions in driving yield under diversified fertilization remains unclear. Through a long-term field experiment comparing no fertilizer (NF), chemical fertilizer alone (CF), chemical fertilizer with low or high straw, pig or cattle manure (CFLS, CFHS, CFP, CFC), we demonstrate that organic-chemical treatments significantly increased SOC, LOC, carbon pool management index (CPMI), and nutrients versus NF, with cattle manure (CFC) exhibiting the strongest improvement. Partial least squares path model (PLS-PM) revealed that LOC strongly influenced soil nutrients and microbial traits, while soil nutrients were the strongest predictor of crop yield, explaining 73% of yield variance. Microbial traits had a marginally negative effect on yield, suggesting potential nutrient competition. Therefore, integrating cattle manure with chemical fertilizers optimizes soil quality and crop yield by enhancing LOC-mediated nutrient availability, demonstrating superior efficacy among organic amendments.