Abstract
Sea-level rise coupled with land subsidence from wetland drainage exposes increasingly large areas of coastal peatlands to seawater intrusion. Seawater contains high concentrations of sulfate (SO42-), which can alter the decomposition of organic matter thereby releasing organic and inorganic solutes from peat. In this study, a flow-through reactor system was used in order to examine the transport of SO42- through peat as well as its effect on solute release. Moderately-decomposed fen peat samples received input solutions with SO42- concentrations of 0, 100, 700 and 2700 mg L-1; sample effluent was analyzed for a variety of geochemical parameters including dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and total dissolved nitrogen (TDN) as well as the concentrations of major cations and anions. The input solution remained anoxic throughout the experiment; however, no signs of a pronounced SO42- reduction were detected in the effluent. SO42- transport in the fen peat resembled non-reactive bromide (Br-) transport, indicating that in the absence of SO42- reduction the anion may be considered a conservative tracer. However, slightly elevated concentrations of DOC and TDN, associated with raised SO42- levels, suggest the minor desorption of organic acids through anion exchange. An increased solute release due to stimulated decomposition processes, including SO42- reduction, was observed for samples with acetate as an additional marine carbon source included in their input solution. The solute release of peats with different degrees of decomposition differed greatly under SO42--enriched conditions where strongly-decomposed fen peat samples released the highest concentrations of DOC, DIC and TDN. Overall, besides a cation-exchange related release of substances adsorbed in the peat, an input of increased SO42- concentrations in a peatland by itself might not lead to an increased solute release in the short-term. However, once SO42- reduction has commenced following long-term anoxic conditions or due to a biologically readily degradable carbon source, a SO42- input can stimulate the release of solutes, including DOC, DIC and TDN, from peat.
Highlights
Over the millennia, large quantities of partly-decomposed organic matter have accumulated in waterlogged peatlands
The differences in their shape are not due to differences in the SO42− input concentration, they are instead a result of differences in the soil pore distribution for the individual peat samples. This effect is verified by nonreactive Br− breakthrough curve (BTC) which show a very similar shape to the SO42− BTCs, as exemplified in Figure 2B for a breakthrough of 100 mg Br− L−1 and 100 mg SO42− L−1. These results suggest that no SO42− reduction occurred and SO42− behaved as a conservative anion during solute transport in the studied fen peat samples
The variations between the parameters of the different peats were minor suggesting that the effect of degree of decomposition and organic matter content on the shape of the BTC is less pronounced than what has been shown in previous studies (Liu et al, 2017)
Summary
Large quantities of partly-decomposed organic matter have accumulated in waterlogged peatlands. It is well known that the drainage of peatlands for agricultural purposes initiates the aerobic decomposition of organic matter in the top layer, resulting in the enhanced emission of greenhouse gases, including carbon dioxide (CO2) and nitrous oxide (N2O) (Kasimir-Klemedtsson et al, 1997) as well as a mobilization of dissolved organic matter (generally measured as dissolved organic carbon, DOC) and other nutrients such as ammonium (NH+4 ) (Zak and Gelbrecht, 2007). The decomposition of organic matter is controlled by the presence of electron acceptors such as sulfate (SO42−), an inorganic and highly mobile form of sulfur, which is used in the metabolism of sulfatereducing bacteria (SRB). Infiltration of seawater into peatlands can drastically increase SO42− concentrations, altering the biogeochemistry of the peat thereby implicating the mineralization of organic matter
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