LEACHING OF NITROGEN FROM FORESTED CATCHMENTS IN FINLAND

This study provides an assessment on the spatial variability of the lo ng-term leaching (8-23 years) of nitrogen and organic carbon from 22 f orested catchments (0.69-56 km(2)). The catchments are located through out Finland excluding the northernmost regions. The Kruunuoja catchmen t is located in a national park; the other catchments represent typica l Finnish forestry land. The leaching from the 21 forestry land catchm ents can be considered to represent average leaching from Finnish fore stry land since the most important forestry practices (ditching, clear -cutting, scarification, and fertilization) since the 1960s have affec ted about 2.4% of the catchment area per year (compare 2.5% in the ent ire country in 1980 and 2% in 1991). Moreover, the mean annual runoff from the catchments, 230-420 mm yr(-1), agree with the mean annual run off from Finland (301 mm yr(-1) from 1931 to 1990). The major part of the nitrogen transported from the catchments consisted of organic nitr ogen (on average 79%). The average inorganic nitrogen proportion ((NO3 -N + NH4-N)/N-tot) was lowest (7.3%) in the Kruunuoja catchment and wa s highest (54%) in the southernmost Teeressuonoja catchment located in the highest anthropogenic nitrogen deposition area. The median C/N ra tio in the study streams was high, ranging from 34 to 66. Nitrate leac hing from the catchments varied between 2.8 (Kruunuoja) and 100 kg km( -2) yr(-1) (Teeressuonoja) and was negatively related to C/N ratio in stream water and latitude. The stepwise multiple regression model sele cted C/N ratio and nitrogen deposition which together explained 72% of the variation in NO3-N leaching. Retention of NO3-N deposition (calcu lated as ((input-output)/input) was high in all catchments, ranging fr om 0.99 in Kruunuoja to 0.67 in Teeressuonoja.