Extensive grasslands as an effective measure for nitrate and phosphate reduction from highly polluted subsurface flow – Case studies from Central Poland

•The effect of nitrogen saturation was not observed despite of high (above 100 mg NO3/L), chronic nitrate inputs.•High (above 1.5 mg PO4/L) phosphate concentrations were reduced by 81% and 76% passing through 45 m and 47 m grassland, respectively.•Biodiversity (expressed as the Shannon Index) increased the efficiency of buffer zones.