HR: 1340h
AN: B13C-0230    [Abstracts]
TI: Response of Soil Organic Carbon to Woody Plant Encroachment: Evidence from Organic Matter Fractionations and Stable Isotopes
AU: * McCulley, R L
EM: mcculley@duke.edu
AF: Duke University, Dept. of Biology, Durham, NC 27708 United States
AU: Jobbagy, E G
EM: jobbagy@agro.uba.ar
AF: CONICET-INTA San Luis, Ruta 7 & Ruta 8 Villa Mercedes 5730, San Luis, 5730 Argentina
AU: Pockman, W T
EM: pockman@unm.edu
AF: University of New Mexico, Dept. of Biology, Albuquerque, NM 87131-1091 United States
AU: Jackson, R B
EM: jackson@duke.edu
AF: Duke University, Dept. of Biology, Durham, NC 27708 United States
AU: Jackson, R B
EM: jackson@duke.edu
AF: Duke University, Nicholas School of the Environment and Earth Sciences, Durham, NC 27708 United States
AB: Soil organic carbon (SOC) storage has been shown to increase, decrease, or exhibit no net change following woody plant encroachment into grasslands. This variability in total SOC response to woody plant encroachment may be the result of climate, soil texture, and species characteristics interacting to impact particular pools of SOC in different ways. Among studies that have found an increase or no net change in SOC with woody plant encroachment, relatively fast cycling, particulate organic matter (POM) pools in shallow soils experience the largest alterations in C content and stable isotope composition. To assess whether this trend is supported where woody plant encroachment is accompanied by losses of SOC, we examined POM fractions from soils collected in sub-humid grassland and adjacent woodland at a Texas site where woody plant dominance has been shown to result in a $\sim$45% loss of SOC. Similar to previous work, the POM fraction $>$500 $\mu$m in size in the woodland contained more C at shallow depths (0-25 cm) than the grassland soil (730 and 290 g C m$^{-2}$ for woodland and grassland, respectively) and was isotopically similar (average $\delta$$^{13}$C = -27.2 %) to woody inputs at the site. At all depths in the woodland, POM pools and total SOC exhibited more negative $\delta$$^{13}$C values than adjacent grassland. Woodland POM pools $>$53 $\mu$m contained as much or more C than the grassland soil; however, the C content of the $>$53 $\mu$m SOC pool was significantly reduced. These results indicate that despite substantial incorporation of woody C inputs into SOC, loss of SOC with woody plant encroachment at this site results from decomposition of grassland derived organic matter residing in the smallest, most recalcitrant SOC pool.
DE: 1600 GLOBAL CHANGE (New category)
DE: 1615 Biogeochemical processes (4805)
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting