Nitrogen relations of natural and disturbed plant communities in tropical Australia

Nitrogen relations of natural and disturbed tropical plant communities in n orthern Australia (Kakadu National Park) were studied. Plant and soil N cha racteristics suggested that differences in N source utilisation occur at co mmunity and species level. Leaf and xylem sap N concentrations of plants in different communities were correlated with the availability of inorganic s oil N (NH4+ and NO3-). In general, rates of leaf NO3- assimilation were low . Even in communities with a higher N status, including deciduous monsoon f orest, disturbed wetland, and a revegetated mine waste rock dump, levels of leaf nitrate reductase, xylem and leaf NO3 levels were considerably lower than those that have been reported for eutrophic communities. Although NO3- assimilation in escarpment and eucalypt woodlands, and wetland, was genera lly low, within these communities there was a suite of species that exhibit ed a greater capacity for NO3- assimilation. These "high-NO3- species" were mainly annuals, resprouting herbs or deciduous trees that had leaves with high N contents. Ficus, a high-NO3- species, was associated with soil exhib iting higher rates of net mineralisation and net nitrification. "Low-NO3- s pecies" were evergreen perennials with low leaf N concentrations. A third g roup of plants, which assimilated NO3- (albeit at lower rates than the high -NO3- species), and had high-N leaves, were leguminous species. Acacia spec ies, common in woodlands, had the highest leaf N contents of all woody spec ies. Acacia species appeared to have the greatest potential to utilise the entire spectrum of available N sources. This versatility in N source utilis ation may be important in relation to their high tissue N status and compar atively short life cycle. Differences in N utilisation are discussed in the context of species life strategies and mycorrhizal associations.