Assessing methane emissions for northern peatlands in ORCHIDEE-PEAT revision 7020

In the global methane budget, the largest natural source is attributed to wetlands, which encompass all ecosystems composed of waterlogged or inundated ground, capable of methane production. Among them, northern peatlands that store large amounts of soil organic carbon have been functioning, since the end of the last glaciation period, as long-term sources of methane (CH4) and are one of the most significant methane sources among wetlands. To reduce uncertainty of quantifying methane flux in the global methane budget, it is of significance to understand the underlying processes for methane production and fluxes in northern peatlands. A methane model that features methane production and transport by plants, ebullition process and diffusion in soil, oxidation to CO2, and CH4 fluxes to the atmosphere has been embedded in the ORCHIDEE-PEAT land surface model that includes an explicit representation of northern peatlands. ORCHIDEE-PCH4 was calibrated and evaluated on 14 peatland sites distributed on both the Eurasian and American continents in the northern boreal and temperate regions. Data assimilation approaches were employed to optimized parameters at each site and at all sites simultaneously. Results show that methanogenesis is sensitive to temperature and substrate availability over the top 75cm of soil depth. Methane emissions estimated using single site optimization (SSO) of model parameters are underestimated by 9gCH4m-2 yr-1 on average (i.e., 50% higher than the site average of yearly methane emissions). While using the multi-site optimization (MSO), methane emissions are overestimated by 5gCH4m-2 yr-1 on average across all investigated sites (i.e., 37% lower than the site average of yearly methane emissions).