COMPARISON OF PARTITION COEFFICIENTS FOR 54Mn AND SOIL-EXTRACTABLE Mn, INCLUDING RELATIONSHIP TO PLANT UPTAKE

Abstract

The objective of this study was to compare the values, statistical characteristics and relationships of partition coefficients (Kd) computed from a short-term study of 54Mn sorption with those computed from DTPA extractions of soil Mn. Of particular interest were the frequency distributions, which must be very well known for use in stochastic modelling of environmental impacts. The study used 69 soils. Sorption of 54Mn, applied with stable 54Mn carrier, was measured after 24 h equilibration in suspension. The Kd for 54Mn was based on the single measurement of 54Mn in solution. Total soil Mn and DTPA + NaHCO3-extractable Mn were also measured and used to compute a Kd from two measurements. The frequency distributions of Kd values measured by either method were strongly skewed and conformed to lognormal, regardless of the statistical properties of the underlying measurements. The geometric mean Kd for 54Mn was 44 L kg−1 and was highest for high pH, high clay content and high organic matter content soils, as expected. The geometric mean Kd for DTPA-extractable Mn was 1070 L kg−1 and was not related in the same way to the soil properties. Clearly, the two Kd measurements were quite different. A significant correlation (r = 0.54, P < 0.001) between the Kd for 54Mn and plant Mn concentrations confirmed that this relationship is useful for modelling applications. The sorption of 54Mn was the best single measurement to predict the plant-availability of soil Mn, determined in a previous study. However, in concert with pH and clay content data, the DTPA + NaHCO3-extractable Mn gave the best correlation to plant uptake. Key words: Kd, frequency distribution, radionuclides, soil test, isotopic exchange