HR: 0830h
AN: B31D-0330    [PDF]
TI: Woody Plant Invasion of Grassland: Changes in Whole-Soil Carbon Storage and Dynamics
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: 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
AB: Woody plant encroachment into grass-dominated ecosystems has been a globally significant land cover change over the past century, but its biogeochemical consequences remain largely unquantified. In the Rio Grande Plains of Texas, grasslands and savannas dominated by C4 grasses (d13C = -14 o/oo) have undergone succession over the past 150 y to subtropical thorn woodlands dominated by C3 trees/shrubs (d13C = -27 o/oo). To evaluate soil organic carbon (SOC) storage and dynamics in these woodlands, we measured the mass and isotopic composition (d13C) of SOC in chronosequences consisting of remnant grasslands (Time 0) and woody plant stands ranging in age from 10-130 y in upland (clusters and groves) and lowland (drainage woodlands) landscape elements. Carbon storage in surface litter increased linearly from 100 g C/m2 in woody plant stands less than 30 y old to nearly 500 g C/m2 in stands more than 60 y old. Similarly, carbon storage in root biomass (0-30 cm) increased linearly from 400 g C/m2 in remnant grasslands to more than 2000 g C/m2 in some of the older (i.e. 60-130 y) woody plant stands. Increased litter and root carbon were associated with significant increases in SOC storage (0-30 cm) from 2000 g C/m2 in grasslands to more than 5000 g C/m2 in older woodlands. Over the past century, SOC accumulation rates (0-30 cm) have ranged from 12.5 g C/m2/y in upland clusters to 41.5 g C/m2/y in low-lying drainage woodlands. d13C of SOC at 0-15 cm decreased exponentially from -18 o/oo in grasslands to -23 o/oo in woodlands older than 60 y. At 15-30 cm, d13C of SOC decreased from -15 o/oo in grasslands to -19 o/oo in woodlands older than 60 y. These C-13 dynamics yielded SOC mean residence times of 40-80 y at 0-15 cm and 150-500 y at 15-30 cm, consistent with results from C-14 dating. Therefore, soils in this region have been a net sink for atmospheric carbon over the past century. Most of this newly sequestered carbon is stored in the upper 15 cm of the profile and has mean residence times less than 100 y. Given the geographically extensive nature of this vegetation shift, changes in SOC dynamics and storage documented here could have significance for regional and global carbon cycles.
DE: 0400 Biogeosciences
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
DE: 1851 Plant ecology
SC: Biogeosciences [B]
MN: 2003 Fall Meeting