34S tracer study of pollutant sulfate behaviour in a lowland peatland

Abstract

Field experiments were carried out in order to assess the practicality and application of 34SO42− as a tracer of the physical and geochemical fate of aerially derived sulfur in peat. Six enclosures were isolated in a lowland peat with high historical acid sulfate inputs at Thorne Moors, UK, and treated with regular additions of 99.9% pure 34SO42− for 12 months. The total 34S sulfate addition resulted in negligible change to the sulfate concentration, but allowed unequivocal change to the isotopic composition of sulfate inputs relative to pre-experiment control data set. Migration and biogeochemical transformations of the 34S tracer were monitored via depth-specific sampling of surface and pore-waters every 6 weeks, and sacrificial sampling of solid peat at 12, 24, and 48 week intervals. Tracer incorporation into the various sulfur forms within the surface and pore-waters, vegetation, organic and inorganic fractions of the peat was apparent through strong positive deviation of δ34S from natural values (in comparison with 18 months control data set for the same site). Consistency within enclosures is good and a detailed model of sulfur cycling within each enclosure can be established but natural variability in the control data and differences between replicate enclosures prevents more quantitative assessment. The 34S tracer was initially rapidly removed from surface waters. The majority of uptake was by living vegetation (5.7–33% of tracer added, mean 17.6%), or through transformation to the organic fraction of the upper peat (25 cm) after rapid bacterial reduction of sulfate to sulfide. Despite penetration of 34S labelled sulfate to deeper pore-waters over time, there was no significant reduction to sulfide or subsequent incorporation into organic or inorganic fraction at these depths (>25 cm); organic and inorganic sulfur, and pore-water sulfide retained their initial unlabelled isotopic compositions. This limitation on sulfur cycling at relatively shallow depth may be attributed to diminished labile organic matter inhibiting the activity of sulfate reducing bacteria or poisoning of sulfate reducers by high dissolved sulfide, after long-term sulfur pollution of this ecosystem.