Stability of iron-carbon complexes determines carbon sequestration efficiency in iron-rich soils

Abstract

The role of iron minerals in soil organic carbon (SOC) stabilization has attracted considerable attention. However, the turnover of Fe-bound organic carbon (Fe–OC) complexes and their impact on the other organic C fractions in natural soils remain unclear, hindering accurate assessments of their C sequestration potential. To address this gap, we prepared 2- and 6-line ferrihydrite-bound ¹³C-glucose (2LFh-Glc and 6LFh-Glc, respectively) with five C loading levels, using free ¹³C-glucose as control. Our aim was to trace and quantify glucose mineralization, SOC priming effects, and net C balance in anaerobic Fe-rich paddy soils. Fh-Glc mineralization and its SOC priming were lower than those of free glucose. Mineralization and SOC priming of 6LFh-Glc were 29% and 67% lower, respectively, compared to 2LFh-Glc. This was attributed to the stronger protective ability of 6LFh for organic C, which limits glucose release and microbial accessibility, thereby inhibiting SOC mineralization. 6LFh-Glc showed 51% higher C sequestration efficiency than 2LFh-Glc (i.e., net soil C balance produced by per unit of C loading). Notably, C sequestration efficiency decreased with increasing C loading. In conclusion, both stability and C loading of Fe-OC complexes are key determinants of C sequestration efficiency in Fe-rich paddy soils, highlighting the importance of Fe-organic C associations in soil C sequestration.