Leaching of nutrients, organic carbon and iron from Finnish forestry land

This study provides an assessment of the spatial variability of the long-term leaching of nutrients, total organic carbon (TOC) and iron (Fe-tot) from 22 forested catchments (0.69-56 km(2)), distributed across all but the northernmost areas of Finland. The natural, unmanaged Kruunuoja catchment is located in a national park, while the other catchments represent Finnish forestry land. The average leaching of P-tot (4.2 kg km(-2) yr(-1)) from the Kruunuoja catchment was small compared to the catchments representing forestry land (on average 10 kg km(-2) yr(-1)). Moreover, P fertilization was the most important predictor for the spatial variation in P-tot leaching (r(2) = 0.45). Leaching of TOC, Fe-tot and N compounds was not closely related to forestry practices. Median C/N ratio in the study streams was high (range 34-66). The average inorganic N proportion and leaching of NO3--N were lowest in the Kruunuoja catchment (7.3 % and 2.8 kg km(-2) yr(-1), respectively) and highest in the southernmost Teeressuonoja catchment (54 % and 100 kg km(-2) yr(-1), respectively) located in the highest N deposition area. The most important forestry practices since the 1960's have affected about 2.4 % of the area of study sites per year (cf. 2 % in the entire country in 1991). Moreover, the mean annual runoff from the catchments (230-430 mm yr(-1)) agrees with the mean annual runoff from Finland (301 mm yr(-1)). Consequently, the results of the study catchments can be used to estimate average total annual leaching from Finnish forestry land: 2,700 t of P-tot, 48,000 t of N-tot, 110,000 t of Fe-tot and 1.5 million t of TOC.