Prairies et qualité de l'eau. Evaluation des risques de lixiviation d’azote et optimisation des pratiques

Pastures and the quality of water. Evaluation of the risks of nitrate leaching and optimisation of practices

The increases in the nitrate contents of water in most regions with intensive animal rearing, together with the European regulations which have now been running for five years, and which are part of the 'Nitrate' direction and later of the general direction on water, have given rise to many studies for the evaluation and forecast of the risks of nitrogen losses, especially in the case of pastures of intensive dairy systems. A synthesis of these studies is proposed here.
The nitrogen cycle is particularly complex on dairy farms, due to the multiplicity of the parts, the diversity and importance of the flows involved, the effects of re-cycling, and the interactions among many factors, themselves diverse and variable : fertilisation and management practices, sward type, special features of the soil and the climate, impacts of the grazing animals, etc. Thanks to studies made in experimental stations in France and elsewhere in Europe, it has been possible to evaluate the amounts and variability of the nitrogen losses through leaching, and to define the crucial explanatory factors, and also to propose some indicators and decision - making aids for an improved efficiency of nitrogen use and for minimizing the leaching losses. The modelling of the processes involved in the nitrogen cycle of the pastures has thus made progress. Studies at the field level are integrating little by little a multi-annual approach, so as to be comprehensive of the perenniality of the sward and of the effect of cash crop - ley successions on the risks of losses.