Losses of nitrogen and phosphorus from agricultural and forest areas in Finland during the 1980s and 1990s.

The temporal changes and spatial variability of phosphorus and nitrogen losses and concentrations in Finland during the period 1981-1997 were studied in 15 small agricultural and forested catchments. In addition, four coastal river basins with high agricultural land use located in southern Finland were included in the study in order to assess the representativeness of agricultural loss estimates from small agricultural catchments. The mean annual loss specific for agricultural land was estimated to be on average 110 kg km(-2) a(-1) for total phosphorus and 1500 kg km(-2) a(-1) for total nitrogen. The results from small agricultural catchments were in agreement with the corresponding loss estimates from rivers, with an average of 137 kg km(-2) a(-1) for total phosphorus and 1800 kg km(-2) a(-1) for total nitrogen. The results from the studied agricultural catchments and rivers during the period 1981-1997 suggest that weather-driven fluctuation in discharge was usually the main reason for changes in nutrient losses, and little or no impact of changes in agricultural production or management practises can be observed. In forested areas the total phosphorus loss (average 9 kg km(-2) a(-1)) and total nitrogen loss (average 250 kg km(-2) a(-1)) were lower than in agricultural areas. In forested catchments the impact of forestry operations, such as clear-cutting and fertilization, and the impact of atmospheric nitrogen deposition can be seen in changes in nutrient losses.