CARBON MOBILIZATION FROM THE FOREST FLOOR UNDER RED SPRUCE IN THE NORTHEASTERN USA

Global climate change may alter soil temperature and moisture conditio ns, increasing the need to understand how these basic factors affect C dynamics. This is particularly important in boreal forests, which oft en have large C pools in the forest floor and mineral horizons. We exa mined the effects of temperature and precipitation frequency on C dyna mics in forest floor horizons from eight red spruce sites in the north eastern U.S. using column leaching experiments. Intact and sieved fore st floor samples were incubated at 3, 10 or 20 degrees C and leached e ither daily, once per week, or twice per week during 14 to 39 days usi ng simulated throughfall solutions (pH 2.7 or 4.0). Leachate DOC and C O2 production were measured along with soil C and N concentrations. Fo r intact samples, losses of C as DOC and as CO2 increased with increas ing temperature, and the increase (Q(10)) was usually greater between 3 and 10 degrees C than between 10 and 20 degrees C. There was a great er response of CO2 to temperature than of DOC (e.g. Howland sieved soi l Q(10)s of 1.9 and 7.2 for CO2 and 1.5 and 2.0 for DOC at 3-10 and 10 -20 degrees C ranges, respectively). More frequent leaching increased steady state DOC mobilization (e.g. 145 and 58 mu g g(-1) forest floor d(-1) for daily and weekly leachings at 10 degrees C, respectively), but not CO2 evolution (e.g. 79 and 74 mu g CO2-C g(-1) forest floor d( -1) for daily and weekly leachings at 10 degrees C, respectively). Acr oss 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 calcu lated 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 selec tive lignin degradation, differences in degree of humification of the material, or position on a west-to-east pollution gradient. Although h igher temperatures and more frequent leaching increased DOC mobilizati on, and higher temperatures increased CO? evolution, both treatments a nd site to site variation illustrate the complexity of the response of forest-floor C pools to manipulations. Copyright (C) 1996 Elsevier Sc ience Ltd