MODELING OF SOIL CARBON FORMS AFTER ORGANIC AMENDMENT UNDER CONTROLLED CONDITIONS

Two different soils were amended with C-14-labelled plant material and incubated under controlled laboratory conditions for 2 yr. The dynami cs of labelled and total (labelled + unlabelled) C remaining in the so il, in the microbial biomass and in the plant residue, were monitored throughout the experiment. In order to fit these results simultaneousl y, a model was defined including five compartments with functioning co ncepts according to earlier proposals and with a relatively simplified mathematical presentation among those used to describe the soil C cyc le. The simultaneous fitting of microbial, plant and total labelled C appears satisfactory in the two soils, with a plausible simulation of the humification process-This model, focusing on the labelled C (added form), has allowed to fit the evolution of soil total C (labelled + u nlabelled). The two fittings reveal the presence of a stable form of c arbon with a half-life longer than that stabilized since the addition of plant material, but shorter than the 'chemically stabilized organic matter' named by Jenkinson and Rayner (Soil Science 123, 298-303, 197 7). Mineralization and humification kinetics were different in the two types of soils. These differences are expressed by model parameters a nd discussed with the presentation of results. In this way, hypothesis were derived in agreement with the soil mineral status and the soil r -arbon forms. Nevertheless, complementary investigations are necessary to verify these hypotheses and perhaps take into account newly endoge nous variables in kinetic equations.