Grasslands play a part in the regulation of the bio-geochemical cycles, especially through the accumulation of organic matter in the soils, the locking-up of atmospheric carbon, and the lowering of the risk of polluting the water systems by nitrates. A better knowledge of the functions and the flows of organic matter under grasslands is essential for the strengthening of their environmental role. The various types of anthropic management applied to grasslands affect the dynamics of soil organic matter (SOM ; MOS in the french text) and therefore also the biogeochemical cycles of C and N, here described. In contrast to the losses from SOM, the C losses begin to be well understood and modelized. The flows from SOM are characterized by a great inertia, so that the use of tools such as the 'Observatoires de Recherche en Environnement' (Environmental Research Observatories) becomes indispensable ; one instance, that of Lusignan, is presented. These tools make it possible to study the extent and speed of renovation of SOM in the soil/plant system under various types of soil utilization and in the present context of climatic change. In this way, the knowledge of the N cycle and particularly that of nitrate leaching can be improved.