ROOT ADAPTATION AND NITROGEN-SOURCE ACQUISITION IN NATURAL ECOSYSTEMS

The capacity for nitrate reduction, as measured by nitrate reductase a ctivity (NRA), was generally low for a range of plant communities in A ustralia (coastal heathland, rainforest, savanna woodland, monsoon for est, mangrove, open Eucalyptus forest, coral cay open forest) and only a loose relationship existed between NRA and leaf nitrogen concentrat ion. This suggests that nitrate ions are not the sole nitrogen source in these communities. Based on N-15 labeling experiments, we found a r ange of tree species exhibiting a pronounced preference for uptake of ammonium over nitrate. Analysis of soil solutions from several forest and heathland communities indicated that ammonium ions were more preva lent than nitrate ions and that soluble forms of organic nitrogen (ami no acids and protein) were present in concentrations similar to those of mineral nitrogen. To determine the extent to which root adaptations and associations might broaden nitrogen source utilization to include organic nitrogen, we assessed the effects of various nitrogen sources on seedling growth in sterile culture. Non-mycorrhizal seedlings of E ucalyptus grandis W. Hill ex Maiden. and Eucalyptus maculata Hook. gre w well on mineral sources of nitrogen, but did not grow on organic sou rces of nitrogen other than glutamine. Mycorrhizal seedlings grew well on a range of organic nitrogen sources. When offered a mixture of ino rganic and organic nitrogen sources at low concentrations, mycorrhizal seedlings derived a significant proportion of their nitrogen budget f rom organic sources. We also demonstrated that a species of the obliga tely non-mycorrhizal genus Hakea, a heathland proteaceous shrub posses sing cluster roots, had the ability to incorporate N-15-labeled organi c sources (e.g., glycine). We conclude that mycorrhizal associations a nd root adaptations confer the ability to substantially broaden the ni trogen source base on some plant species.