Effect of granite crystal grain size on soil properties and pedogenic processes along a lithosequence

• Two acid soils formed from granite of different crystal grain sizes were compared.
• The mineralogical and chemical compositions of the two granites were identical.
• The pedogenesis diverged among the two soils only due to the particle size distribution.
• The hydraulic properties were modified and played a major role in mineralogical weathering.
• The soil carbon, the nutrients and the aluminium dynamic of the soils were impacted.

This study generated new insight into the effect of bedrock grain sizes on pedogenesis under identical topographic and climatic environment. The physico-chemical and mineralogical properties of two adjacent Typic Dystrochrepts (USDA, 1999) respectively developed from fine and coarse textured granites were compared. This research study was performed in the Morvan Mountains (France). Analysis of the two underlying bedrocks revealed similar chemical and mineralogical properties, with the crystal grain size being the only parameter which differed. In these soils, bedrock played the key role in the particle size fraction distribution, the main factor controlling water retention in the soils. Weathering reactions of clay minerals were more marked in the fine textured granite soil. In this soil, both clay mineral swelling and mica transformation into expansible phyllosilicates were greater compared to the coarse textured granite soil. In the clay fraction of the fine textured granite soil, there were smaller amounts of low crystallised Fe and Al minerals, with higher carbon content in the topsoil, as compared to the coarse granite soil. The exchangeable cation analysis of the fine textured granite soil revealed a higher proportion of base saturation and a smaller proportion of Altit than in coarse textured granite soil which could be explained by preferential leaching of Al cations. The decrease in exchangeable Al, the higher swelling of the smectitic layer and the smaller proportion of Fe and Al-oxy-hydroxides could be explained by the complexing acid conditions, which increased mineral weathering and Al leaching. The higher water retention increased the time of contact between minerals and the soil solution, and the higher carbon content in the topsoil could have enhanced the complexing acid conditions.