Effectiveness of Unfertilized Buff er Strips for Reducing Nitrogen Loads from Agricultural Lowland to Surface Waters

Unfertilized buff er strips (BS) are widely accepted to reduce nitrogen (N) loads from agricultural land to surface water. However, the relative reduction of N load or concentration (BS eff ectiveness, BSE), varies with management and local conditions, especially hydrogeology. We present novel experimental evidence on BSE for 5-m-wide grass BS on intensively drained and managed plain agricultural lowland with varying hydrogeology. We selected characteristic sites for fi ve major hydrogeological classes of the Netherlands and installed paired 5-m-wide unfertilized grass (BS) and reference (REF) treatments along the ditch. Th e REF was managed like the adjacent fi eld, and BS was only harvested. Treatments were equipped with reservoirs in the ditch to collect and measure discharge and fl ow proportional N concentration for 3 or 4 yr. In addition, N concentration in upper groundwater was measured. We found a statistically signifi cant BSE of 10% on the peat site. At the other sites, BSE for N was low and statistically insignifi cant. Low BSE was explained by denitrifi cation between adjacent fi eld and ditch, as well as by the site-specifi c hydrologic factors including low proportion of shallow groundwater fl ow, downward seepage, low residence time in the BS, and surface runoff away from the ditch. We emphasize that a REF treatment is needed to evaluate BSE in agriculture and recommend reservoirs if drainage patterns are unknown. Introduction of a 5-m-wide BS is ineff ective for mitigating N loads from lowland agriculture to surface waters. We expect more from BS specifi cally designed to abate surface runoff