© 1996 Heron Publishing—Victoria, Canada
Root adaptation and nitrogen source acquisition in natural ecosystems
Matthew H. Turnbull (1), Susanne Schmidt (2), Peter D. Erskine (2), Suanne Richards (2) and George R. Stewart (2, 3)
1. Department of Plant and Microbial Sciences, University of Canterbury, Christchurch, New Zealand / 2. Department of Botany, The University of Queensland, Brisbane, Queensland 4072, Australia / 3. Author to whom correspondence should be addressed / Received October 25, 1995
Summary
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, rainforest, 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 labeling experiments, we found a range 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 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, we assessed the effects of various nitrogen sources on seedling growth in sterile
culture. Non-mycorrhizal seedlings of Eucalyptus grandis W. Hill ex Maiden and Eucalyptus maculata Hook. 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.
We also demonstrated that a species of the obligately non-mycorrhizal genus Hakea, a heathland proteaceous shrub possessing cluster roots, had the ability to incorporate 15N-labeled organic sources (e.g., glycine). We conclude that mycorrhizal associations and root adaptations confer the ability
to substantially broaden the nitrogen source base on some plant species.
Keywords:
ammonium, cluster root, mycorrhiza, nitrate, nitrate reductase.
