Long-term manure application increased soil organic carbon and nitrogen mineralization through accumulation of unprotected and physically protected carbon fractions

Soil organic carbon (SOC) and nitrogen (N) mineralization are important biogeochemical processes associated with soil fertility. These processes are influenced by physically, chemically, and biologically stabilized SOC fractions, the mechanisms of which are not well known. The present study was conducted to evaluate the combined effect of manure and mineral fertilizers on the contents of SOC fractions to promote the mineralization of SOC and N. Treatments included: i) no fertilizer control (CK); ii) a combination of mineral N, phosphorus, and potassium fertilizers (NPK); iii) manure alone (M); iv) manure combined with NPK (MNPK); and v) a high dose of manure combined with NPK (hMNPK). The combined uses of manure and mineral fertilizers (MNPK and hMNPK) enhanced the accumulation of the unprotected coarse particulate organic carbon (C) fraction (cPOC) by 44%–72% compared to CK. Manure applications (M, MNPK and hMNPK) enhanced physically microaggregate-protected organic C (µagg), physicochemically protected organic C within the microaggregate-derived silt (µsilt) fraction (H-µsilt), and physicobiochemically protected organic C within the µsilt fraction (NH-µsilt) by 30%–56%, 62%–150%, and 27%–51%, respectively. In contrast, all chemically and biochemically protected SOC fractions showed a minor response to manure application. Accumulation of cPOC, µagg, H-µsilt, and physicochemically protected organic C within the microaggregate-derived clay fraction (H-µclay) significantly contributed to the mineralization of SOC and N, resulting in a significant increase in rice grain yield with long-term manure application. In summary, long-term combined use of manure and mineral fertilizers improved SOC accumulation in unprotected and physically protected fractions, which enhanced SOC and N mineralization and benefited soil productivity in a rice-wheat cropping system.