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Stable isotope systematics of C, N and S were studied in soils of 5 European forest ecosystems. The sites were located along a North–South transect from Sweden to Italy (mean annual temperatures from +1.0 to +8.5 °C, atmospheric deposition from 2 to 19 kg N ha −1 a −1 , and from 6 to 42 kg S ha −1 a −1 ). In Picea stands, the behavior of C, N and S isotopes was similar in 3 aspects: (1) assimilation favored the lighter isotopes 12 C, 14 N and 32 S; (2) mineralization in the soil profile left in situ residues enriched in the heavier isotopes 13 C, 15 N and 34 S; and (3) NO 3 –N as well as SO 4 –S in soil solution was isotopically lighter compared to the same species in the atmospheric input. In this study, emphasis was placed on S isotope profiles which so far have been investigated to a much lesser extent than those of C and N. Sulfate in monthly samples of atmospheric input had systematically higher δ 34 S ratios than total soil S at the 0–5 cm depth, on average by 4.0‰. Sulfate in the atmospheric input had higher δ 34 S ratios than in deep (>50 cm) lysimeter water, on average by 3.2‰. Organic S constituted more than 50% of total soil S throughout most of the profiles (0–20 cm below surface). There was a tendency to isotopically heavier organic S and lighter inorganic SO 4 –S, with ester SO 4 –S heavier than C-bonded S at 3 of the 5 sites. With an increasing depth (0 to 20 cm below surface), δ 13 C, δ 15 N and δ 34 S ratios of bulk soil increased on average by 0.9, 4.2 and 1.6‰, respectively, reflecting an increasing degree of mineralization of organic matter. The isotope effects of C, N and S mineralization were robust enough to exist at a variety of climate conditions and pollution levels. In the case of S, the difference between isotope composition of the upper organic-rich soil horizon (lower δ 34 S) and the deeper sesquioxide-rich soil horizons (higher δ 34 S) can be used to determine the source of SO 4 in streams draining forests. This application of δ 34 S as a tracer of S origin was developed in the Jezeři catchment, Czech Republic, a highly polluted site suffering from spruce die-back. In 1996–1997, the magnitude and δ 34 S of atmospheric input (20 kg S ha −1 a −1 , 5.8‰) and stream discharge (56 kg S ha −1 a −1 , 3.5‰) was monitored. Export of S from the catchment was 3 times higher than contemporary atmospheric input. More than 50% of S in the discharge was represented by release of previously stored pollutant S from the soil. Stable isotope systematics of Jezeři soil S (mean of 2.5‰ in the O+A horizon, 4.8‰ in the B horizon, and 5.8‰ in the bedrock) suggests that most of the soil-derived S in discharge must come from the isotopically light organic S present in the upper soil horizon, and that mineralized organically-cycled S is mainly flushed out during the spring snowmelt. The fact that a considerable proportion of incoming S is organically cycled should be considered when predicting the time-scale of acidification reversal in spruce die-back affected areas. |
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