Australian mulga ecosystems -: 13C and 15N 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 seven 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), suggesting lower water stress and poorer water use efficiency in carbon acquisition by the former than latter groups of taxa, Corresponding values for delta(15)N of the above growth and life forms lay within the range 7.5-15.5 parts 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 of 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 prolific nodulation of the legumes, indicated symbiotic inputs of tired N in this unusual situation. delta(15)N signals of lichens, termites, ants and grasshoppers from mulga of Western Australia provided evidence of N, fixation in certain termite colonies and by a cyanobacteria-containing species of lichen, Data are discussed in relation to earlier evidence of nitrophily and water availability constraints on nitrate utilization by mulga vegetation.