HR: 14:40h
AN: B53D-04    [Abstracts]
TI: Soil Carbon Stabilization During Tropical Reforestation
AU: * Marin-Spiotta, E
EM: aurios@nature.berkeley.edu
AF: University of California, Berkeley, Dept. of Environmental Science, Policy and Management 151 Hilgard Hall, Berkeley, CA 94705 United States
AU: Silver, W L
EM: wsilver@nature.berkeley.edu
AF: University of California, Berkeley, Dept. of Environmental Science, Policy and Management 151 Hilgard Hall, Berkeley, CA 94705 United States
AU: Ostertag, R
EM: ostertag@hawaii.edu
AF: University of Hawaii, Hilo, Department of Biology 200 W. Kawili St., Hilo, HI 96720 United States
AB: Land use change can affect global atmospheric carbon (C) concentrations by changing the quantity and residence time of C stored in plant biomass and in soils. Currently, large areas of agricultural and pastureland in the Neotropics are being abandoned and replaced by secondary forests. The prevalence of secondary forests has focused attention on reforestation as a potential C sink. Our research examines the importance of physical and chemical mechanisms of soil C storage during forest re-growth. Using a long-term successional chronosequence, we sampled replicate pastures, old growth forests, and forests re-growing on pastures abandoned 10, 20, 30, 60 and 80 years ago. We tested a two-compartment, isotopic mixing-model to determine contributions of C$_{3}$-C (forest) and C$_{4}$-C (pasture) to the total soil C pool at each site. There were no significant differences in the relative proportions of C$_{3}$ and C$_{4}$-C when using the isotopic values of fresh leaf litter, forest-floor leaf material or roots. Using site specific „$^{13}$C values or average pastures and old-growth forests as end-members also did not affect the estimates of soil C gain and loss. After 10 years of reforestation, half of the pasture-derived C was lost from the top 10 cm of mineral soil. In the following decades, the loss of C$_{4}$-C was slower, following an exponential decay model, so that all pasture-derived C would be lost after 100 years of forest re-growth. Loss of C$_{4}$-C was compensated by a gain of new forest C, resulting in no net change in soil C content down to 1 m depth during 80 years of succession. Although changes in the bulk soil C stock were undetectable, land use change can affect the distribution of different C fractions and hence, residence time. We have used a density fractionation procedure to separate the bulk soil C into a free light fraction, a free occluded fraction released after the disruption of aggregates, and a mineral-associated heavy fraction. The pasture sites had lower quantities of the light fractions as a percentage of total soil mass than the forest sites, suggesting that these fractions may be depleted faster in the pastures. Litter quality may be the controlling factor in determining turnover rates in pastures versus forests, as we found no significant differences in the structural stability of soil macroaggregates with land use, while litter chemistry did differ. Pasture tissues had significantly lower concentrations of hydrophobic compounds and lignin, which play an important role in soil C stabilization. At all sites, the lighter fraction tended to be depleted in „$^{13}$C relative to the mineral-associated C, suggesting a forest-C origin or a selective accumulation of plant compounds during decomposition. A better understanding of soil C dynamics during reforestation will enhance our capacity to rehabilitate degraded soils and improve our assessment of their role as C sinks.
DE: 1040 Isotopic composition/chemistry
DE: 1600 GLOBAL CHANGE (New category)
DE: 1615 Biogeochemical processes (4805)
DE: 0330 Geochemical cycles
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting