A MODEL FOR THE IMPACT OF SOIL SOLUTION CA-AL RATIO, SOIL-MOISTURE AND TEMPERATURE ON TREE BASE CATION UPTAKE

A model for tree base cation uptake has been developed, dependent on the soil solution concentration of Al3+, divalent base cations such as Ca2+, Mg2+ and H+ ions, modelled with a Mikaelis-Menten type of expression based on the molar BC:Al ratio, where BC is the sum of the divalent non-toxic base cations Ca2+ and Mg2+. The expression has the form of counteracting adsorption isoterms for BC and Al to the tree root. The effect of toxic levels of Hg and Pb is incorporated into the model, using root adsorption as the mechanism, and parameterization against experimental data. The expression is modified with an expression accounting for the effect of heavy metal toxicity and soil water content. The dependence of the uptake rate on soil moisture content can be shown to have the form of a Freundlich adsorption isotherm for water. The available data indicate an activation energy of 47 kJ-1 mol for base cation uptake to trees. Data from the literature was used to estimate the rate coefficients and ion selectivity coefficients for typical coniferous and decideous trees in Sweden and Germany. The model indicates that Ca2+ and Mg" is effective in mitigating Al3+ toxicity to tree roots, and that increasing the Ca2+ + Mg2+ soil solution concentration has a promotive effect on base cation uptake. Above a certain limit base cation uptake becomes independent of the solution base cation concentration. This is consistent with field observations, and may be developed to become a tool for assessing the impact of soil chemistry changes on forest growth rate and health status. Field data from the Swedish Forest Survey indicate that uptake depend on the square root of the soil solution base cation availability originating from weathering and deposition input, which is consistent with the BC:Al expression of the model.