Interfacial Molecular Fractionation Induces Preferential Protection of Biorefractory Organic Matter by Ferrihydrite

Abstract

Minerals are known to protect dissolved organic matter (DOM) against biodegradation and thus play an important role in the long-term persistence of organic carbon (OC) in soils. However, as a super-mixture of organic compounds, it is expected that not all components in DOM can be protected by minerals due to selective adsorption. The present study examined how molecular fractionation of plant-derived DOM at ferrihydrite–water interface affects their biodegradation. The results of Fourier transform ion cyclotron resonance mass spectrometry analysis showed that highly unsaturated and oxygenated moieties in DOM displayed high affinities, while highly saturated compounds rich in aliphatic groups exhibited low affinities to ferrihydrite, resulting in the molecular fractionation of DOM at the water–mineral interface. Comparison of the biodegradation of DOM extracted from different phases after adsorption on ferrihydrite indicated that compounds with lower affinities to ferrihydrite had higher degradation degrees, while compounds with higher affinities to ferrihydrite had lower degradation degrees. Considering the protective role of minerals on adsorbed OC, molecular fractionation at the ferrihydrite–water interface leads to inherently refractory OC being more stable, while labile OC is more easily degraded in soils.