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 34 SO 4 2-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 34 SO 4 2-for 12 months. The total 34 S 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 34 S 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 d 34 S 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 34 S 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 34 S 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 porewater 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.

The effects of hydrologic fluctuation and sulfate regeneration on mercury cycling in an experimental peatland

“…The increasing strength of the relationship between sulfate concentrations and the length of the drawdown period is not surprising given that other studies have found that the sulfate that appears during a rewetting event comes from the oxidation of organic sulfur compounds stored in the peat [ Eimers et al , ; Mandernack et al , ; Mörth et al , ]. Isotopic studies of sulfur cycling in peat have found that sulfate added to peatland mesocosms is predominantly incorporated into the organic‐sulfur fraction of the peat matrix through bacterial sulfate reduction and plant uptake [ Bartlett et al , ; Chapman and Davidson , ] and that the sulfate released during rewetting events has a light isotopic signature relative to atmospheric deposition, suggesting reoxidation of sulfur from the “lighter” carbon‐bound sulfur pool [ Mandernack et al , ; Mörth et al , ].…”

DOC-dynamics in a small headwater catchment as driven by redox fluctuations and hydrological flow paths – are DOC exports mediated by iron reduction/oxidation cycles?

“…In contrast to the pore water, in the case of discharge the analysis of the hydrograph and the separation of sources based on event water contributions demonstrated that this inverse relation can be explained by the fact that sulfate and DOC have different hydrological sources, at least in summer: waters of high DOC concentrations and due to reducing conditions low in or devoid of sulfate were released from the wetland waters (wetland pore water, shallow wetland groundwater) during discharge events, while during these events the contribution of deeper groundwater -as the main source of sulfate -decreased. Only during winter months, DOC and sulfate correlated in a positive manner, probably due to changes in hydrological flow paths (e.g., more surface flow components), less activity of reductive processes and also reduced plant uptake (Bartlett et al, 2009). On the other hand there was a strong positive correlation of DOC and iron in pore water, groundwater, and surface water.…”

Geochemical evolution of organic-rich shales with increasing maturity: A STXM and TEM study of the Posidonia Shale (Lower Toarcian, northern Germany)

“…Consistently with previous results (Horsfield, 1997), Wickensen organic matter appears to mainly consist of immature kerogen containing variable amount of organic sulphur moieties. Such sulphur incorporation into organic matter is believed to rapidly occur during bacterial sulphate reduction at the early stages of sediment burial (Sinninghe Damsté; and de Leeuw, 1990;Hebting et al, 2006;Vandenbroucke and Largeau, 2007;Bottrell et al, 2009;Lepot et al, 2009), on time-scales that can be of less than a year (Bartlett et al, 2009). Reactive iron is usually considered to outcompete organic matter for sulphur incorporation, due to faster formation of pyrite (Berner, 1984).…”