Incorporation of the degradation products of 14C, 15N-glycine in various forms of organic carbon and nitrogen in two acid soils

Abstract

The incorporation of the degradation products of 14C, 15N-glycine in the endogenous organic matter of two Cambisols, one developed over acid schists and the other over gabbros, was studied by aerobic incubation for 42 days at 28°C, of unenriched samples and samples enriched with glycine, and analysis of the C and N organic forms in the samples. The incorporation of exogenous C depended on soil type and on the nature of the added substrate. Thus, the addition of glycine increased the proportion of water-soluble C, stimulated microbial oxidation of unhumified organic matter and favoured the synthesis of early humification compounds at the expense of unmineralized and unhumified organic matter; and also favoured the formation of organo-clay compounds (possibly due to adsorption of the glycine or its metabolites on clay) and (where stabilizing agents were present) of insoluble humin. In both soils, the distribution of endogenous N was unaffected by incubation. 15N from glycine was incorporated mainly among identifiable forms (NH + 4, amides, hexosamines and α-amino acids) in the schist soil, which may possibly be related to ammonification being the dominant mineralization process. In the gabbro soil, 15N from glycine was incorporated mainly in hydrolyzable unidentified forms which is probably related to the dominance of nitrification in the mineralization process. The virtual absence of isotoplc excess in the hydrolysis residue and the large isotoplc excesses in the first two hydrolysis stages (the shortest and gentlest) showed that added N was considerably more labile than endogenous N. The step-wise hydrolysis procedure used extracted the more labile soil N much more selectively than does conventional continuous hydrolysis.