A meta-analysis of soil carbon fraction response to organic amendments in annual croplands

Organic amendments are key sources of nutrients in croplands and play an important role in maintaining soil organic carbon (SOC) stocks. Stabilization of SOC is mediated by environmental and management factors and may differ between mineral-associated organic carbon (MAOC) and particulate organic carbon (POC). To quantify these patterns, we conducted a global meta-analysis evaluating SOC, MAOC, and POC responses to long-term organic amendments (animal manure or crop residues) in annual cropping systems. Overall, organic amendments increased SOC, POC, and MAOC relative to unamended controls. When analyzed using effect sizes, POC exhibited greater proportional responses than MAOC or bulk SOC, reflecting the smaller baseline size of the POC pool. Responses across all SOC fractions were greatest when control treatments did not receive mineral fertilizer. Amendment effects were strongest in surface soils (