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.