Climate-Linked Si-Fe Dynamics and Their Associations With Soil Mineral-Associated Organic Carbon

Abstract

Iron (Fe) (hydr)oxides and silicate minerals are key controls on the formation of mineral-associated organic carbon (MAOC) in soils—a relatively persistent fraction of soil organic carbon (SOC). Silicon (Si) is essential for silicate mineral formation, and the interactions between Fe and Si are widespread in natural environments. However, the interactive effects of Fe (hydr)oxides and Si on MAOC remain poorly understood. Here, we determined Fe and Si speciation in soils across diverse ecosystems by selective extraction, and estimated their individual and interactive effects on MAOC. Our results show that pedogenic and biogenic amorphous Si are positively associated with MAOC accumulation, while more mobile Si forms (including dissolved and weakly adsorbed Si) are negatively associated with the formation of Fe-bound organic carbon (Fe-OC). Although amorphous Fe oxides serve as the dominant carriers of Fe-OC complexes, increasing metal oxide-associated Si is linked to a reduced contribution of Fe oxides to MAOC. Additionally, temperature and precipitation are associated with variations in Si and Fe speciation, likely through changes in soil weathering stage and pH, and these patterns are further linked to differences in MAOC accumulation. Our results highlight climate-linked Si-Fe dynamics and interactions as critical but previously underappreciated factors related to SOC stabilization, and we suggest that Si speciation should be integrated into land management and SOC sequestration strategies across diverse ecosystems.