No evidence for trace metal limitation on anaerobic carbon mineralization in three peatland soils

Peatlands store roughly one-third of the terrestrial soil carbon and release the potent greenhouse gas methane (CH4) to the atmosphere, making these wetlands among the most important ecosystems in the global carbon cycle. Despite their importance, the controls of anaerobic decomposition of organic matter to carbon dioxide (CO2) and CH4 within peatlands are not well understood. It is known, however, that the enzymes responsible for CH4 production require cobalt, iron and nickel, and there is a growing appreciation for the potential role of trace metal limitation in anaerobic decomposition. To explore the possibility of trace metal limitation in peatlands, we washed 3 peat soils with either PbCl2, to remove available trace metals, or distilled water. Following these washes, we added trace metals (as CoCl2, CuCl2, FeCl2, PbCl2 and NiCl2) to each soil. We measured anaerobic CH4 and CO2 production in laboratory incubations over 4 weeks before adding glucose as a labile carbon source and measuring CH4 and CO2 production for an additional 4 weeks. In all 3 soils, neither CH4 nor CO2 production were limited by individual trace metals, even following the wash with PbCl2 to remove available metals. Further, in response to the addition of a labile carbon substrate, all soils supported increased rates of CH4 and CO2 production without progressive trace metal limitation. Taken together, our findings suggest that individual trace metals may not be limiting to anaerobic decomposition in many peatland soils.