Root growth patterns of two nitrogen-fixing trees under landfill conditions

The root growth patterns of two N2-fixing trees: Acacia confusa, Casuarina equisetifolia, in two completed landfill sites and two control sites were studied. The topsoils of the four sites were compacted by machinery (bulk density 1.5 to 2.2 g cm-3) and no additional plant medium was added for tree growth. Both Acacia confusa and Casuarina equisetifolia had shallow root plates in all four sites. Their roots were mostly confined to the upper 15 cm of topsoil and did not penetrate further down. Dehydrogenase activities were high in the upper 15 cm of topsoil and low at deeper levels. Nitrogen fixing activities were found only in the upper 15 cm of topsoil. These two species of tree showed they were suited to grow on completed landfill sites, to fix nitrogen under landfill conditions and the 1 m thick landfill cap is unlikely to be damaged by the growth of their root plates.