Soil properties, Hydrology, Farm Management and Weather Conditions Determine the Variability in Nitrate Leaching on Grassland-based Dairy Farms.

Intensive dairy farming in Western Europe is associated with high nitrate leaching to aquatic systems. In the project ‘Cows & Opportunities’, farm management strategies are developed to reduce nitrate leaching on 16 commercial pilot dairy farms on different soil types in the Netherlands. To evaluate these management strategies, N flows and nitrate concentrations were regularly monitored. For a sound evaluation, insight is required in the interactive effects of farm management and environmental factors on nitrate leaching. The objective of this paper is to analyze the effects of  farm management, soil properties, hydrology and weather on nitrate concentrations in the upper groundwater on sandy soils. Data from 9 dairy farms were analyzed at three scales: whole farm, crop and sampling point. For crop and sampling point we distinguished two crops: grassland and silage maize. Regression models were used to identify the factors affecting nitrate concentration in groundwater. At farm level, nitrate concentration was best described by (unit and parameter estimate in brackets): N surplus of the farm balance (kg ha^-1; 0.18), water soluble organic matter in groundwater (mg l^-1; -0.51), fraction grassland within the farm (unitless; -45) and grazing intensity (hours ha^-1; 0.09). Nitrate concentration under grassland, was best described by a similar model. Nitrate concentration under maize was best described by total N input to the soil (kg ha^-1; 0.12), mean highest groundwater level (cm below soil surface; 0.27), depth of presence of soil organic matter (cm below soil surface; 0.20), C:N ratio in the top soil (unitless; -5.4) and cumulative precipitation between November 1 and February 28 (mm; -0.2). Based on this analysis, we can quantify the effects of restricting grazing, crop rotation, management of catch crops, plowing up grassland, and fertilizer management (amount and timing) on nitrate concentrations in the upper groundwater.