HR: 0830h
AN: B31D-0331    [PDF]
TI: Woody Plant Invasion of Grassland: Storage and Turnover of Carbon in Soil Physical Fractions
AU: * Liao, J D
EM: jade@neo.tamu.edu
AF: Department of Rangeland Ecology and Management, Texas A&M University, College Station, TX 77843-2126 United States
AU: Boutton, T W
EM: boutton@neo.tamu.edu
AF: Department of Rangeland Ecology and Management, Texas A&M University, College Station, TX 77843-2126 United States
AU: Jastrow, J D
EM: jdjastrow@anl.gov
AF: Environmental Research Division, Argonne National Laboratory, Argonne, IL 60439 United States
AB: Woody plant encroachment is common in many grassland and savanna regions around the world. In the Rio Grande Plains of southern Texas, subtropical thorn woodlands dominated by C3 trees and shrubs (d13C = -27 o/oo) have largely replaced C4 grasslands (d13C = -14 o/oo) over the past 150 y. This vegetation change has resulted in increased soil organic carbon (SOC) storage. To elucidate mechanisms of SOC sequestration and turnover in this system, we separated soil organic matter into specific physical (size, density) fractions and determined natural d13C values of carbon in these fractions. Using a chronosequence approach, soils were collected from remnant grasslands (Time 0) and from woody plant stands ranging in age from 10-130 y. The free light-fraction (density less than 1 g/cm3) increased from 1 o/o of whole-soil weight in grasslands to 2-4 o/o of whole-soil weight in wooded landscape elements. The macroaggregate fraction (greater than 250 um) of soil in the 0-15 cm depth increment increased from less than 10 o/o of whole-soil dry weight in grasslands to more than 30 o/o of whole-soil weight in older woodlands. In contrast, the microaggregate fraction (53-250 um) decreased from 80 o/o of whole-soil weight in grasslands to 60 o/o of whole-soil weight after 80-130 y of woodland development. The decrease in microaggregates with increasing stand age likely reflects their incorporation into the macroaggregate fraction. Carbon contents (g C within a fraction per kg of whole soil) of all soil physical fractions (except free silt and clay) increased linearly with increasing woodland age, and were greatest in macroaggregates. Based on changes in natural C-13 abundance, mean residence times (1/k) for microaggregate carbon (326 y) were significantly greater than those for macroaggregate carbon (76 y), indicating that the older carbon associated with microaggregates is biochemically recalcitrant and/or physically protected. These results indicate that the interactions between SOC and soil aggregation may provide a mechanistic explanation for carbon processes and dynamics following land cover changes in terrestrial ecosystems.
DE: 0400 Biogeosciences
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
DE: 1851 Plant ecology
SC: Biogeosciences [B]
MN: 2003 Fall Meeting