• MAOC plays a key role in SOC stabilization promoted by organic manure substitution. • Organic manure substitution enhances iron oxide amorphization in bulk soil and MAOM. • Organically complexed Fe oxides (Fe PP ) show higher C stabilization capacity than other Fe oxides. • Co‑precipitation with Fe PP enables carbon stability without elevated greenhouse gas emissions. Organic amendments regulate soil organic carbon (SOC) and mineral-associated organic carbon (MAOC) stability by altering iron oxides. However, most sequential extraction studies for iron oxides have focused on bulk soil instead of mineral-associated organic matter (MAOM), leaving fraction-specific responses within MAOM unclear under varying substitution ratios. Here, a 12-year field experiment was conducted with four treatments including conventional fertilization (CF), 25% organic N substitution (OS25), 50% organic N substitution (OS50) and 100% organic N substitution (OS100), to investigate the responses of iron oxides and associated organic carbon in bulk soil and MAOM. Results revealed that organic manure substitution markedly increased SOC and MAOC by 8.42%–23.73%, relative to CF. The application of organic fertilizer facilitated the transformation of crystalline iron oxides into poorly crystalline forms, notably promoting the formation of organically complexed iron oxides (Fe PP ), which became the dominant iron oxide phase. Notably, Fe PP -associated organic carbon (OC PP ) exhibited a non‑linear response across substitution ratios, declining at moderate levels before increasing sharply at OS100 (13.53%-35.22% higher than other treatments). The molar ratios of organic carbon to iron supported that Fe PP stabilizes organic carbon primarily via co‑precipitation, a mechanism that enabled maximal organic carbon stabilization at OS100 without increasing greenhouse gas emissions. Consistent with this, linear mixed-effects and automated linear modeling revealed that variables related to organically complexed iron oxides were the dominant predictors of the MAOC/SOC ratio in MAOM. Overall, organic manure substitution enhances MAOC formation by promoting iron oxide transformation in MAOM and organo-mineral binding, with implications for soil carbon stabilization.
Xian et al. (Thu,) studied this question.