Biogeochemistry and ecology of peatlands: response to global change and linking the C, N and P cycles

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Peatlands cover ~12% of the land surface of Canada, store ~100 Gt of C, emit 5 - 10 Tg CH4 /yr and are significant sources of DOC (~10 Tg/yr) to water bodies. Although we have made significant progress in measuring and modeling these fluxes, it is clear that further advances depend on a better identification of the role of nutrients, such as N and P, in influencing C cycling in peatlands, through increased atmospheric loading and through drainage and accelerated rates of mineralization. Our long-term NPK fertilization experiment at the Mer Bleue peatland has shown shrub replacement of mosses, no increase in photosynthesis despite N uptake, and increased respiration resulting in reduced C uptake or possibly loss, contrary to the expectation of fertilization. Analysis of foliar tissues shows that the Mer Bleue is N/P co-limited and responds in a conservative manner to increased fertilization. The stoichiometric cascade of C, N and P from plants through senescent leaves and litter to the peat profile shows that much of the N is conserved, resulting in a relatively narrow C:N ratio in peat and large rates of N accumulation (0.5 – 1.0 g m2/yr), yet the C:P increases as peat forms - P is drawn out of the peat (C:P 1500 to 2000:1) and maintained in the upper 0.5 m, accumulating at only 15 mg m2/yr. The peatland stoichiometry of P is very different from Canadian forest litter/soils, but the processes responsible remain unknown and this is a focus of my proposal. The sources of the large amount of N accumulated in peatlands are unknown and our estimates of N2 fixation by cyanobacteria at Mer Bleue are small compared to the accumulation rate. Our fertilization experiment provides an opportunity to test whether increasing N deposition (or P addition) affects N2 fixation rates. We shall test mosses and peat from the fertilization plots and rhizospheres for N2 fixation potential; methanotrophs are known to have N2 fixation potentials and we shall quantify this by laboratory incubations of peat profiles varying in water table position and CH4 concentration. Rates of litter and peat decomposition are important controls on peatland biogeochemistry and sources of DOC, CO2 and CH4. We are using peatland sites near Montreal, thawing permafrost in the NWT, pocosins in North Carolina and restiad bogs in New Zealand to broaden our understanding on the controls of litter and peat decomposition rates and release of N and P.