Temporal Changes in Soil Carbon Storage Across a Managed Red Spruce Forest Chronosequence Reveal Low Carbon Stabilization Capacity of a Podzol Soil

Forest harvesting alters the long term balance of inputs and outputs, potentially affecting the quantity of carbon stored in a forest soil. Using a successional chronosequence, we sampled the top 50 cm of mineral soil in a mature old growth forest, an immature old growth forest, a 45 year old clearcut, a 15 year old clearcut, and a recent clearcut all separated by no more than 5 km and differing only in their time since harvest. Soils sampled from each site were subjected to a density fractionation method to separate soil carbon into three fractions: a free light fraction, an intra-aggregate or occluded light fraction, and a mineral associated heavy fraction. Fractions were analyzed for % C and, δ13C, and carbon storage was estimated for each fraction. Temporal trends in storage for the whole soil and all fractions described using a gamma function, showed that losses in the whole soil reached a minimum 30 years post harvest, at which point they were approximately 50% of the reference condition. In each of the three fractions losses were of a similar magnitude but minima occurred at different times post harvest. Losses were greatest below the dominant rooting zone (>20 cm) and in the mineral associated heavy fraction and stable isotopic signatures suggest accelerated decomposition as the mechanism of loss. Our findings are consistent with a low stabilization capacity and suggest that a significant proportion of the soil carbon pool in the top 50 cm of mineral soil is capable of cycling on decadal time scales in response to environmental perturbations.