The humus layer determines SO42− isotope values in the mineral soil

  • Giesler, Reiner
  • Mörth, Carl-Magnus
  • Mellqvist, Elin
  • Torssander, Peter

  • 1Department of Forest Ecology, Swedish University of Agricultural Sciences, S-901 83, Umeå, Sweden.

    2Department of Geology and Geochemistry, Stockholm University, S-106 91, Stockholm, Sweden.

    3Department of hydrogeology, Geological survey of Sweden, Box 670, S-751 28, Uppsala, Sweden.

*Author for correspondence;e-mail: [email protected]; Phone: +46-90-786-50-06; Fax: +46-90-786-77-50

Biogeochemistry 74(1):p 3-20, May 2005.

Biocycling of sulfur (S) has been proposed to play an important role in the recovery of ecosystems following anthropogenic S deposition. Here, we investigated the importance of the humus layer in the biocycling of S in three forested catchments in the Gårdsjön area of southwestern Sweden with differing S inputs and S isotope signature values. These experimental sites consisted of two reference catchments and the Gårdsjön roof experiment catchment (G1), where anthropogenic deposition was intercepted from 1991 until May 2002 by a roof placed over the entire catchment area. Under the roof, controlled levels of deposition were applied, using a sprinkler system, and the only form of S added was marine SO42− with a δ 34SSO4 of +19.5‰.

We installed ion exchange resin bags at the interface between the humus layer and mineral soil at each of the catchments to collect SO42− passing through the humus. The resin bags were installed on four occasions, in 1999 and 2000, covering two summer and two winter periods. The ions collected by each bag during these sampling periods were then eluted and their 34SSO4 values and SO42− concentrations determined. The most striking result is that the average 34SSO4 value in the resin bags was more than 12‰ lower compared to that of the sprinkler water in the G1 roof catchment. There was no increasing trend in the isotope value in the resin bag SO42− despite that the roof treatment has been on-going for almost 10 years; the average value for all resin bags was +7.1‰. The highest 34SSO4 values found in the G1 roof catchment were between +11‰ and +12‰. However, these values were all obtained from resin bags installed at a single sampling location. Throughfall and resin bag 34SSO4 values were more similar in the two reference catchments: about +7.5‰ in both cases. There was, however, an increase in resin bag 34SSO4 values during the first winter period, from about +7‰ to +9‰. The resin bag 34SSO4 value was linearly and positively related (r2 = 0.26, p < 0.001) to the amount of SO42− extracted from the resin bags, if relatively high amounts (>50 mmol m−2) were excluded. High amounts of resin bag SO42− seemed to be related to groundwater inputs, as indicated by the 34SSO4 value. Our results suggest that rapid immobilization of SO42− into a large organic S pool may alter the S isotope value and affect the 34SSO4 values measured in the mineral soil and runoff.

Copyright ©2005 Kluwer Academic Publishers