Climate change is likely to significantly affect the hydrology, ecology, and ecosystem function of peatlands, with potentially important but unclear impacts on mercury mobility within and transport from peatlands. Using a full‐factorial mesocosm approach, we investigated the potential impacts on mercury mobility of water table regime changes (high and low) and vegetation community shifts (sedge‐dominated, Ericaceae‐dominated, or unmanipulated control) in peat monoliths at the PEATcosm mesocosm facility in Houghton, Michigan. Lower and more variable water table regimes and the loss of Ericaceae shrubs act significantly and independently to increase both total Hg and methylmercury concentrations in peat pore water and in spring snowmelt runoff. These differences are related to enhanced peat decomposition and internal regeneration of electron acceptors which are more strongly related to water table regime than to plant community changes. Loss of Ericaceae shrubs and an increase in sedge cover may also affect Hg concentrations and mobility via oxygen shuttling and/or the provision of labile root exudates. Altered hydrological regimes and shifting vegetation communities, as a result of global climate change, are likely to enhance Hg transport from peatlands to downstream aquatic ecosystems.
Peatland decomposition may be altered by hydrology and plant functional groups (PFGs), but exactly how the latter influences decomposition is unclear, as are potential interactions of these factors. We used a factorial mesocosm experiment with intact 1 m 3 peat monoliths to explore how PFGs (sedges vs Ericaceae) and water table level individually and synergistically affect decomposition processes. Decomposition was measured using litter bags at three depths filled with cellulose strips to mimic decomposition of a simple plant-derived structure, and Sphagnum tissue to simulate decomposition of the most abundant recalcitrant material in peatlands. We also analyzed the potential activity of five hydrolytic extracellular enzymes at an intermediate depth. We found lowered water table reduced activity of several enzymes and increased cellulose and Sphagnum decomposition. Presence of Ericaceae reduced decomposition of the recalcitrant Sphagnum tissue, whereas higher activity of chitinase was found in the combined presence of sedges and Ericaceae.We found no relationship between any potential enzyme activity and Sphagnum decomposition rate.Overall our results showed a dominating role of water table controlling decomposition processes, as well as support for the hypothesis that the presence of mycorrhizal Ericaceae can slow decomposition processes of complex plant tissues in peatlands.
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