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 (<30 cm), when manure was applied rather than crop residues, and in studies with cumulative C inputs exceeding 15 Mg ha − 1 . Analysis of absolute SOC stock changes (Δ Mg ha − 1 ) revealed that MAOC accumulated carbon at approximately six times the rate of POC with increasing cumulative C inputs (0.06 vs. 0.01 Mg C ha −1 per Mg C input), indicating a substantially greater capacity for MAOC formation in annual croplands. These results demonstrate that repetitive C inputs drive the formation of MAOC, a more stable carbon pool critical for long‐term carbon storage.