Affiliation: | (1) United States Department of Agriculture-Forest Service, North Central Research Station, 410 MacInnes Drive, Houghton, MI 49931, USA;(2) Department of Forest, Rangeland, and Watershed Stewardship and Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO 80523, USA;(3) United States Department of Agriculture-Forest Service, Rocky Mountain Research Station, 240, West Prospect Street, Fort Collins, CO 80526, USA;(4) Department of Natural Resources conservation, Holdsworth Natural Research Center, University of Massachusetts, Amherst, MA 01003, USA;(5) Department of Natural Resources and Environmental Management, University of Hawaii at Manoa, Honolulu, HI 96822, USA |
Abstract: | Only a small fraction of the carbon (C) allocated belowground by trees is retained by soils in long-lived, decay-resistant forms, yet because of the large magnitude of terrestrial primary productivity, even small changes in C allocation or retention can alter terrestrial C storage. The humid tropics exert a disproportionately large influence over terrestrial C storage, but C allocation and belowground retention in these ecosystems remain poorly quantified. Using mass balance and 13C isotope methods, we examined the effects of afforestation and fertilization, two land-use changes of large-scale importance, on belowground C cycling at a humid tropical site in Hawaii. Here we report that in unfertilized plots, 80% of the C allocated belowground by trees to roots and mycorrhizae was returned to the atmosphere within 1 year; 9% of the belowground C flux was retained in coarse roots and 11% was retained as new soil C. The gains in new soil C were offset entirely by losses of old soil C. Further, while fertilization early in stand development increased C storage in the litter layer and in coarse roots, it reduced by 22% the flux of C moving through roots and mycorrhizae into mineral soils. Because soil C formation rates related strongly to rhizosphere C flux, fertilization may reduce an already limited capacity of these forests to sequester decay-resistant soil C. |