Variability of 40K isotopic composition in forest soils under different environmental conditions

Radiopotassium isotopic composition (40K/K, %) of several forest soils did not show a constant value of generally known 1.17 × 10−2 %, but they were varied significantly from 0.4 × 10−2 to 1.3 × 10−2 % at different locations under different environmental conditions. Surface portion of a soil (2–4 cm in depth) gave always lower 40K/K values compared with those of deeper soil layer (35–40 cm in depth). Ion exchange of K+ with NH4+ did not affects the 40K/K value in any soils, which revealed with chemical leaching experiments in the laboratory. Some plant species showed much lower 40K/K values than those in the surface soil. Possible reasons for varying 40K/K values obtained in this study may result from a dynamic behavior of potassium in soil, probably due to biological activity including root uptake and decomposing soil organic matter by microorganisms in the forest floor.