Fire is an important pathway for carbon (C) loss from boreal forest ecosystems and has a strong effect on ecosystem C balance. Fires can range widely in severity, defined as the amount of vegetation and forest floor consumed by fire, depending on local fuel and climatic conditions. Here we explore a novel method for estimating fire severity and loss of C from fire using the atmosphere to integrate ecosystem heterogeneity at the watershed scale. We measured the δ¹³C and Δ¹⁴C isotopic values of CO₂ emitted from an experimental forest fire at the Caribou-Poker Creek Research Watershed (CPCRW), near Fairbanks, Alaska. We used inverse modeling combined with dual isotope measurements of C contained in aboveground black spruce biomass and soil organic horizons to estimate the amount of C released by this fire. The experimental burn was a medium to severe intensity fire that released, on average, about 2.5 kg Cm⁻², more than half of the C contained in vegetation and soil organic horizon pools. For vegetation, the model predicted that approximately 70–75% of pools such as needles, fine branches, and bark were consumed by fire, whereas only 20–30% of pools such as coarse branches and cones were consumed. The fire was predicted to have almost completely consumed surface soil organic horizons and burned about half of the deepest humic horizon. The ability to estimate the amount of biomass combusted and C emission from fires at the watershed scale provides an extensive approach that can complement more limited intensive ground-based measurements.

The response of large stores of carbon in boreal forest soils to global warming is a major uncertainty in predicting the future carbon budget. We measured the temperature dependence of decomposition for upland boreal peat under black spruce forest with sphagnum and feather moss understory using incubation experiments. CO₂ efflux rates clearly responded to temperature, which ranged from −10° to +8°C by ∼2°C increments. At temperatures below 0°C, significant decomposition was observed in feather moss peat but not in wetter sphagnum peat. Above 0°C, decomposition was exponentially related to temperature, corresponding to a Q(10) (the ratio of the rate of CO₂ evolution at one temperature divided by that at a temperature 10°C cooler) of 4.4 for feather moss and 3.1 for sphagnum peat. The greatest change in CO₂ evolution rate with temperature occurred between −2° and 0°C, which coincided with the phase transition of soil water. We saw no large change in the rate of CO₂ evolution between incubation experiments separated by a 6 month storage period for feather moss peat. Stable C isotope measurements of evolved CO₂ and the rate of change of CO₂ evolution with time suggest different substrates are used to sustain heterotrophic respiration above and below freezing. Radiocarbon signatures of CO₂ respired from both types of peat reflected significant contributions from C fixed in the last 35 years (“bomb” ¹⁴C) as well as C fixed prior to 1950. We observed no change in the Δ¹⁴C of respired CO₂ with temperature. Isotopic signatures of peat components showed that a combination of substrates must contribute to the CO₂ evolved in our incubations. Decomposition of fine roots (which made up less than 7% of the total peat C) accounted for ∼50% of respired CO₂ in feather moss peat and for ∼30% of respired CO₂ in sphagnum peat. Fine-grained (<1 mm), more humified material that makes up 60–70% of the bulk peat organic carbon contributed significantly to heterotrophic respiration (∼30% in feather moss and ∼50% in sphagnum moss peat), despite slow decomposition rates. Increased temperatures caused enhanced decomposition from all pools without changing their relative contributions. Because the contribution of peat decomposition is a small portion of total soil respiration at the study site, increased respiration rates would be difficult to measure as increased fluxes in the field. Nonetheless, sustained warming could lead to significant loss of C from these peat layers.