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 dynamics of labelled and total (labelled + unlabelled) C remaining in the soil, in the microbial biomass and in the plant residue, were monitored throughout the experiment. In order to fit these results simultaneously, a model was defined including five compartments with functioning concepts according to earlier proposals and with a relatively simplified mathematical presentation among those used to describe the soil C cycle. 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 + unlabelled). The two fittings reveal the presence of a stable form of carbon 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, 1977). Mineralization and humification kinetics were different in the two types of soils. These differences are expressed by model parameters and 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 endogenous variables in kinetic equations.