Adsorptive fractionation of dissolved organic matter (DOM) by mineral soil: Macroscale approach and molecular insight

Adsorption of dissolved organic matter (DOM) to mineral surfaces is an important process determining DOM bioavailability and carbon sequestration in soils. However, little is known about preferential adsorption of DOM at the molecular level. In this study, DOM originating from composted biosolids was analyzed in order to elucidate DOM adsorptive fractionation by clay soil. Structural changes in DOM due to adsorption to soil were studied using two complementary approaches: (i) macroscale analysis including resin separation and (ii) molecular characterization using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Both approaches demonstrated consistency regarding the DOM adsorptive fractionation. Resin separation showed preferential adsorption of the hydrophobic acid (HoA) fraction by soil surfaces, with up to 70% of total adsorbed carbon; this fraction was apparently responsible for low DOM desorption. FT-ICR MS data demonstrated preferential adsorption of polyphenols, which are components of the HoA fraction. Adsorption of highly oxidized, saturated “carbohydrate-like” molecules was also observed, which might be a result of adsorption of the hydrophilic neutral (HiN) fraction. DOM exhibited concentration-dependent fractionation: enhanced adsorption of highly oxidized compounds at low DOM concentrations, and selective adsorption of less oxidized components at higher DOM concentrations, suggesting that adsorptive fractionation of DOM depended on the extent of its loading. Our findings suggest that a significant amount of carbon originating from the applied DOM was irreversibly stabilized by mineral surfaces. The study demonstrates that both DOM chemical heterogeneity and DOM concentration need to be considered in order to predict DOM reactivity and carbon stabilization in soils.