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. The ¹⁴C signature of respired CO₂ respired from O horizon decomposition depended on the age of C substrates. These varied with time since fire, but consistently had Δ¹⁴C greater (averaging ~120[per thousand]) than autotrophic respiration. The Δ¹⁴C of autotrophically respired CO₂ in young stands equaled those expected for recent photosynthetic products (70[per thousand] in 2003, 64[per thousand] in 2004). CO₂ respired by black spruce roots in stands >40 years old had Δ¹⁴C up to 30[per thousand] higher than recent photosynthates, indicating a significant contribution of C stored at least several years in plants. Decomposition of O horizon organic matter made up 20% or less of soil respiration in the younger (<40 years since fire) stands, increasing to ~50% in mature stands. This is a minimum for total heterotrophic contribution, since mineral soil CO₂ had Δ¹⁴C close to or less than those we have assigned to autotrophic respiration. Decomposition of old organic matter in mineral soils clearly contributed to soil respiration in younger stands in 2003, a very dry year, when Δ¹⁴C of soil respiration in younger successional stands dropped below those of the atmospheric CO₂