Nitrogen Cycling Feedback on Carbon Dynamics Leads to Greater CH4 Emissions and Weaker Cooling Effect of Northern Peatlands

Northern peatlands have been a carbon sink since their initiation. This has been simulated by existing process-based models. However, most of these models are limited by lacking sufficient processes of the N cycle in peatlands. Here, we use a peatland biogeochemistry model incorporated with N-related processes of fixation, deposition, gas emission, loss through water flow, net mineralization, plant uptake and litterfall to project the role of the peatlands in future radiative forcing (RF). Simulations from 15-ka BP to 2100 are conducted driven by CMIP5 climate forcing data of IPSL-CM5A-LR and bcc-csm1-1, including warming scenarios of RCP 2.6, RCP 4.5 and RCP 8.5. During the Holocene, northern peatlands have an increasing cooling effect with RF up to −0.57 W m⁻². By 1990, these peatlands accumulate 408 Pg C and 7.8 Pg N. Under warming, increasing mineral N content enhances plant net primary productivity; the cooling effect persists. However, RF increases by 0.1–0.5 W m⁻² during the 21st century, mainly due to the stimulated CH₄ emissions. Northern peatlands could switch from a C sink to a source when the annual temperature exceeds −2.2 to −0.5°C. This study highlights that the improved N cycle causes higher CO₂-C sink capacity in northern peatlands. However, it also causes a significant increase in CH₄ emissions, which weakens the cooling effect of northern peatlands in future climate.