The studies where carried out on two permanent grasslands located in France, in the Central Pyrenees (Ercé) and in the Massif-Central (Gramond). Our results show that there is an inverse relationship between the total biomass and a synthetic indicator of the water regime, namely, the number of decades with a reserve RFU=0 on the horizon 0-5 cm. easy to use. The results also revealed a marked reduction of the nitrogen nutrition index (IN) when the mineral balance becomes negative. This result suggests that the IN is able to account for the depletion of the level of reserves in nitrogen as a result of changing balances accumulated over time. However, the question arises to what extent this development does not depend on water conditions that have prevailed over the last 2 years (2003 and 2004)?