Role of clay minerals on soil organic matter stabilization and humification

The hypothesis that Maillard reactions contribute to the formation of new soil organic matter (SOM) was investigated. Specific objectives of the research were to determine whether smectites (type of smectite and their associated saturating metal cations) and synthetic Al-substituted goethites catalyze the Maillard reaction leading to the formation of humic-like compounds under abiotic conditions and to determine the distribution of newly formed humic materials into mineralogical distinct clay-size fractions. To investigate clay catalysis of the Maillard reaction, four smectites and four synthetic Al-substituted goethites were incubated with arginine (or glycine peptides) + glucose solutions for 21 days at 37°C under abiotic conditions. The results indicate that smectites and to a lesser extent goethites were capable of catalyzing dehydration of glucose to form furfural compounds and levulinic acid. Glucose dehydration was affected by the type of saturating cation on the clay; Al3+ enhanced monosaccharide dehydration whereas Fe3+ inhibited monosaccharide dehydration. In goethites + peptide + glucose systems, as Al substitution on goethites increased, cleavage of the peptide bond increased. The transformation products in the systems (sorbed C + N) are believed to be co-polymerization products of furfural compounds and levulinic acid and amino containing compounds. To determine the relationship between clay mineralogy and the distribution of newly formed humic materials, a Monona soil was fractionated into mineralogical distinct fractions. The Monona soil had been previously labeled with 14C through the decomposition of labeled Oat roots. The total organic C and total N content in the fine clay (dominated by interstratified smectite/illite) and coarse clay (dominated by quartz) fractions were similar. However, the new 14C preferentially accumulated in the fine clay fraction relative to the coarse clay fraction. Scanning electron micrographs reveal diffuse humic materials on the surfaces smectites in the fine clay fractions, suggesting that new humic materials are preferentially forming or accumulating on soil smectite surfaces. This research demonstrates that Maillard reactions may be a major pathway for the incorporation of new C and N into SOM and for the formation of new humic substances.