Alkalinity effects of different tree litters incubated in an acid soil of NSW, Australia

The impact of trees on processes involved in soil acidification is not clearly understood. This study has measured the cation and anion composition of leaf litter from 28 tree species including both indigenous species and those exotic to Australia, which varied widely in composition. The excess cation content was calculated and shown to be highly correlated with ash alkalinity, determined by ashing and titration. Calcium (Ca) was the dominant cation in these samples and was highly correlated with ash alkalinity. Four rates of ground litter samples were incubated with an acid soil and after eight weeks soil pH and extractable cations were determined. The pH was strongly influenced by the addition of litter, and the increase in pH was linearly related to the amount of ash alkalinity added. Extractable Ca in the soil was highly correlated with Ca added in the litter. A second property of litter measured was the ability of water extracts from the material to complex aluminium (Al). This was assessed by reacting leaf litter extract with Al solution under standardised conditions and measuring the uncomplexed (monomeric) Al using the 8-hydroxyquinoline 15 sec reaction. Species varied markedly in complexing ability. Using the two properties, complexing ability and ash alkalinity, a framework is developed in which the effects of different species on nutrient cycling and podzolisation processes in relation to soil acidification are discussed, and predictions compared with available evidence from field studies. This index for assessing the potential impact of the removal/addition plant material could be used to predict the long- term consequences of cut and carry agroforestry systems on soil chemical properties.