Metal-bound carbon and nutrients across hydrologically diverse boreal peatlands

Boreal peatlands store abundant carbon (C) belowground because of saturated conditions and cold temperatures, which inhibit the enzymatic release of dissolved organic carbon (DOC) from organic matter. However, metals may also bind DOC, as well as nitrogen (N) and phosphorus (P), and their impact may vary among peatlands with differing hydrology. To assess variation of metal-C-nutrient interactions within and among peatlands and with depth, we sampled cores from seven peatlands in the Marcell Experimental Forest, Minnesota, including bogs, poor fens, and a rich fen. We extracted peat with sodium sulfate to release elements bound with exchangeable metals such as calcium (Ca) or aluminum (Al), and with sodium dithionite to release elements bound with the redox-active metals iron (Fe) and manganese (Mn). We compared extracted elements to long-term peat porewater measurements. Mean DOC extracted by sulfate or dithionite in the bogs and poor fens was 5 or 8 times greater, respectively, than porewater DOC, and in the rich fen it was 8 or 38 times greater. Similarly, N and P extracted by sulfate and dithionite were 10–24 times higher than porewater in the bogs and poor fens and 7–55 times higher in the rich fen. The ratio and absolute values of redox-sensitive and ion-exchangeable elements varied by element among peatland types and with peat depth and values were not always greater in fens than bogs. We conclude that both redox-active (Fe) and non-redox-active (Ca and Al) metals bind important pools of peatland C and nutrients regardless of peatland hydrologic type and despite the very low total mineral content of these boreal peats.