title: Nitrogen relations of natural and disturbed plant communities in tropical Australia.
abstract: Nitrogen relations of natural and disturbed tropical plant communities in northern Australia (Kakadu National Park) were studied. Plant and soil N characteristics suggested that differences in N source utilisation occur at community and species level. Leaf and xylem sap N concentrations of plants in different communities were correlated with the availability of inorganic soil N (NH+4and NO-3). In general, rates of leaf NO-3assimilation were low. Even in communities with a higher N status, including deciduous monsoon forest, disturbed wetland, and a revegetated mine waste rock dump, levels of leaf nitrate reductase, xylem and leaf NO-3levels were considerably lower than those that have been reported for eutrophic communities. Although NO-3assimilation in escarpment and eucalypt woodlands, and wetland, was generally low, within these communities there was a suite of species that exhibited a greater capacity for NO-3assimilation. These "high- NO-3species" were mainly annuals, resprouting herbs or deciduous trees that had leaves with high N contents. Ficus, a high-NO-3species, was associated with soil exhibiting higher rates of net mineralisation and net nitrification. "Low-NO-3species" were evergreen perennials with low leaf N concentrations. A third group of plants, which assimilated NO-3(albeit at lower rates than the high-NO-3species), and had high-N leaves, were leguminous species. Acacia species, common in woodlands, had the highest leaf N contents of all woody species. Acacia species appeared to have the greatest potential to utilise the entire spectrum of available N sources. This versatility in N source utilisation may be important in relation to their high tissue N status and comparatively short life cycle. Differences in N utilisation are discussed in the context of species life strategies and mycorrhizal associations.
authors.affiliation: Department of Botany, The University of Queensland, Brisbane, QLD 4072, Australia, , , , , , AU.
authors.forename: S
authors.initials: S
authors.lastname: Schmidt
authors.affiliation: Department of Botany, The University of Queensland, Brisbane, QLD 4072, Australia, , , , , , AU.
authors.forename: G R
authors.initials: GR
authors.lastname: Stewart
authors.affiliation: Department of Botany, The University of Queensland, Brisbane, QLD 4072, Australia, , , , , , AU.
authors.forename: M H
authors.initials: MH
authors.lastname: Turnbull
authors.affiliation: Department of Botany, The University of Queensland, Brisbane, QLD 4072, Australia, , , , , , AU.
authors.forename: P D
authors.initials: PD
authors.lastname: Erskine
authors.affiliation: Environmental Research Institute of the Supervising Scientist, Environmental Protection Agency, Locked Bag 2, Jabiru NT 0886, Australia, , , , , , AU.
authors.forename: N
authors.initials: N
authors.lastname: Ashwath
keywords: Key words Nitrate; Nitrogen utilisation; Tropical monsoon forest; Tropical woodland; Xylem sap