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EFFECTS OF ELEVATED CO2 AND TEMPERATURE ON SOIL CARBON DENSITY FRACTIONS IN A DOUGLAS FIR MESOCOSM STUDY
JOHNSON, M. G. AND P. T. RYGIEWICZ. EFFECTS OF ELEVATED CO2 AND TEMPERATURE ON SOIL CARBON DENSITY FRACTIONS IN A DOUGLAS FIR MESOCOSM STUDY. Presented at 2nd International Conference on Mechanisms of Organic Matter Stabilization and Destabilization in Soils, Asilomar, CA, October 09 - 13, 2005.
We conducted a 4-year full-factorial study of the effects of elevated atmospheric CO2 and temperature on Douglas fir seedlings growing in reconstructed native forest soils in mesocosms. The elevated CO2 treatment was ambient CO2 plus 200 ppm CO2. The elevated temperature treatment was ambient temperature plus 4 degrees C. The CO2 source was depleted in 13C and provided a stable isotope tracer of newly fixed C. At the end of the study soil samples were collected by horizon and density fractionated. Density fractionation provides a means of separating kinetic C fractions with the denser fractions representing older more recalcitrant, mineral associated C. Soil C concentration and �13C were measured. C concentration decreased both with depth in the soil and as the fraction density increased. There was a 30 difference between the �13C of the light and heavy density fractions with the heavy fractions being more enriched in 13C. Elevated CO2 led to an increased depletion in 13C in the uppermost horizons and in the lightest density fractions. Elevated CO2 appears to affect the chemistry of the lightest density fractions, which are more like plant material and may be more rapidly cycling compared to the C in the heavier density fractions. Overall, elevated CO2 had no effect on total soil C while elevated temperature led to an increase in total soil C.