Organic carbon stabilization in the fractal pore structure of Andosols

Despite the outstanding potential of Andosols for soil organic carbon (SOC) sequestration, the importance of the different mechanisms involved in protecting SOC against mineralization is still unclear. Previous studies have described allophane particles as having a fractal structure. One hypothesis is that SOC is adsorbed or trapped in this fractal structure and could be less available to microbes and enzymes. This paper aims to determine the fractal structure of allophanic soils, especially in mesopores (2–50 nm), and to establish a correlation between the SOC bioavailability and the fractal features of the allophane aggregates. The present study confirms the fractal structure of allophane aggregates. The structural features of the pores, measured using N2 adsorption–desorption curves and SAXS for the nano-scale range, showed correlation with SOC bioavailability. SOC bioavailability decreased with the extent of the fractal structure.

Research highlights
► Allophane aggregates have large mesoporous volumes (scale 2–50 nm).
► Allophane aggregates have a fractal structure (2–50 nm).
► SOC bioavailability decreases with the extent of this fractal structure.
► This peculiar mesopore structure can explain the high SOC stocks in Andosols.