Abstract of a publication based on research funded in part by the Australian Flora Foundation
Turnbull, M. H. Schmidt, S. Erskine, P. D. Richards, S. Stewart, G. R. (1996). Tree Physiology. 16: 11/12, 941-948.
The capacity for nitrate reduction, as measured by nitrate reductase activity (NRA), was generally low for a range of plant communities in Australia (coastal heathland, rain forest, savanna woodland, monsoon forest, mangrove, open Eucalyptus forest, coral cay open forest) and only a loose relationship existed between NRA and leaf nitrogen concentration. This suggests that nitrate ions are not the sole nitrogen source in these communities. Based on 15N labelling experiments, a range of tree species were found 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 prevalent than nitrate ions and that soluble forms of organic nitrogen (amino 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, the effects of various nitrogen sources on seedling growth in sterile culture were assessed. Non-mycorrhizal seedlings of Eucalyptus grandis and E. maculata grew well on mineral sources of nitrogen, but did not grow on organic sources of nitrogen other than glutamine. Mycorrhizal seedlings grew well on a range of organic nitrogen sources. When offered a mixture of inorganic and organic nitrogen sources at low concentrations, mycorrhizal seedlings derived a significant proportion of their nitrogen budget from organic sources. It was also shown that a species of the obligately non-mycorrhizal genus Hakea, a heathland proteaceous shrub possessing cluster roots, had the ability to incorporate 15N-labelled organic sources (e.g., glycine). It is concluded that mycorrhizal associations and root adaptations confer the ability to substantially broaden the nitrogen source base on some plant species.