Peatlands and their carbon dynamics in northern high latitudes from 1 1990 to 2300: A process-based biogeochemistry model analysis

. Northern peatlands are a large C sink during the Holocene, but whether they will keep being a C sink 7 under future climate change is uncertain. This study simulates the responses of northern peatlands to future climate until 8 2300 with a Peatland version Terrestrial Ecosystem Model (PTEM). The simulations are driven with two sets of CMIP5 9 climate data (IPSL-CM5A-LR and bcc-csm1-1) under three warming scenarios (RCP2.6, 4.5 and 8.5). Peatlands expansion, 10 shrink, accumulation and decomposition are modeled. In the 21 st century, northern peatlands are projected to be a C source 11 of 1.2-13.3 Pg C under all climate scenarios except for RCP 2.6 of bcc-csm1-1 (a sink of 0.8 Pg C). During 2100-2300, 12 northern peatlands under all scenarios are a C source under IPSL-CM5A-LR scenarios, being larger sources than bcc-csm1-1 13 scenarios (5.9-118.3 vs. 0.7-87.6 Pg C). The peatland being C sources are due to: 1) water table depth (WTD) becomes 14 deeper and permafrost thaw increases decomposition rate; 2) net primary production (NPP) does not increase much as 15 climate warms because peat drying suppresses net N mineralization and 3) as WTD deepens, peatlands switches from moss- 16 herbaceous dominated to moss-woody dominated, while woody plants require more N for productivity. Under IPSL-CM5A- 17 LR scenarios, northern peatlands remain as a C sink until pan-Arctic annual temperature reaches -2.6 - -2.89°C, while this 18 threshold is -2.09 - -2.35°C under bcc-csm1-1 scenarios. This study predicts an earlier northern peatland sink to source shift 19 than previous estimates in the literature and emphasizes the vulnerability of northern peatlands to climate change. Peteet (2019). this study, the of the Holocene simulation as the initial condition for the future simulation.