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Changing sources of soil respiration with time since fire in a boreal forest

Radiocarbon signatures (Δ14C) of carbon dioxide (CO2) provide a measure of the age of C being decomposed by microbes or respired by living plants. Over a 2‐year period, we measured Δ14C of soil respiration and soil CO2 in boreal forest sites in Canada, which varied primarily in the amount of time since the last stand‐replacing fire. Comparing bulk respiration Δ14C with Δ14C of CO2 evolved in incubations of heterotrophic (decomposing organic horizons) and autotrophic (root and moss) components allowed us to estimate the relative contributions of O horizon decomposition vs. plant sources. Although soil respiration fluxes did not vary greatly, differences in Δ14C of respired CO2 indicated marked variation in respiration sources in space and time.