COMPARING MODELED AND OBSERVED EFFECTS OF ASH ADDITIONS ON CHEMISTRY OF A HIGHLY ACID SOIL

The chemical composition of soil solutions (field percolates collected in situ and laboratory saturation extracts) and the amount of salt-ex tractable cations were measured at several microsites (unburnt, modera tely burnt, and intensely burnt ashbeds) after a fuel reduction burnin g in a subalpine Eucalyptus pauciflora forest. Soil samples were colle cted 1, 58, 375, 745, and 1095 days after the fire, and soil percolate s were obtained on 17 occasions during the initial year. A model of co upled equilibria, which includes insoluble salts, multiple cation exch ange, and inorganic complexation, was used to describe soil chemical c hanges after fire and ash addition. The model was able to describe tem poral changes in cation concentrations of held percolates and soil exc hangeable cations. Measurements of extractable cations using unbuffere d salt solution on samples taken immediately after fire suggest immedi ate changes in exchangeable cations which were related to solubilisati on of cations from ash, and not to changes in exchangeable cations in soils. Modelling suggests that under natural conditions the difference s in solubility of cations in ash result in slow changes in exchangeab le cations extending over a period of 6 years or more. The time requir ed to reacidify the surface layer of ashbed soil was estimated to be 4 5 years when annual acid input was 0.5 kmol H+/ha.