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 clea rly understood. This study has measured the cation and anion composition of leaf litter from 28 tree species including both indigenous species and tho se exotic to Australia, which varied widely in composition. The excess cati on content was calculated and shown to be highly correlated with ash alkali nity, determined by ashing and titration. Calcium (Ca) was the dominant cat ion in these samples and was highly correlated with ash alkalinity. Four ra tes of ground litter samples were incubated with an acid soil and after eig ht weeks soil pH and extractable cations were determined. The pH was strong ly influenced by the addition of litter, and the increase in pH was linearl y 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 li tter measured was the ability of water extracts from the material to comple x aluminium (Al). This was assessed by reacting leaf litter extract with Al solution under standardised conditions and measuring the uncomplexed (mono meric) Al using the 8-hydroxyquinoline 15 sec reaction. Species varied mark edly in complexing ability. Using the two properties, complexing ability an d ash alkalinity, a framework is developed in which the effects of differen t species on nutrient cycling and podzolisation processes in relation to so il acidification are discussed, and predictions compared with available evi dence 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 proper ties.