Reduced nitrate concentrations in shallow ground water under a non-fertilised grass buffer strip

In this paper the suitability of a buffer strip to reduce nitrate concentrations in the upper groundwater was tested for a sandy arable soil in The Netherlands during two consecutive leaching seasons. The bufferstrip was a 3.5 m wide unfertilised grass strip adjacent to a ditch on an arable field. In total 24 groundwater wells were installed in 4 transects perpendicular to the ditch to determine Cl, NO₃ and δ¹⁵N concentrations. Piezometers were installed to assess the groundwater flow, which was in the direction of the ditch with small downward leakage across a peat layer at about 3 m depth. Nitrogen was dominantly present as nitrate (NO₃). The NO₃-N concentrations under the bufferstrip were significantly lower than under the adjacent arable field. The lower concentrations were due to dilution, uptake by grass and denitrification. Nitrate was actively removed in the bufferstrip, since the Cl/NO₃ ratios were higher in the bufferstrip than in the remainder of the field. Furthermore, δ¹⁵N data indicated that denitrification occurred in the groundwater and increased with decreasing distance to the ditch. NO₃-N loads to the ditch were estimated at 8.5 kg ha-¹yr-¹, which is relatively low for this area. We can, however, not determine whether these relatively low NO₃-N loads were causally related to the reduced NO₃-N concentrations in the bufferstrip. Nevertheless, the results of the present study are promising and justify additional research on the efficiency of bufferstrips to reduce NO₃ concentrations in shallow groundwater, and subsequently reduce NO₃ loading of surface water, under Dutch conditions.