Significant Metal-Bound Organic Carbon Across Hydrologically Diverse Peatlands

Peatlands store a significant amount of organic carbon (C) belowground as partially decomposed plant matter, which is thought to accumulate due to anaerobic conditions and low temperatures that impede solubilization of dissolved organic carbon (DOC) by extracellular enzymes. However, metals and minerals may also influence peatland DOC solubilization and this role may vary with depth and peatland type: fens receive metal-rich groundwater while bogs do not. We sampled peat cores from four bogs, two poor fens, and one rich fen in Minnesotas Marcell Experimental Forest. We extracted samples with 0.05 M sodium dithionite and 0.05 M sodium sulfate to quantify iron (Fe), Fe-associated C, and other ions associated with Fe and C. Dithionite reduces Fe(III) to Fe(II) and releases C associated with Fe as DOC. Sulfate acts as a control extraction to account for ion-exchangeable C. Solutes in the dithionite and sulfate extractions were expressed on a porewater concentration basis to compare the concentration of potentially soluble DOC bound by metals to porewater DOC from long-term observations. Cluster analysis based on extractable metals (Fe, Ca, Mg, and Al) showed the rich fen samples were distinct from the bog and poor fen samples, which were similar. However, dithionite-extracted DOC at 0-10 cm was similar between the bogs (40.7 ± 7.0 mmol C/L) and the rich fen (54.3 ± 30.7 mmol C/L) despite the difference in Fe between these sites (1.9 ± 0.5 mmol Fe/L and 6.2 ± 3.9 mmol Fe/L, respectively) and was greater than the sulfate-extracted DOC (13.5 ± 2.3 mmol C/L and 10.9 ± 5.5 mmol C/L, respectively). The extracted DOC became more variable with depth in the bogs (50-75 cm; dithionite: 48.8 ± 32.0 mmol C/L; sulfate: 30.7 ± 22.4 mmol C/L) and fen (50-75 cm; dithionite: 94.9 ± 74.9 mmol C/L; sulfate: 24.5 ± 19.0 mmol C/L). The sulfate-extracted DOC was consistently higher than average porewater DOC at 0 cm (5.2 ± 1.8 mmol C/L) and 50 cm (4.7 ± 1.8 mmol C/L), indicating a likely role for divalent cations (Ca2+, Mg2+) in binding DOC. The high extracted DOC compared to porewater DOC across these hydrologically diverse peatlands indicates the key role of metals regardless of water sources. In addition to anaerobic conditions and low temperatures, metal-C interactions are another important mechanism controlling DOC availability and decomposition in peatlands.