HR: 14:55h
AN: B12E-06    [PDF]
TI: Soil Mineralogical Control of Aggregate-Protected Carbon in a Mature Secondary Conifer Forest in Northern California
AU: * Rasmussen, C
EM: crasmussen@ucdavis.edu
AF: University of California, Davis, Soil Science Graduate Group Land, Air, and Water Resources Department One Shields Avenue, Davis, CA 95616 United States
AU: Torn, M S
EM: mstorn@lbl.gov
AF: Center for Isotope Geochemistry Lawrence Berkeley National Laboratory, One Cyclotron Road MS 90-1116, Berkeley, CA 94720 United States
AB: Forest systems have the potential to act as sinks for atmospheric carbon dioxide, as shown by the role of secondary forest growth in the North American C sink. We investigated the role of soil mineralogy as a key parameter in controlling long-term soil C storage and aggregate-protection of C in secondary growth forests of California. Soil pedons were sampled to a depth of one meter on granitic and andesitic parent materials in 80 year old ponderosa pine forests. Soil samples were characterized for total C and N, pH, clay content and clay mineralogy. The latter were determined by X-ray diffraction (XRD) and selective dissolution (SD) techniques. The andesitic soils contain 50% more C than the granitic soils (19.6 vs 12.8 kg m-2) mainly due to significant differences in surface horizon C content. No significant difference was observed in subsurface C concentration between parent materials. XRD data indicates only slight differences in clay species between parent materials. In both soils, clay crystallinity increases with depth, with an increased dominance of kaolinite and gibbsite, while amorphous material decreases with depth. The main soil mineralogy difference between the parent materials is greater crystalline (38.7 vs 22.8 g kg-1) and short range order (1.0 vs 0.5 g kg-1) iron oxide content in the andesite derived soils. Chemical and physical indices of C differed significantly between parent materials. Both soils had similar amounts of Na-pyrophosphate (pH 10) extractable C (Cp), but significantly more Cp in the andesite soil was stabilized in Al and Fe organo-metal complexes ([Alp+Fep]/Cp; 1.0 vs 0.7). In addition, a greater portion of the total C was contained in aggregate fractions in the andesitic soils (17.8 vs 9.4 gC kg soil-1). Radiocarbon analyses will be carried out on three C fractions separated by density and ultrasonic dispersion techniques to isolate inter-aggregate, intra-aggregate and organo-mineral pools for modeling C dynamics. Preliminary data suggest a longer mean residence time of C in andesite than granite derived soils. We hypothesize that the enhanced aggregate C storage and slower turnover time of C in andesitic soils is a function of iron oxides acting to stabilize soil aggregates and of short range order aluminosilicates providing greater reactive surface area to bind organic compounds. These results suggest that the potential for natural soil C storage and sequestration management in secondary growth forests may be sensitive to parent material and soil development.
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting