Transformation of organic matter from maize residues into labile and humicfractions of three European soils as revealed by C-13 distribution and CPMAS-NMR spectra

The dynamics of incorporation of fresh organic residues into the various fr actions of soil organic matter have yet to be clarified in terms of chemica l structures and mechanisms involved. We studied by C-13-dilution analysis and CPMAS-C-13-NMR spectroscopy the distribution of organic carbon from mix ed or mulched maize residues into specific defined fractions such as carboh ydrates and humic fractions isolated by selective extractants in a year-lon g incubation of three European soils. The contents of carbohydrates in soil particle size fractions and relative delta C-13 values showed no retention of carbohydrates from maize but rather decomposition of those from native organic matter in the soil. By contrast, CPMAS-C-13-NMR spectra of humic (H A) and fulvic acids (FA) extracted by alkaline solution generally indicated the transfer of maize C (mostly carbohydrates and peptides) into humic mat erials, whereas spectra of organic matter extracted with an acetone solutio n (HE) indicated solubilization of an aliphatic-rich, hydrophobic fraction that seemed not to contain any C from maize. The abundance of C-13 showed t hat all humic fractions behaved as a sink for C from maize residues but the FA fraction was related to the turnover of fresh organic matter more than the HA. Removal of hydrophobic components from incubated soils by acetone s olution allowed a subsequent extraction of HA and, especially, FA still con taining much C from maize. The combination of isotopic measurements and NMR spectra indicated that while hydrophilic compounds from maize were retaine d in HA and FA, hydrophobic components in the HE fraction had chemical feat ures similar to those of humin. Our results show that the organic compounds released in soils by mineralization of fresh plant residues are stored mai nly in the hydrophilic fraction of humic substances which are, in turn, sta bilized against microbial degradation by the most hydrophobic humic matter. Our findings suggest that native soil humic substances contribute to the a ccumulation of new organic matter in soils.