MINERALIZATION OF A NITROGEN-BEARING ORGANIC SUBSTRATE MODEL C-14, N-15-GLYCINE IN 2 ACID SOILS

The kinetics of the mineralization of organic matter was studied in two Cambisols, one developed over acid schists and the other over gabbros, by aerobic incubation, for 42 days at 28-degrees-C, of unenriched samples and samples enriched with the model substrate C-14, N-15-glycine. The schist soil proved to have considerable capacity for mineralization of both endogenous and exogenous C and N, whereas the mineralization capacity of the gabbro soil depended on the source of substrate, being similar to that of the schist soil for the C and N of recently added labile glycine but only between one-third and one-half that of the schist soil for endogenous C and N. In the schist soil N mineralization chiefly involved ammonification; in the gabbro soil nitrification was the dominant process. The response to addition of glycine, which was rapid, intense and brief in the schist soil, was slower, less intense and longer-lasting in the gabbro soil, possibly due to the latter's high silt and Al gel contents making the added substrate temporarily unavailable to micro-organisms. It may be concluded from the data obtained that the mineralization of endogenous C and N in the gabbro soil is chiefly limited by the scarcity of available labile C, which is used by heterotrophic micro-organisms as a readily-assimilable source of energy.