Fractal structure in natural gels: effect on carbon sequestration in volcanic soils

Allophanic soils are interesting in terms of environmental properties especially because of their potentialities as sinks for “greenhouse gases” by the way of C sequestration. These volcanic soils contain amorphous clays (allophanes) and exhibit higher organic carbon content than the one measured in other clay soils. We measured the C content of a set of allophanic soils and showed that the C content is positively correlated to the allophane content. We also measured the part of organic matter transformed into CO₂ during a respiration experiment and showed that the decomposition is lowered as the soils allophane content increases. Allophane aggregates are very close to the synthetic gels: high specific surface area large pore volume, fractal structure, large water content and important irreversible shrinkage during drying. In this work we characterized by Small Angle X-Ray Scattering (SAXS) the fractal structure of the allophane aggregates at the nano scale. We hypothesized that the peculiar structure and the associated low accessibility of the allophanic soils could explain the high organic carbon content and the associated poor transformation into CO₂. The tortuous structure of the allophane aggregates plays the role of a labyrinth which fix and traps soil organic carbon.