Regional variations in diffuse nitrogen losses from agriculture in the Nordic and Baltic regions

This paper describes nitrogen losses from, and the characteristics of. 35 selected catchments (112 to 2000 ha) in the Nordic and Baltic countries. Average annual losses of N in 1994-1997 ranged from 5 to 75 kg ha(-1). Generally, the lowest losses were observed in the Baltic countries and the highest in Norway. The N losses were also characterised by significant within-country and interannual variations, particularly in the Norwegian catchments. An important finding of the study is that the average nutrient losses varied greatly among the catchments studied. The main explanations for this variability were water runoff, fertiliser use (especially the amount of manure), soil type and erosion (including stream bank erosion). However, there were several exceptions, and it was difficult to find general relationships between the individual factors. For example, there was poor correlation between nitrogen losses and surpluses. Therefore, the results suggest that the observed variability in N losses cannot have been due solely to differences in farm management practices, although the studied catchments do include a wide range of nutrient application levels, animal densities and other relevant elements. There is considerable spatial variation in the physical properties (soil. climate, hydrology, and topography) and the agricultural management of the basins, and the interaction between and relative effects of these factors has an important impact on erosion and nutrient losses. In particular, hydrological processes may have a marked effect on N losses measured in the catchment stream water. The results indicate that significant differences in hydrological pathways (e.g. the relationship between fast- and slow-flow processes) lead to major regional differences in N inputs to surface waters and therefore also in the response to changes in field management practices. Agricultural practices such as crop rotation systems, nutrient inputs and soil conservation measures obviously play a significant role in the site-specific effects, although they cannot explain the large regional differences observed in this study. The interactions between agricultural practices and basic catchment characteristics, including hydrological processes, determine the final losses of nitrogen to surface waters, hence it is necessary to understand these interactions to manage diffuse losses of agricultural nutrients efficiently.