Potential Vulnerability of Deep Carbon Deposits of Forested Swamps to Drought

Climate warming is resulting in increases in the frequency and intensity of summer droughts in the Great Lakes–St. Lawrence forest region (Ontario, Canada), raising concerns for the fate of C stores. We hypothesized that deeper peat historically existing beneath the water table would produce significant CO2 efflux during summer droughts. To test this hypothesis, we collected saturated peat cores, partitioned them into depth intervals, incubated the peat under conditions that resulted in peat drying, and monitored daily CO2 production together with potential drivers of CO2 production, including peat quality, microbial biomass, and microbial extracellular enzyme activity. Peat CO2 production (μmol CO2 min−1 g−1 dry soil) was highest in the top 30 cm of the peat profile, with the highest production at intermediate volumetric water content (VWC). Peat substrates fuelling CO2 production had quotients of C to N of