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

Radiocarbon signatures (DC) 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 DC 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 DC with DC 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 DC of respired CO2 indicated marked variation in respiration sources in space and time. The C 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 DC greater (averaging 120%) than autotrophic respiration. The DC of autotrophically respired CO2 in young stands equaled those expected for recent photosynthetic products (70% in 2003, 64% in 2004). CO2 respired by black spruce roots in stands 440 years old had DC up to 30% 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 (o40 years since fire) stands, increasing to 50% in mature stands. This is a minimum for total heterotrophic contribution, since mineral soil CO2 had D C 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 DC of soil respiration in younger successional stands dropped below those of the atmospheric CO2.