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

Abstract

Abstract Radiocarbon signatures (Δ 14 C) of carbon dioxide (CO 2 ) provide a measure of the age of C being decomposed by microbes or respired by living plants. Over a 2‐year period, we measured Δ 14 C of soil respiration and soil CO 2 in boreal forest sites in Canada, which varied primarily in the amount of time since the last stand‐replacing fire. Comparing bulk respiration Δ 14 C with Δ 14 C of CO 2 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 Δ 14 C of respired CO 2 indicated marked variation in respiration sources in space and time. The 14 C signature of respired CO 2 respired from O horizon decomposition depended on the age of C substrates. These varied with time since fire, but consistently had Δ 14 C greater (averaging ∼120‰) than autotrophic respiration. The Δ 14 C of autotrophically respired CO 2 in young stands equaled those expected for recent photosynthetic products (70‰ in 2003, 64‰ in 2004). CO 2 respired by black spruce roots in stands >40 years old had Δ 14 C 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 (<40 years since fire) stands, increasing to ∼50% in mature stands. This is a minimum for total heterotrophic contribution, since mineral soil CO 2 had Δ 14 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 Δ 14 C of soil respiration in younger successional stands dropped below those of the atmospheric CO 2 .