Nitrogen fluxes in the plant component of the 'De Marke' farming system, related to groundwater nitrate content

Abstract

Sandy areas in the Netherlands are mainly used for intensive dairy farming. In conventional farming systems, losses of nitrate to the groundwater are high, threatening other valuable functions of the rural area, like conservation of nature and 'production' of drinking water. Possibilities were examined to improve farm management in such a way that the desired upper groundwater quality could be realized without reducing milk production intensity (11,900 kg milk ha-1) and without exporting manure. A prototype system was established on an experimental farm. Nitrogen fluxes, including mineralization, were studied in detail. Actual nitrogen surplus of the plant component of that system decreased from 337 kg ha-1 (conventional system) to 128 kg ha-1, which still is 49 kg ha-1 higher than estimated in the design phase. Of this 128-kg surplus probably about 40 kg accumulated as organic N in the root zone, 29 kg denitrified and 59 kg was leached as nitrate. Nitrate content of the upper groundwater decreased from 220 to 55 mg 1-1, which is close to the target value of 50 mg 1-1. Because accumulation is a finite process and groundwater nitrate content is still too high, the N surplus has to be further reduced by at least 40 kg ha-1. Calculations suggest scope for such a reduction.