Leaching of Nutrients, Organic Carbon and Iron from Finnish Forestry Land

Abstract

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 km²), distributed across all but the northenmost 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 (r² = 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 NO ⁻₃ -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.