Carbon mobilization from the forest floor under red spruce in the Northeastern U.S.A.

Global climate change may alter soil temperature and moisture conditions, increasing the need to understand how these basic factors affect C dynamics. This is particularly important in boreal forests, which often have large C pools in the forest floor and mineral horizons. We examined the effects of temperature and precipitation frequency on C dynamics in forest floor horizons from eight red spruce sites in the northeastern U.S. using column leaching experiments. Intact and sieved forest floor samples were incubated at 3, 10 or 20°C and leached either daily, once per week, or twice per week during 14 to 39 days using simulated throughfall solutions (pH 2.7 or 4.0). Leachate DOC and CO₂ production were measured along with soil C and N concentrations. For intact samples, losses of C as DOC and as CO₂ increased with increasing temperature, and the increase (Q₁₀) was usually greater between 3 and 10°C than between 10 and 20°C. There was a greater response of CO₂ to temperature than of DOC (e.g. Howland sieved soil Q₁₀s of 1.9 and 7.2 for CO₂ and 1.5 and 2.0 for DOC at 3-10 and 10-20°C ranges, respectively). More frequent leaching increased steady state DOC mobilization (e.g. 145 and 58 μg g^-1 forest floor d^-1 for daily and weekly leachings at 10°C, respectively), but not CO₂ evolution (e.g. 79 and 74 μg CO₂-C g^-1 forest floor d' 'for daily and weekly leachings at 10°C, respectively). Across the eight sites DOC loss and CO₂ evolution varied by factors of 3.6 and 4.0, respectively. Both CO₂ evolution and DOC in leachates calculated as fluxes were correlated (r = 0.73 and 0.87 respectively, n = 8) with the C-to-N ratios of the samples (C-to-N ratios ranged from 27 to 58), which could be explained by N limitations that triggered selective lignin degradation, differences in degree of humification of the material, or position on a west-to-east pollution gradient. Although higher temperatures and more frequent leaching increased DOC mobilization, and higher temperatures increased CO₂ evolution, both treatments and site to site variation illustrate the complexity of the response of forest-floor C pools to manipulations.