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Summary The aim was to characterize dissolved organic matter in soils under different tree species. Molecular size distribution and chemical composition of dissolved organic carbon and nitrogen were determined in water extracts from humus layers and mineral soils taken from silver birch (Betula pendula Roth.), Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) stands. Concentrations of tannins and 15 phenolic acids in the humus layers were measured. Per unit of organic matter, the concentrations of dissolved organic C and N were larger in birch and spruce humus layers than in the pine humus layer. In the underlying mineral soil, the concentrations of dissolved organic C were similar at all sites, but the concentration of dissolved organic N was greater in spruce and pine soils than in birch soil. In all soils, the 10–100 kDa fraction was the most abundant molecular size group and hydrophobic acids the most abundant chemical group of dissolved organic C. In all humus layers, hydrophobic acids and hydrophilic bases were the major components of dissolved organic N. There were only minor differences in the concentrations of total tannins in the humus layers under different tree species. Small‐molecule tannins (about < 0.5 kDa) were most abundant in the birch humus, and large‐molecule tannins in the pine humus. Coniferous humus contained more ferulic and p‐coumaric acids than did the birch humus. The concentrations of 3,4 and 3,5‐dihydroxybenzoic acid, vanillic acid and 4‐hydroxybenzoic acid were similar in all soils.

Rate of sediment accumulation of extractable organic halogen (EOX) and of 12 trace elements (As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sn, V, and Zn) were analyzed retrospectively over 100 years in the recipient area of pulp mills in Lake Saimaa and in two nonrecipient forest lakes in southeastern Finland. The sediments were dated by 210 Pb and 137 Cs activities. From 1880s to 1940s, <1 mg of EOX-Cl accumulated m -2 year -1 in the sediment of southeastern Lake Saimaa, but accumulation increased to 1350 mg of EOX-Cl m -2 year -1 from the 1950s to the 1980s at 3 km and to 500 mg at 5 km from the discharge point of bleaching pulp mills. Abandoning elemental chlorine in bleaching and adopting biological treatment for wastewaters in the 1990s associated with a decreasing the sediment accumulation of EOX to <400 mg of EOX-Cl m -2 year -1 at both sites. The deposition rates of inorganic elements were similar in all three lakes from 1880s to 1940s, 5-40 mg m -2 year -1 for Mn and Zn, 1-5 mg m -2 year -1 for Cr, Cu, Ni, Pb, Sn, and V, and below 1 mg m -2 year -1 for As, Cd, Co, and Hg. After the 1950s, the sediment accumulation rates of Cd, Cr, Cu, Hg, Ni, Pb, Sn, Zn, and carbon increased 2-5-fold in the recipient lake and those of Pb, Hg, and Sn also in the nonrecipient lakes. From each ton of adsorbable organic halogen (AOX) discharged by the mills, 0.6-0.1 mg of EOX-Cl m -2 accumulated in the Lake Saimaa sediments at 3-5 km from the mills. From the slope of sediment accumulation vs time, the half-life of the EOX of pulp mill origin in the sediment was estimated as 60-80 years. The rates of sediment accumulation of Hg during the decades when organomercurials were used as slimicides show that sediment was not the main sink for the Hg.