Distribution of cation exchange capacity between organic matter and mineral fractions in acid forest soils (Vosges mountains, France)

Abstract

SummaryThe cation exchange capacity (CEC) of 18 samples (<2 mm) of five acid soils developed on granite or gneiss was measured before and after H2O2 treatment from pH‐unbuffered extraction solution (1 N KCl). The pH in water of the soils ranged from 3.4–4.7. The samples were separated into seven fractions (<2 μm, 2–20 μm, 20–50 μm, 0.05–0.2 mm, 0.2–0.5 mm, 0.5–1 mm and 1–2 mm) and the CEC of each measured. Thus the contribution of each size fraction and of organic matter to the CEC was obtained. From CEC measurements on the different fractions and particle size distribution data, a balance calculation was carried out to verify the methodology. According to the hypotheses used and soil characteristics, the CEC of organic matter was found to vary from 35 to 165 cmolc kg−1 and so represented from 10 to 85% of the total soil CEC in the upper soil horizons. In these sandy soils developed on granite or gneiss which were frequently affected by hydrothermal alteration, the CEC of silt and sand fractions was large. It can represent from 35 to 80% of the total soil mineral CEC. The specific contribution of the 0.2–2 mm fraction can reach 50% of the total soil mineral CEC with values ranging from 0.5–5.2 cmolc kg−1. The mineralogical description showed that hydrothermal white micas and neoformed smectites (precipitated during weathering) were present in all the fractions even in the coarsest ones. In some subsoils, albite grains containing smectite have a CEC as large as 21 cmolc kg−1. This study shows that the CEC of acid soils was not always located in the organic matter and in the clay fraction. The sand fraction can contribute substantially to the soil CEC.