Evaluation of the effects of agricultural practices on nitrogen fluxes at the scale of an intensive husbandry watershed

Abstract

One of the reasons for the deterioration in the water quality in Brittany is the effluent discharged from stock breeding farms. To optimise the management of this waste, methods for analysing and assessing agricultural practices were drawn up and applied in a 6 km2 catchment area in North Finistère with intensive stock breeding (beef and pigs) with levels exceeding the limits. The aims were to quantify the levels of nutrients (N, P) and to propose and assess changes in farming practices that would reduce these levels, in particular in water. The 12 pig units studied revealed very marked differences in breeding performance, feeding practices and therefore in nitrogen and phosphorus levels in the effluent. The introduction of feeding practices that were less damaging to the environment, such as two phase feeding for pig farms, would considerably reduce nitrogen and phosphorus levels in effluent. However the scope for improvement varies considerably depending on the farm. Levels in cattle farms were also excessively high and considerable improvements could be made in particular by better pasture management. The annual excess nitrogen level, calculated from individual plots, was 210 kg/ha. It was found in the soil in autumn and for the whole winter. Better fertilisation management could be expected to reduce the nitrogen level by 25%. There was very little room for improving the phosphorus level as the soil already had a high phosphorus content. Nitrogen losses, estimated using the NBVI model from soil measurements, were higher for annual crops than for pasture. We were able to link nitrogen levels in the soil in this basin with output flows using a relatively simplified model. The excess nitrogen was reduced by 40% in the outflow. This considerable reduction applied to agricultural ground (denitrification/metabolisation) and also underground water (about 18% of the excess), probably in the wetlands, which cover 23% of the area of the basin. However, upstream as well as downstream, the current structure of the basin is not conducive to their action as there is a dense drainage network and deep ditches which channel the water that is most heavily loaded with nitrates coming from the unsaturated zone.