Modelling soil carbon dynamics with various cropping sequences on the rolling pampas

The two-compartment model of Henin-Dupuis (1945) was tested to track the medium-term (13 years) evolution of C reserves in the silty soils of the rolling pampas in Argentina for various crop rotations. The coefficient of annual mineralisation, theoretically constant in the model, depends in fact on the amount of organic residues returned. The model is thus open to question. A simple model with three carbon fractions (harvest residues, active fraction and stable fraction) is proposed. It enables the changes in the soil organic reserves to be well simulated. However, a simple statistical fit of the three parameters of the model gives an infinite number of solutions. Use of the C-13 natural abundance method allows evolution of the young and old soil carbon fractions to be separated and tracked, and to determine the three parameters unambiguously. The results show that the stable fraction represents most of the initial soil organic C reserve: 60-68 %. The humification parameter increases in proportion to the lignin concentration of the crop residues. Intensification of soil tillage accelerates the mineralisation of the soil organic matter. ((C) Inra/Elsevier, Paris.)