Leaching of cations and sulphate after mechanical site preparation at a boreal forest clear-cut area

It is generally assumed that site preparation after forest harvesting has an impact on nutrient leaching to watercourses, but the scientific evidence is limited. We studied the effects of soil harrowing on cation (Ca2+, Mg2+, K+, H+, Al3+, Fe3+) and sulphate (SO42−) fluxes through podzolic soil horizons in a boreal forest in Finland. The Norway spruce (Picea abies (L.) H. Karst.) dominated forest had been stem-only harvested in 1996 and soil harrowing with a disc-plower carried out in 1998. Three different micro-sites were formed: shallow furrows (25% of whole area), low ridges (30%) and undisturbed surface (45%). Zero-tension lysimeters were installed below the organic (O), eluvial (E) and illuvial (B) horizons at each of these micro-sites and leachates collected from 1999 to 2003. The leaching of cations and SO42− was the highest under the ridges and lowest under the furrows, and mainly the result of increased concentrations, indicating that organic matter as a source and mechanism was involved in cation and SO42− leaching. Leaching fluxes (kg ha− 1) from below the B-horizon for entire treated area over the five-year study period were 6.26 kg ha− 1 for Ca2+, 11.40 kg ha− 1 for K+, 1.97 kg ha− 1 for Mg2+, 1.00 kg ha− 1 for Al3+, 0.06 kg ha− 1 for H+, 0.30 kg ha− 1 for Fe3+ and 7.48 kg ha− 1 for SO42− - S. Deposition inputs for the same period were 6.51 kg ha− 1 for Ca2+, 7.02 kg ha− 1 for K+, 1.24 kg ha− 1 for Mg2+, 0.96 kg ha− 1 for Al3+, 0.83 kg ha− 1 for Fe3+ and 12.65 kg ha− 1 for SO42− - S. Assuming the fluxes from the undisturbed micro-site represent those from an unprepared clear-cut area, Fe3+ leaching increased by 112% and those of Al3+, H+, Ca2+, Mg2+ and K+ were 47%, 27%, 21%, 21% and 12%, respectively. The leaching of SO42− - S was not affected by site preparation. Since nitrate leaching was negligible, the increased leaching of cations can be attributed to the increased concentrations of dissolved organic carbon and charge-countering organic anions. The results show that soil harrowing increases cation (including important mineral nutrients, Ca, Mg and K) leaching losses from the soil; more so than clear cutting, and in relation to the extent and intensity of soil disturbance.