Nitrate leaching from dairy farming on sandy soils : case studies for experimental farm De Marke

The main problem with nitrogen (N) at dairy farms on sandy soils in the Netherlands is the leaching of nitrate to the groundwater. Experimental farm De Marke was set up in 1991 on poor sandy soils with the objective to develop a prototype of an economically feasible farming system with acceptable nutrient losses. For the quantification of the inputs and outputs of the De Marke farming system, measurements have been carried out on subsystems of the farm. This thesis uses the data collected for assessing the performance of the crop-soil-nitrogen subsystem.Monitoring of soil moisture conditions and nitrate concentrations was carried out during the years 1991-1995 at six experimental sites. With these data, studies were carried out on nitrate leaching affected by land use in relation to the occurring soils at the farm and the groundwater regimes, using simulation models. The use of simulation models can only be valid when the right input data are used. Soil physical characteristics are important input parameters and for the monitoring sites a comparison was made using soil physical characteristics from either laboratory measurements or from the Staring series as input. It was found that simulation results were not significantly different, implying that the Staring series could be used in studies like these for simulating the unsaturated water flow regime in sandy soils.Cattle grazing at the experimental farm was reduced to eight hours per day, but urine-affected areas had great influence on the nitrate measurements. At one 'wet' site the probability of exceeding the EC-directive for drinking water (11.3 mg/l nitrate-N) under a urination deposited in either July or September was respectively 10 and 25%. At the dry site the directive will be exceeded under any urine patch in almost 100% of the years, affecting the field average concentration. In this field careful grazing management would result in less nitrate leaching, but the environmental goals would not be reached.A precision agriculture technique which allows adapting fertilization to urine-affected areas resulted in considerable reductions of simulated nitrate concentrations. A rise of the water table usually also resulted in a decrease in simulated nitrate concentrations. The combined effect of non-fertilization of urine patches and the raising of groundwater levels usually resulted in higher simulated reductions of nitrate concentrations than the single options.Supplementary irrigation management options for grazed grassland were selected. A change in application volume from 25 to 15 mm per irrigation event resulted in higher irrigation efficiencies, lower annual water use and only small changes in the transpiration ratio T a /T p . The different irrigation strategies had no significant effect on nitrate concentrations of the two dry fields studied (fields 9 and 11). For the evaluation of environmental effects it was advised to assess the actual nitrate concentrations and not only the water fluxes, which potentially cause solute leaching (i.e. the leaching potential).For the upscaling of the data collected at six sites within the farm to whole farm level, the information of the soil survey of De Marke was used. The probability of exceeding the threshold value of 11.3 mg/l nitrate-N (EC-directive) during the period of summer 1991 - spring 1995 was 63% for the whole farm with marked differences between years, crops and hydrological conditions. Considering temporal variability due to weather conditions, the average simulated nitrate-N concentration at a depth of 1 m for the whole farm was 15.1 mg/l and the probability of exceeding the EC-directive for drinking water (11.3 mg/l) at the same depth was 67%. It would be easier to meet both the environmental and agricultural goals of De Marke on most other sandy soils than those of De Marke itself. Allocation of land use with the highest nitrate leaching risk to the least vulnerable soils within one farm and vice versa could well reduce the farm average nitrate losses to the groundwater.