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Soil Chemistry

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

^a Dep. of Soil, Water, and Environmental Science, Univ. of Arizona, 1177 E. Fourth St., P.O. Box 210038, Shantz Bldg. #38, Tucson, AZ 85721-0038
^b Lawrence Berkeley National Lab., Center for Isotope Geochemistry, One Cyclotron Road MS 90-1116, Berkeley, CA 94720
^c Land, Air and Water Resources Dep., Univ. of California, One Shields Avenue, Davis, CA 95616

^* Corresponding author (crasmuss{at}ag.arizona.edu)

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 organomineral association with Al-humus complexes and SRO Al minerals control the variation in soil C dynamics in these systems.

Abbreviations: AMS, accelerator mass spectrometry • AN, andesite-granite • DI, deionized • GR, granite • HIV, hydroxy-interlayered vermiculite • MRT, mean residence time • PSA, particle-size analysis • SPT, sodium polytungstate • SRO, short-range order • subscript d, citrate dithionite extract • subscript o, ammonium oxalate extract • subscript p, sodium pyrophosphate extract • XRD, x-ray diffraction

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				C. Rasmussen, R. J. Southard, and W. R. Horwath Soil Mineralogy Affects Conifer Forest Soil Carbon Source Utilization and Microbial Priming Soil Sci. Soc. Am. J., June 8, 2007; 71(4): 1141 - 1150. [Abstract] [Full Text] [PDF]

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