Organic amendments enhance aggregate stability of yellow soils in southwest China: roles of SOC characteristics and Fe oxides

Soil structure is the foundation of soil functions, and its widespread degradation poses significant threats to soil health. By 13 C nuclear magnetic resonance (NMR) spectroscopy and PLS-PM model, this study aimed to investigate how soil binding agents (SOC and Fe oxide) dynamically reshape soil structure after organic materials incorporation. Four treatments were implemented in a one-year field trial: conventional fertilization (CK), charcoal + rapeseed cake + husk (BC), rapeseed cake + husk (RS), and N fertilizer reduction (DP). Results show that treatments incorporating organic materials significantly improved the soil structure during maize maturity, RS treatment was more effective, resulting in increases of 27.15%, 32.26%, and 9.12% in aggregate MWD, GMD, and R 0.25 , compared with CK. And organic amendments enhanced aggregate-associated OC during the silking and maturity stages, particularly within the 8–2 mm aggregates (17.05–104.23%). In addition, organic amendment effects on aggregate‑associated OC accumulation differed across growth stages: RS outperformed BC at jointing and silking, while BC was superior at maturity. Compared with free-form Fe oxides (Fed) and complex Fe oxides (Fep), amorphous Fe oxides (Feo) exhibited a stronger influence on aggregate stability. We further observed that, apart from SOC quantity, the inherent characteristics of SOC also play a key role in aggregate stability. A/OA, aliphaticity, hydrophobicity, and alkyl C abundance of SOC favored the formation of 8 –2 mm aggregates, whereas O-substituted aromatic C, O-alkyl C, and SOC aromaticity were more strongly associated with the formation of 2–0.25, 0.25–0.053, and < 0.053 mm aggregates. This study provides new insights into SOC characteristics and aggregate formation in the yellow soils of southwest China.