Nitrogen flows in grazed pastures. I. Balances on cultivation and nitrate leaching under pastures.

摘要

After a general presentation of the importance of nitrogen flows both at the whole-farm scale and on the scale of separate fields, the main experimental results obtained in Western France regarding the leaching of nitrogen from grazed pastures are synthesized. These studies show the diversity of nitrogen flows, the complexity of the nitrogen cycle in those systems, and the management difficulties encountered for the control of nitrate leaching. The control of nitrogen fertilization appears to be only one aspect of the problem, as other flows such as animal feed complementation, stocking rate, sward type, etc. have also to be taken into account. A synthetic parameter such as stocking rate (expressed as number of grazing days per hectare per year) seems to be a good indicator of nitrate leaching. Below 550 grazing days/ha per year, the nitrate concentration in leached water is below the limit (50 mg/litre) fixed for suitability for drinking. Above 550 grazing days/ha per year, there is a rapid increase in nitrate concentration. Several practices to reduce nitrate leaching are described.