Buffer zones as a sink for sediment and phosphorus between the field and stream: Danish field experiences

Rill erosion, surface runoff and storage of sediment and phosphorus in buffer zones were investigated during three winters on ca. 140 arable field slope units in twenty places in Denmark covering all landscape types, climate gradients and dominant soil types. The dominant soils are Alfisols and Spodosols, with textural composition typically ranging from sand to loam. The average slope is 7% (range: 2-20%) and median buffer zone width was 8.3 m (range: 0.6-125 m). The geometric mean annual rill erosion was 0.33 m3 ha(-1) equalling to 495 kg sediment ha(-1) and 0.25 kg P ha(-1). The deposition of sediment on the field, in the buffer zone and delivery of soil to the stream was surveyed within the 140 slope units following three winters. Deposition of sediment in the buffer zone was observed in 31% (1997/98), 31% (1998/99) and 29% (1999/2000) of the slope units. Delivery of soil across the edge of the stream was observed in 23% (1997/98), 17% (1998/99) and 25% (1999/2000) of the 140 slope units. Median dissolved P concentration in surface runoff was 0.18 mg Pl(-1) (range: 0.029-16.294 mg Pl(-1)). A probability model was developed to predict the efficiency of different widths of buffer zones.