Climate warming and elevated CO2 alter peatland soil carbon sources and stability

Peatlands are an important global carbon (C) reservoir storing at least one-third of global soil organic carbon (SOC), but little is known about the stability of these vast C stocks under climate change. Here, we examine the impact of four years of warming (+0, +2.25, +4.5, +6.75, +9 °C) and two years of elevated atmospheric CO₂ concentration (eCO₂) on the molecular composition of SOC to infer SOC sources (microbe-, plant- and fire-derived) and stability in a boreal peatland. We show that while warming alone decreased plant- and microbe-derived SOC due to enhanced decomposition, warming combined with eCO₂ increased plant-derived SOC compounds. Further, using biopolymers distinct to either leaf/needle (cutin) or root (suberin), we observed increasing root-derived inputs and declining leaf-derived C inputs into SOC under warming and eCO₂. Unsurprisingly, SOC derived from historical pyrolysis (pyrogenic C) was unaffected by warming or eCO₂. The decline in SOC compounds with warming and gains from new root-derived C under eCO₂, suggest that warming and eCO₂ may shift peatland C budget towards pools with faster turnover. Together, our results indicate that climate change may increase inputs and enhance decomposition of SOC potentially destabilising C storage in peatlands.