Influence of allophane and organic matter contents on surface properties of Andosols

The aim of this study was to use nitrogen gas adsorption to study the complex surface properties of a wide range of Andosol Ah and Bw horizons; N₂ gas adsorption not only provides specific surface area, SSA, but it also yields complementary information about micropore volume and hydrophilic and/or hydrophobic surface properties. Total SSAs were positively related to micropore SSA which was, in its turn, positively interrelated to the dimensionless parameter C of the Brunauer‐Emmett‐Teller (BET) equation (micropore N₂ filling) and microporous allophane content. The more allophanic the Andosol horizon sample, the larger were its total, micropore and mesopore SSAs. On the other hand, strong negative exponential relationships were obtained between either total or micropore or mesopore SSA and soil organic carbon content, with a SSA threshold at an organic carbon content of c. 8–10%, the SSAs being extremely small at larger organic carbon content values. Both SSA_BET and C_BET decreased non‐linearly as a function of the organic C/allophane ratio, with either a SSA_BET or a C_BET threshold at an organic C/allophane ratio value of 3–5, above which both SSA_BET and C_BET were very small (mostly the aluandic Andosol Ah horizons). The more the soil allophanes are assumed to be coated, the more hydrophobic the soil surfaces become and the smaller both SSA_BET and C_BET become; nitrogen gas has a permanent quadrupole moment and therefore acts as a polar probe when surfaces are sufficiently hydrophobic. The antagonistic roles played by allophanes and organic matter in both the SSAs and the values of the dimensionless parameter C in the BET equation were also highlighted by (i) multiple linear regressions between the SSAs and both allophane and organic carbon contents and (ii) principal‐component analysis of SSA_BET, C_BET and soil constituent (allophane, ferrihydrite and organic C) contents.