Mineral assemblage and aggregates control carbon dynamics in a California conifer forest

Uncertainty about the effects of climate change on terrestrial soil organic 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. Granite and AN soils contained similar clay mineral assemblages as determined by x-ray diffraction (XRD), dominated by vermiculite, hydroxy-interlayered vermiculite (HIV), 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. Andesite-granite 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 (MRT) > 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.