HR: 1340h
AN: B13C-0229    [Abstracts]
TI: Assessing Impacts of Land Use Change on Carbon Dynamics via Soil Organic Matter Distribution and Stable Isotopic Composition
AU: * Billings, S
EM: sharonb@ku.edu
AF: University of Kansas, 2101 Constant Ave., Lawrence, KS 66047 United States
AB: The conversion of vast amounts of land in Midwestern North America from native tallgrass prairie to agricultural uses promoted the loss of large amounts of soil carbon (C) from this region. Largely due to fire suppression in the region, much rural land not used for crops is comprised of successional forests; many other parcels consist of non-native, cool season grasses. In an effort to assess the status of soil C recovery following agricultural use on these lands, this study explores the C distribution and dynamics in soils supporting two land cover types - cool season grasslands and successional forests. Soil from the top 15 cm in the mineral profile were collected from eight sites representing the same soil type and similar land use histories at the University of Kansas' Nelson Environmental Study Area, within the Kansas Field Station and Ecological Reserves. Four of the sites support cool-season grasses that are maintained by mowing; the remaining four sites support successional forest that has developed for at least 35 y. Soils were subjected to size fractionation, and all fractions as well as bulk soil were analyzed for total C, total nitrogen (N), and C and N isotopic signatures. Soils also underwent long-term incubations, during which cumulative respiration and net N mineralization were assessed. There was no difference between land cover types in bulk soil C or N. The largest size fraction (212 to 2000 mm) from grassland soil had higher total C (45.5\pm8.67 vs. 29.4\pm3.36 mg g$^{-1}$) and higher total N (3.0\pm0.4 vs. 2.3\pm0.2 mg g$^{-1}$) than forested soils' largest fraction; for soil aggregates sized 63 to 212 mm, grassland soil exhibited a near-significant trend of lower total C (P=0.06) and significantly lower total N (2.3\pm0.2 vs. 3.0\pm0.3 mg g$^{-1}$) than the corresponding forest soil fraction. The smallest size fraction ($<$63 mm) exhibited no difference in total C and N between land cover types. The largest size fractions from both land cover types exhibited similar \delta$^{13}$C signatures, reflective of the current C3 vegetation at all eight sites. Smaller size fractions reflected more of a C4 signature in the grassland soils, suggesting that these sites have not experienced soil organic matter turnover to the extent that forested soils have. Net N mineralization data from the incubations are consistent with this suggestion. After 35 years of forest development at these sites, forested soils appear better able to process organic matter into more recalcitrant fractions than cool season grasslands. In conjunction with the larger stocks of biomass C in forests compared to grasslands, these results suggest that if land management decisions permit, allowing forest succession to take place on these soils may be a sound practice for sequestering relatively more atmospheric C.
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