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Stable isotope ratios were used as a tracer for S flow and transformations in an irrigation experiment with 5 different German forest soils. Seventy-five lysimeters constructed from soil cores, 15 from each site, were irrigated over 20 months with SO 4 -rich artificial canopy throughfall, simulating 3 different S input levels: 35 kg S ha −1 in treatment I, 63 kg S ha −1 in treatment II, and 131 kg S ha −1 in treatment III. The δ 34 S value of the irrigation SO 4 was more than 22‰ higher than those of total S in the untreated soils. Mass and isotope balances for different soil S compounds were used to assess the patterns and mechanisms of S retention in individual soil horizons and their dependence on S deposition levels. Independent of the S deposition level, on average 12±5 kg ha −1 of the applied S were bound organically by the microbial biomass in all soils. Immobilization of irrigation SO 4 occurred predominantly in the topsoil horizons with the formation of C-bonded S being more prevalent than the synthesis of organic sulfates. Tracer retention via formation of organic soil S compounds accounted for up to 50% of the irrigation SO 4 in treatment I, from 16 to 25% in treatment II, and less than 20% in treatment III. The dominant process of inorganic S retention in the soils appeared to be adsorption of SO 4 , but precipitation of aluminum hydroxy sulfate minerals constituted a second potential inorganic retention process in some soils. Sulfate adsorption increased with increasing sesquioxide content of the soils and with increasing S deposition rates. In soils with high sesquioxide contents, typically more than 70% of the irrigated S was retained inorganically, whereas in the soil with the lowest sesquioxide content, generally less than 50% of the labeled irrigation S was detected in inorganic form. In the latter soil, the sesquioxide content was not high enough to fully adsorb the elevated SO 4 inputs in treatments II and III. Consequently, increased tracer S export with the seepage water SO 4 was observed in the experimental variants with elevated SO 4 deposition rates. In soils with high sesquioxide contents, the elevated SO 4 inputs in treatments II and III were fully retained in the soil horizons in inorganic form during the 20 months of the experiment and thus increased seepage water export of labeled SO 4 was not observed. The ability to inorganically retain tracer S in the mineral soil horizons was identified as the major factor regulating the extent of tracer S export with the seepage water at 60 cm depth. The high retention of labeled S in all soils combined with the comparatively low recovery of irrigation SO 4 with the seepage water implies that the mean transit time of S in the uppermost 60 cm of the acid forest soils varies between several years and many decades, much longer than previously thought. |
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