Variation of runoff and nitrogen leaching in a small agricultural catchment in southern Finland

Long term monitoring data from 1981–2000 was used to study nitrogen (N) losses from a small (15.4 km²) agricultural catchment in southern Finland. The annual loads of total N, NO₃-N and NH₄-N varied considerably during the study period. The mean total N load was 820 kg km⁻² yr⁻¹. More than half of the annual N load was in the form of NO₃-N, while the relative contribution of NH₄-N was low, which is common in Finnish agricultural fine-textured soils. The measured annual N loads were highly dependent on runoff. The highest annual N load (1310 kg km⁻²) was observed in 1984, in accordance with the highest annual runoff (616 mm). Due to mild weather conditions during the winters since 1989, the relative proportion of winter runoff was higher than earlier. So far, no decrease could be seen in annual N loads, even though most of the farmers are participating in the Finnish Agri-Environmental Programme established in 1995. However, the amount of NH₄-N load seems to have remained very low (less than 50 kg km⁻² yr⁻¹) in Savijoki since 1995, which may reflect changes in agricultural practices and a decrease in N deposition.