Adsorptive fractionation of corn, wheat, and soybean crop residue derived water-extractable organic matter on iron (oxy)hydroxide

Plant biomass is the primary source material for the formation of soil organic matter, which comprises the largest terrestrial pool of the global C cycle. Adsorption of water-extractable organic matter (WEOM) to soil mineral surfaces is a critical step in the process of organic matter accumulation. In this study, we examined the molecular fractionation of WEOM derived from field-grown corn, wheat, and soybean crop residues upon adsorption to iron (oxy)hydroxide (FeOOH) mineral using ultrahigh resolution mass spectrometry. The results show that aromatic, N-containing aliphatic as well as lignin-like molecules with higher O/C atomic ratios have preferential affinity for FeOOH surfaces. Lignin-like molecules with low and high numbers of O atoms were adsorbed, while those with intermediate O numbers were not adsorbed. This pattern is likely due to two different mechanisms of adsorption that is dependent on molecular size: smaller molecules with low O numbers bond through an inner-sphere ligand exchange mechanism, and the larger molecules with high O numbers bond through the formation of multiple H-bonds between the WEOM and FeOOH surface functional groups. Adsorption of WEOM to soil mineral surfaces has wide ecosystem implications, since adsorbed organic matter molecules are now believed to be more protected from microbial decomposition reactions. This study shows that WEOM chemical composition is an important factor controlling its adsorption to mineral surfaces. Understanding these soil and crop chemical interactions at the molecular level will be increasingly important for developing production systems that maintain high SOM levels and soil health in the decades ahead.