Charge characteristics of forest soils derived from sedimentary rocks in Kinki District, Japan, in relation to pedogenetic acidification process

Abstract

Charge characteristics of forest soils derived from sedimentary rocks in the Kinki District, western Japan, were investigated with special reference to the process of pedogenetic acidification. Charge characteristics of the soils were analyzed based on the coefficients a, b, and c in the regression equation log CEC = a pH + b log C + c, after CEC determination at different ionic concentrations (C) and pH values. The values of (a + b) /2 and c of the B horizons of forest soils with a thermic soil temperature regime (TSTR) were similar to those of Ultisols or Oxisols, whereas the values for the B horizons of forest soils with a mesic soil temperature regime (MSTR) were comparable to those of the B horizons of Andisols. Unlike in some Oxisols or Andisols, the variable positive charge, or AEC, of the soils studied here was very low even in the low pH range, presumably because it was neutralized by the permanent negative charge of 2 : 1 minerals in the soils. Multiple regression followed by principal component analysis revealed that the variable charge characteristics of the soils were the highest among soils with high scores for the “amorphous materials” factor and low scores for the “acidity” factor. In contrast, the CEC of the soils increased with a decrease in the “amorphous materials” factor and an increase in the “acidity” factor. Since removal of organic matter did not affect appreciably the CEC, the variable charge characteristics were assumed to be derived from amorphous sesquioxides and / or interlayered materials. It is concluded that the processes of pedogenetic acidification directly affected the charge characteristics of these forest soils as follows: 1) During the formation of the B horizons, accumulation of amorphous compounds and / or interlayered Al hydroxides resulted in the development of a variable charge. Amorphous compounds, which were preferentially formed in the B horizon of soils with MSTR, contributed significantly to the development of variable charge characteristics. 2) During extensive soil acidification, the E and upper B horizons of the forest soils occasionally lost amorphous and / or interlayered compounds. As a result, the permanent negative charge of the expansible 2 : 1 minerals became predominant in these soils.