The influence of sulfur deposition rates on sulfate retention patterns andmechanisms in aerated forest soils

Stable isotope ratios were used as a tracer for S flow and transformations in an irrigation experiment with 5 different German forest soils. Seventy-f ive lysimeters constructed from soil cores, 15 from each site, were irrigat ed over 20 months with SO4-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 delta S-34 value o f the irrigation SO4 was more than 22%0 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 individua l 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 wer e bound organically by the microbial biomass in all soils. Immobilization o f irrigation SO4 occurred predominantly in the topsoil horizons with the fo rmation of C-bonded S being more prevalent than the synthesis of organic su lfates. Tracer retention via formation of organic soil S compounds accounte d for up to 50% of the irrigation SO4 in treatment I, from 16 to 25% in tre atment II, and less than 20% in treatment III. The dominant process of inor ganic S retention in the soils appeared to be adsorption of SO4, but precip itation of aluminum hydroxy sulfate minerals constituted a second potential inorganic retention process in some soils. Sulfate adsorption increased wi th increasing sesquioxide content of the soils and with increasing S deposi tion rates. In soils with high sesquioxide contents, typically more than 70 % of the irrigated S was retained inorganically, whereas in the soil with t he lowest sesquioxide content, generally less than 50% of the labeled irrig ation S was detected in inorganic form. In the latter soil, the sesquioxide content was not high enough to fully adsorb the elevated SO4 inputs in tre atments II and III. Consequently, increased tracer S export with the seepag e water SO4 was observed in the experimental variants with elevated SO4 dep osition rates. In soils with high sesquioxide contents, the elevated SO4 in puts in treatments TI and III were fully retained in the soil horizons in i norganic form during the 20 months of the experiment and thus increased see page water export of labeled SO4 was not observed. The ability to inoganica lly retain tracer S in the mineral soil horizons was identified as the majo r 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 th e comparatively low recovery of irrigation SO4 with the seepage water impli es that the mean transit time of S in the uppermost 60 cm of the acid fores t soils varies between several years and many decades, much longer than pre viously thought. (C) 2001 Elsevier Science Ltd. All rights reserved.