Effect of grass buffer strips on nitrate export from a tile-drained field site

Vegetated buffer strips may reduce nutrient inputs from agricultural land into surface waters. Their effectiveness at tile-drained fields, though, remains unclear. The objective of this study was to quantify nitrate reduction in the groundwater underneath a buffer strip, to evaluate the effect of buffer strip width, and to assess their impact on nitrate loading in a drainage ditch. The study site was a tile-drained lowland field on glacial till in north-eastern Germany. The investigated grass buffer strips were 7, 3 and 1 m wide. Water levels and nitrate concentrations in 29 monitoring wells and in the adjacent ditch were measured during three winter half-years. Groundwater nitrate loads were calculated based on a simple Darcy approach. Initial nitrate concentrations in the shallow groundwater entering the buffer strip were up to 98 mg L-1, with median values ranging between 2 and 36 mg L-1. Within the buffer strip, these concentrations decreased by 56-98 %. We assume that this reduction was caused by denitrification processes in two study years and dilution after snowmelt in the third year. The width of the buffer strip did not have any influence on the nitrate reduction. Presumably, site characteristics and the hydraulic conductivity are of greater importance. The groundwater nitrate load was reduced by the buffer strip, but the contribution of the groundwater to total ditch nitrate load was minor. We conclude that possible positive effects of buffer strips on groundwater quality do not ameliorate surface water quality at tile-drained field sites.