Monday, November 13, 2006 - 11:30 AM
119-4
Clay-Humic Complexes in Soil Microaggregates of a Prairie Soil.
Microaggregates (5 to 50 μm) in Midwestern prairie soils are composed primarily of intimate associations of diffuse filamentous humic substances and smectite. The humic material coats surfaces of the smectites and bridges from one smectite quasicrystal to another and between different locations on the same quasicrystal thus holding the microaggregates together. Sodium saturation and dispersion readily separates humic substances from the smectite surfaces, thus suggesting that the humic substances are bonded to smectite surfaces primarily by cation bridging involving polyvalent cations (primarily Ca). The humic substances associated with smectite surfaces in soil microaggegates have a C:N ratio of about 10 and are a mixture of alkyl, O-alkyl, and carboxylic C with little aromatic C. This humic material has modern radio C dates and is relatively bioavailable during incubations. Discrete particles of rigid organic C are physically associated with coarse clay particles (primarily quartz, feldspars, kaolinite and 10 Å-illite). Some of these discrete particles appear to be entrapped or enmeshed within the microaggregates while others exist as discrete particles separate from the microaggregates. The discrete C particles have a C:N ratio >15 and are dominated by single and condensed ring aromatic structures but also contain some aliphatic and carboxylic C. Radio C dates of the discrete C particles are old (70 to 730 YBP) and during incubations C in the discrete particles is biologically less available than the C associated with the smectite surfaces. These discrete C particles are believed to have a charcoal core that has adsorbed small biogenic organic molecules while in the soil.
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