Monday, November 13, 2006 - 11:00 AM
119-2

Soil Mineral Assemblage and Aggregates Control Soil Carbon Dynamics in a California Conifer Forest.

Craig Rasmussen, Univ of Arizona, Soil, Water, and Environmental Science, 520 Shantz Bldg, PO Box 210038, Tucson, AZ 85721

Uncertainty about the effects of climate change on terrestrial soil organic carbon (C) stocks has generated interest in clarifying the processes that underlie soil C dynamics.  We investigated the role of soil mineralogy and aggregate stability as key variables controlling soil C dynamics in a California conifer forest.  We characterized soils derived from granite (GR) and mixed andesite-granite (AN) parent materials from similar forest conditions.  GR and AN soils contained similar clay mineral assemblages as determined by X-ray diffraction (XRD), dominated by vermiculite, hydroxy-interlayered vermiculite, kaolinite and gibbsite.  However, AN soils contained significantly more Al in Al-humus complexes (6.2 vs. 3.3 kg m-2) and more crystalline and short range order (SRO) Fe oxyhydroxides (30.6 vs. 16.8 kg m-2) than GR soils.  AN pedons contained nearly 50% more C relative to GR soils (22.8 vs. 15.0 kg m-2).  Distribution of C within density and aggregate fractions (free, occluded, and mineral associated C) varied significantly between AN and GR soils.  In particular, AN soils had at least twice as much mineral associated C relative to GR soils in all horizons.  Based on 14C measurements, occluded C mean residence time > mineral C > free C in both soil types, suggesting a significant role for aggregate C protection in controlling soil C turnover.  We found highly significant, positive correlations between Al-humus complexes, SRO Al minerals and total C content.  We suggest that a combination of aggregate protection and organo-mineral association with Al-humus complexes and SRO Al minerals control the variation in soil C dynamics in these systems.