Amendment of saline-alkali soils promotes the formation and stability of iron-bound organic carbon

Iron oxides protect soil organic carbon (SOC) over the long term, but the effects of saline-alkali soil amendments on iron-bound organic carbon (Fe-OC) remain unclear. Using data from Northeast China, this study examined the effects of desulfurized gypsum and polyaspartic acid calcium (PASP-Ca) on Fe-OC. Compared to the control, Fe-OC increased by 2.7 times with desulfurized gypsum and 3.9 times with PASP-Ca. Both amendments shifted the association of Fe-OC from adsorption to co-precipitation, thereby enhancing the formation and stability of Fe-OC and consequently promoting SOC sequestration. Both measures also facilitated the transformation of free iron minerals into complexed forms, reducing microbial carbon binding while maintaining the pattern of preferential binding of iron minerals with aromatic-C. Key factors driving these changes include improved soil nutrients, water content, and microorganisms, along with reduced soil pH. This study offers valuable insights into the effects of saline-alkali soil amendments on Fe-OC and SOC stabilization.