CHARACTERIZATION OF HUMIC SUBSTANCES ISOLATED FROM CLAY- AND SILT-SIZED FRACTIONS OF A CORN RESIDUE-AMENDED AGRICULTURAL SOIL1

Abstract

In a small-plot field study on a Waukegan silt loam soil, annual applications of 20 g N m−2 were made each May for 8 years before planting corn (Zea mays L.). Subplots were fertilized with 0.8 g 15N m−2. Soil treatment in the fall either incorporated the chopped corn stover after grain harvest, using a rototiller, or the stover was removed from the plots. Soil samples taken in the fall were ultrasonicated, separated into clay- and silt-sized fractions, and extracted exhaustively with 0.1 mol L−1 sodium pyrophosphate (Na4P2O7) + 0.1 mol L−1 NaOH (pH 12.6). Humic (HA) and fulvic (FA) acids were isolated using the International Humic Substances Society (IHSS) procedures. A variety of analytical methods were employed. The most useful information was obtained from amino acid (AA) and neutral sugar (NS) analyses, and from cross polarization magic angle spinning (CPMAS) 13C-NMR and δ13C data. Overall, the corn residue amendments did not have a large effect on the composition of the humic substances (HS) from the different sized separates, but there were differences in the relative abundance of some AA and NS in the HAs and FAs. The NMR and δ13C data provided evidence of some compositional differences and extent of humification between the HS from the clay- and silt-sized separates. The conclusion reached is, therefore, that the silt-sized particles were microaggregates of clay-sized particles, and the HS in these microaggregates were partially protected from bioalteration. These HS bore greater resemblance to the plants of origin than did those associated with the clays. The composition of the HAs and that of the FAs were similar to that of the Mollisol soil standard of the IHSS, but they were different from humic samples from other non-Mollisol soil types.