Australian mulga ecosystems - C-13 and N-15 natural abundances of biota components and their ecophysiological significance

Samples of recently produced shoot material collected in winter/spring from common plant species of mulga vegetation in eastern and Western Australia were assayed for C-13 and N-15 natural abundance. C-13 analyses showed only three of the 88 test species to exhibit C-4 metabolism and only one of sev en succulent species to be in CAM mode, Non-succulent winter ephemeral C-3 species showed significantly lower mean delta(13)C values (- 28.0 parts per thousand) than corresponding C-3-type herbaceous perennials, woody shrubs or trees (- 26.9, - 25.7 and - 26.2 parts per thousand, respectively), sugg esting lower water stress and poorer water use efficiency in carbon acquisi tion by the former than latter groups of taxa, Corresponding values for del ta(15)N of the above growth and life forms lay within the range 7.5-15.5 pa rts per thousand. delta(15)N of soil NH4+ (mean 19.6 parts per thousand) at a soft mulga site in Western Australia was considerably higher than that o f NO3- (4.3 parts per thousand). Shoot dry matter of Acacia spp. exhibited mean delta(15)N values (9.10 +/- 0.6 parts per thousand) identical to those of 37 companion non-N-2-fixing woody shrubs and trees (9.06 +/- 0.5 parts per thousand). These data, with no evidence of nodulation, suggested little or no input of fixed N-2 by the legumes in question, However, two acacias and two papilionoid legumes from a dune of Hind-blown, heavily leached sand bordering a lake in mulga in Western Australia recorded delta(15)N values in the range 2.0-3.0 parts per thousand versus 6.4-10.7 parts per thousand for associated non-N-2-fixing taxa. These differences in delta(15)N and pro lific nodulation of the legumes, indicated symbiotic inputs of tired N in t his unusual situation. delta(15)N signals of lichens, termites, ants and gr asshoppers from mulga of Western Australia provided evidence of N, fixation in certain termite colonies and by a cyanobacteria-containing species of l ichen, Data are discussed in relation to earlier evidence of nitrophily and water availability constraints on nitrate utilization by mulga vegetation.