Iron plays an important role in molecular fractionation of dissolved organic matter at soil-water interface

Adsorption of dissolved organic matter (DOM) onto soils plays an important role in the mobility and stabilization of organic carbon in soils; however, little attention has been paid to changes in the molecular components of soil DOM during adsorption on soils. In the present study, molecular fractionation of DOM induced by adsorption on a red soil was investigated using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The results indicated that compounds high in unsaturation or polarity or rich in oxygen had a high affinity to soil surfaces, while aliphatic compounds with few oxygenated groups and low polarity compounds were preferentially retained in solution. Among soil fractions with different particle sizes, the fine clay fraction with high iron content and surface area was the main contributor to the adsorptive fractionation of DOM. Comparison of the molecular fractionation of DOM derived from adsorption on soil with iron removed and on soil minerals with various iron contents and surface areas further indicated that iron containing minerals in the soil provided the major adsorptive sites and determined the molecular fractionation of DOM at the soil-water interface. The results provide molecular information for further understanding mechanisms underlying the persistence and mobility of organic carbon in soils.