N-15 NATURAL ABUNDANCE OF PLANT AND SOIL COMPONENTS OF A BANKSIA WOODLAND ECOSYSTEM IN RELATION TO NITRATE UTILIZATION, LIFE FORM, MYCORRHIZAL STATUS AND N2-FIXING ABILITIES OF COMPONENT SPECIES

Studies of the variation in deltaN-15 values for plants from a fire-prone Banksia woodland in South West Australia showed that pioneer herbaceous, non-mycorrhizal species which were active in nitrate reduction and storage, had the highest values (1.81 parts per thousand). A detailed study of one such species Ptilotus polystachus demonstrated a close correspondence between the deltaN-15 values of soil nitrate, xylem nitrate and leaf total nitrogen, suggesting an exclusive reliance on nitrate ions as nitrogen source. These pioneer species also showed a preponderance of the chloroplastic isoform of glutamine synthetase while woody species generally had higher activity associated with the cytosolic isoform. The group comprising monocotyledonous hemicryptophytes and geophytes contained species with slightly positive deltaN-15 values and moderately active in nitrate reduction and storage. Nitrogen-fixing species had the lowest deltaN-15 values (-0.36 parts per thousand), irrespective of their apparent utilisation of nitrate. However, woody resprouter species which had low levels of nitrate reduction and storage had deltaN-15 values which fell within the range of values obtained for the miscellaneous assemblage of N2-fixing species. Consequently, N-15 abundance values failed to distinguish N2 fixing from non-fixing woody species, and therefore, could not be used in the ecosystem to determine the dependence of putative nitrogen fixing species on N2 fixation. The study demonstrated complex patterns of nitrogen ultilization in the ecosystem in which exploitation of different nitrogen resources related to plant life form and the physiological attributes of nitrogen assimilation by component species.