The implications of Fe speciation for the humic substance stability of ternary Fe(III)-montmorillonite-humic substance systems

Mineral-organic matter (OM) associations play an important role in determining the long-term retention of OM in soils. However, the retention mechanisms of OM in cation-mineral-OM systems remain unclear. Taking into account the dominance of montmorillonite (Mnt) in the soil of the temperate zone, we investigated the stability of humic substances (HSs) in the Fe(III)-Mnt-HS system using thermal analysis. The HS degradation started at similar to 387 degrees C in the Fe(III)-Mnt-HS system, which was higher than that of the Fe(III)-HS system (290 degrees C). The formed ferrihydrite (Fhy) mainly contributed to the enhanced labile OM retention through adsorption and/or co-precipitation, whereas Mnt inhibited the initial formation and subsequent transformation of Fhy, thus improving the stability of OM. These results suggest that the HS stability in Fe(III)-clay-HS systems depends on the Fe speciation affected by clay minerals, and this finding provides insights into OM-mineral interactions in temperate-zone soils.