Iron‑carbon complex types and bonding forms jointly control organic carbon mineralization in paddy soils

The crucial role of iron (Fe) oxides in stabilizing soil organic carbon (SOC) is well recognized, but their effects on SOC mineralization remain poorly understood. To address this knowledge gap, we evaluated the effects of four typical Fe-bound OC (Fe-OC) complexes including adsorbed ferrihydrite (Fh)- and goethite (Goe)- ¹³C, coprecipitated Fh/Goe-¹³C and ¹³C-glucose as the control, on OC mineralization during an 80-day anaerobic incubation in a paddy soil. ¹³C-tracing indicated that Fe-¹³C complexes significantly stimulated CO₂ emissions from both the input ¹³C and SOC compared with glucose alone. In contrast, the addition of Fh- and Goe-C complexes consistently inhibited CH₄ emissions by 72-91 % and 21-61 % compared with glucose addition, respectively. Fe-OC complexes reduced the CO₂ equivalent by 62-71 % and 17-41 % in soils with Fh-C and Goe-C complexes, respectively. We concluded that Fe crystallinity and its bonding forms with organic carbon jointly control SOC mineralization. The coprecipitated Goe-C complexes had the lowest OC mineralization rate and highest OC residence time among four Fe-OC complexes. These findings highlighted that promoting the formation of coprecipitated well-ordered minerals would increase SOC sequestration by reducing OC mineralization and mitigating the global warming effect in paddy management.