Deposition and Leaching of Sulphate and Base Cations in a Mixed Boreal Forest in Eastern Finland

Monthly fluxes of sulphate (SO₄ ^2-) and base cations(Ca²⁺, Mg²⁺, K⁺) were studied from 1993 to 1996 as precipitation passed through forest vegetation and surfacesoil layers in an area receiving low and declining levels of atmospheric sulphate pollution. The canopy was dominated by mature Norway spruce (Picea abies Karsten) and the soilwas a podzol developed on glacial till material. The mean annual bulk deposition of SO₄ ^2- collected in the open was 136 mol_c ha^-1 and that of Ca²⁺, Mg²⁺ and K⁺ was 44, 11 and 25 mol_c ha^-1, respectively. The annual total throughfall deposition of SO₄ ^2- was 318 mol_c ha^-1 and that of Ca²⁺, Mg²⁺ and K⁺ was 151, 64 and 181 mol_cha^-1, respectively. Sulphate was the dominant anion accompanying the base cations leached from the canopy. More than half (58%) of the annual total throughfall deposition ofSO₄ ^2- was retained by the O-horizon and only 15% leached from below the B-horizon. The annual leaching of Ca²⁺, Mg²⁺ and K⁺ from below the B-horizon was14, 25 and 9% of the annual total throughfall deposition, respectively. The transport of base cations through the soil was predominantely countered by SO₄ ^2- anions.