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

Abstract

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 13C and 15N natural abundance. 13C analyses showed only three of the 88 test species to exhibit C4 metabolism and only one of seven succulent species to be in CAM mode. Non-succulent winter ephemeral C3 species showed significantly lower mean δ13C values (- 28.0‰) than corresponding C3-type herbaceous perennials, woody shrubs or trees (- 26.9, - 25.7 and - 26.2‰, respectively), suggesting lower water stress and poorer water use efficiency in carbon acquisition by the former than latter groups of taxa. Corresponding values for δ15N of the above growth and life forms lay within the range 7.5-15.5‰. δ15N of soil NH4+ (mean 19.6‰) at a soft mulga site in Western Australia was considerably higher than that of NO3- (4.3‰). Shoot dry matter of Acacia spp. exhibited mean δ15N values (9.10 ± 0.6‰) identical to those of 37 companion non-N2-fixing woody shrubs and trees (9.06 ± 0.5‰). These data, with no evidence of nodulation, suggested little or no input of fixed N2 by the legumes in question. However, two acacias and two papilionoid legumes from a dune of wind-blown, heavily leached sand bordering a lake in mulga in Western Australia recorded δ15N values in the range 2.0-3.0‰ versus 6-4-10.7‰ for associated non-N2-fixing taxa. These differences in δ15N, and prolific nodulation of the legumes, indicated symbiotic inputs of fixed N in this unusual situation. δ15N signals of lichens, termites, ants and grasshoppers from mulga of Western Australia provided evidence of N2 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.