Sulfur isotope dynamics in two central european watersheds affected by high atmospheric deposition of SO x

Abstract

Sulfur fluxes and δ 34S values were determined in two acidified small watersheds located near the Czech–German border, Central Europe. Sulfur of sulfate aerosol in the broader region (mean δ 34S of 7.5‰ CDT) was isotopically heavier than sulfur of airborne SO 2 (mean δ 34S of 4.7‰). The annual atmospheric S deposition to the Jezeřı́ watershed decreased markedly in 1993, 1994, and 1995 (40, 33, and 29 kg/ ha · yr), reflecting reductions in industrial S emissions. Sulfur export from Jezeří via surface discharge was twice atmospheric inputs, and increased from 52 to 58 to 85 kg/ha · yr over the same three-year period. The δ 34S value of Jezeřı́ streamflow was 4.5 ± 0.3‰, intermediate between the average atmospheric deposition (5.4 ± 0.2‰) and soil S (4.0 ± 0.5‰), suggesting that the excess sulfate in runoff comes from release of S from the soil. Bedrock is not a plausible source of the excess S, because its S concentration is very low (<0.003 wt.%) and because its δ 34S value is too high (5.8‰) to be consistent with the δ 34S of runoff. A sulfur isotope mixing model indicated that release of soil S accounted for 64 ± 33% of sulfate S in Jezeřı́ discharge. Approximately 30% of total sulfate S in the discharge were organically cycled. At Načetı́n, the same sequence of δ 34S IN > δ 34S OUT > δ 34S SOIL was observed. The seasonality found in atmospheric input (higher δ 34S in summer, lower δ 34S in winter) was preserved in shallow (<10 cm) soil water, but not in deeper soil water. δ 34S values of deeper (>10 cm) soil water (4.8 ± 0.2‰) were intermediate between those of atmospheric input (5.9 ± 0.3‰) and Nac̆etín soils (2.4 ± 0.1‰), again suggesting that remobilization of soil S accounts for a significant fraction (roughly 40 ± 10%) of the S in soil water at Načetı́n. The inventories of soil S at both of these sites are legacies of more intense atmospheric pollution during previous decades, and are large enough (740 and 1500 kg S/ha at Jezeřı́ and Načetı́n, respectively) to supply significant sulfur fluxes to runoff for several more decades. The ongoing release of this stored soil S may significantly delay the recovery of water quality under declining atmospheric S deposition. Analysis of possible scenarios that would result in different S isotope composition of rainfall, runoff and soil suggested that biologic S isotope fractionation must be involved. Mineralization of organic soil S was recorded in two opposite but complementary vertical isotope trends: while soil water had lower δ 34S values in deeper horizons, bulk soil had higher δ 34S values in deeper horizons.