Nutrient movement into soils beneath eucalypt and exotic conifer forests in southern central Victoria

Above-ground nutrient return to the soil was estimated by studying forest floor leachates for up to 2 years in two eucalypt forests (Eucalyptus obliqua and E. regnans) and two nearby conifer plantations (Pinus radiata and Pseudotsuga menziesii) near Narbethong in southern central Victoria. Forest floor leachate volumes were recorded approximately once a month and samples were analysed for pH and potassium, sodium, magnesium, and calcium concentrations. The volume of forest floor leachate was greater in the eucalypts than in the conifers. This was attributed to greater interception losses of water by the conifers. Forest floor leachate chemistry underwent regular seasonal fluctuations which were similar for each of the four forest types. Acidity was highest in spring and autumn and lowest in summer. Cation concentrations were highest in autumn and lowest in spring. This behaviour was attributed to changes in climate and in rates of litter decomposition. In contrast to cation concentrations, cation quantities entering the soil in forest floor leachate showed no pronounced seasonal variations although they tended to be high in winter. The quantities of cations added to the soil in forest floor leachate varied with forest type, and were significantly higher in E. obliqua than in the comparable P. radiata stand. Greater quantities of acid were added to the soil under P. radiata. The P. radiata soil contained lower quantities of exchangeable cations than the E. obliqua soil. This was consistent with the observed differences in forest floor leachate chemistry. Unlike the P. radiata soil, the P. menziesii soil did not appear to be depleted in exchangeable cations. It was concluded that P. radiata had depleted the exchangeable cation reserves in the soil studied but that the full significance of this is still unknown.