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 me dium-term (13 years) evolution of C reserves in the silty soils of the roll ing pampas in Argentina for various crop rotations. The coefficient of annu al 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 frac tion and stable fraction) is proposed. It enables the changes in the soil o rganic reserves to be well simulated. However, a simple statistical fit of the three parameters of the model gives an infinite number of solutions. Us e of the C-13 natural abundance method allows evolution of the young and ol d soil carbon fractions to be separated and tracked, and to determine the t hree parameters unambiguously. The results show that the stable fraction re presents most of the initial soil organic C reserve: 60-68 %. The humificat ion parameter increases in proportion to the lignin concentration of the cr op residues. Intensification of soil tillage accelerates the mineralisation of the soil organic matter. ((C) Inra/Elsevier, Paris.)