Recent trends and patterns of nutrient concentrations in small agricultural streams in Sweden.

Abstract

Long-term median total phosphorus (TotP) and total nitrogen (TotN) concentrations were 0.08 and 5.5 P/N (mg L(-1)), respectively, in 10 agricultural streams monitored since 1988 in Sweden. The areas of the respective catchments are 2-20 km2. The period 1992/2002 was characterised by stable hydrological conditions without any flow trends in nearly all of the streams. The highest average TotP concentration, 0.17 mg P L(-1), was found in a small agricultural stream in the largest Swedish agricultural plain. The soil texture is here characterised by a large specific surface area of the soil particles, and the agriculture by cereal production. The second highest average TotP concentration, 0.14 mg P L(-1), was measured in the surface water from a catchment characterised partly by clay soils and by production of potato, spring cereals and grass. This catchment had twice as many fields with a calculated high risk for P losses compared with another monitored catchment in the same watershed (River Rönneå). There was a significant downward TotP trend during 1992/2002 of 0.0012 mg P L(-1) yr(-1) (Sen's slope estimator) in the catchment where many fields risk P losses and which had a reduced P manure application rate of -20% during 1995/2000. In recent years practically no manure has been spread during autumn. Bypass flow of nitrate through one soil has been suggested to influence the LOWESS (LOcally WEighted Scatterplot Smoothing) fitting curve of TotN. Total nitrogen concentration decreased in most of the catchment. The average downward slope was similar to a general TotN reduction of 0.069 mg N L(-1) yr(-1). During the period 1992/2002 this was equal to slightly more than 10 per cent. Cultivation of catch crops was relatively uncommon until 2002, but this practice is expected to expand to larger areas during 2003 and in the future.