Sulfur concentrations and d34S ratios were determined in vertical soil profiles at 18 forested sites throughout the Czech Republic (Central Europe), in first-order stream baseflow and in forest floor moss. In addition, sulfate concentrations and d34S ratios were monitored in bulk precipitation, spruce throughfall, soil water (depth of 0, 10, 30, 70 and 90 cm) and surface discharge at CER and NAC, two severely polluted catchments located in the northern Czech Republic, from 1992 to 1994. All 18 forested sites exhibited an increase in d34S ratios from topsoil to mineral soil. Of these sites, 15 sites were characterized by a smooth positive isotope signal with the highest d34S at the deepest level sampled. The remaining 3 sites had a d34S maximum in the second deepest horizon. With increasing soil depth, the generally positive d34S signals accompanied a negative S concentration gradient. Vertical d34S signals in soil may result from
(i) mixing of sulfur from two isotopically distinct sources, bedrock and atmosphere, and/or (ii) isotope fractionation processes within the soil profile. The latter mechanism requires a different degree of openness of the system toward the lighter isotope 32S and heavier isotope 34S (Krouse, 1986). Isotope signatures of accessory sulfides in bedrock were approximated by stream baseflow d34S during a 5-year hydrological minimum. Mean d34S ratio in bedrock was +7.4, higher than d34S of the deepest soil horizon sampled (mean of +4.2). We propose that the convergence of soil and bedrock d34S at depths sampled (0 to 70 cm) was fortuitous and not caused by an increasing admixture of bedrock S from horizon A to C : d34S increased (up to +3.0) with depth even at a site where bedrock shale had an extremely negative d34S of -13.3 . In the presentation possible mechanisms of redistribution of S isotopes witin the soil profile will be discussed (cf. Novak et al., 1994; Novak et al., subm.).
In the two intensively monitored catchments, throughfall [SO42-], up to 80 mg L-1 in winter and as low as 7 mg L-1 in summer, was higher than [SO42-] in bulk precipitation (annual average 6 mg L-1). There was a distinct seasonality in S isotope abundances, with lower d34SBULK in summer (+4 per mil CER, + 6 per mil NAC) and lower d34STF in winter (+3 per mil CER, +4 per mil NAC). Wintertime d34SBULK was around +8 per mil (+10 per mil) at CER (NAC), summertime d34STF was close to +7 per mil at both CER and NAC. For only a 1-month period in spring, bulk precipitation S became isotopically lighter than throughfall S. Suction lysimeters (depth 30 and 90 cm) yielded higher sulfate concentrations and lower d34S ratios compared to both bulk and throughfall precipitation. Little seasonality was observed in [SO42-] at 30 cm (around 40 mg L-1); at 90 cm [SO42-] was higher in winter (70 mg L-1) than in summer (45 mg L-1). d34S at 90 cm was <+5 per mil in 1993 and up to +7.5 in 1994. Zero tension lysimeters (0, 10, 30, 70 cm) generally recovered more concentrated soil water from greater depths but always more diluted compared to suction lysimeters. In 2 months when all four depths yielded enough soil water for analysis, [SO42-] increased while d34S decreased in the order 0-10-30-70 cm. Similar to suction lysimeters, d34S in zero tension lysimeters was lower than in bulk and throughfall precipitation, with values mostly in the range of +4 to +6 per mil. CER surface discharge (drainage area 261 ha) had [SO42-] higher than spruce throughfall except for winter months. There was a slight negative correlation between [SO42-]DIS (averaging 53 mg L-1) and d34SDIS (ranging between +3.5 and +8.4 per mil). Even though 80% of the CER catchment is deforested, d34SDIS were lower than d34SBULK indicating an admixture of low- d34S sulfur dioxide captured by spruce canopies. Sulfur fluxes at CER and NAC are characterized by distinct isotope compositions and can therefore be used to trace S pathways and transformations in the forest soil (Novak et al., 1996).
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