273 NITROGEN ECONOMY IN ARID AND SEMI-ARID PLANT COMMUNITIES. Part III. The Symbiotic Nitbogen-fixing Organisms. By N. C. W. Beadle, Department of Botany, University of New England, Armidale, N.S.W. [Read 29th July, 1964.] Synopsis. Of the 80 legumes investigated from the arid and semi-arid areas of eastern Australia, at least 68 produce effective rhizobial nodules (Mimosaceae and Papilionaceae) ; Caesalpiniaceae do not nodulate. Herbaceous species usually nodulate freely and many of them fix highly significant amounts of nitrogen. Some species of Acacia nodulate sporadically and tardily in the seedling condition and this is probably of significance with regard to regeneration, especially on eroded soils. Five species of Bhizobium occur and these are distributed discontinuously. Most soils contain at least one species of RhizoMum. Rhizobial populations are likely to be reduced or removed by soil movement under the action of wind. Introduotion. In a previous publication (Beadle and Tchan, 1955) the widespread death of Acacia aneura (mulga), particularly in western New South Wales, and the consequent depletion of the sandy soils of their organic matter by the winnowing action of the wind were discussed as an introduction to the nitrogen economy of arid and semi-arid soils. Tchan and Beadle (1955) assessed quantitatively the possible accessions of soil nitrogen by non-symbiotic organisms. The present paper deals qualitatively with the native legume-rhizobia systems, the data including nodulation records chiefly from field observations, the approximate distribution of the rhizobia of certain of the common native legumes, and an assessment of the significance of the various species as nitrogen-fixers in the plant communities in which they occur. Data on the introduced legumes (chiefly Trifolieae) will be presented in another paper. The area covered in the field survey lies west of the 15-inch isohyet in western New South Wales and Queensland, between the Murray River and the highway linking Charleville and Windorah; isolated records come also from the Lake Eyre and Lake Amadeus Basins, and the Alice Springs and Oodnadatta districts. The plant com-munities in the main area of study are mentioned in Table 1; they have been described by Beadle (1948). Relatively little work has been done on the inland legumes; some data on a few of the species, or on other species of common inland genera found in more humid regions, have been investigated with regard to nodulation (listed by Bowen, 1956, and unpublished data of Allen and Allen in personal communications). NODtTLATION. The roots of about 80 legume species have been examined in the field for the occurrence of nodules and 68 of these are reported on (Table 1). The remaining twelve, on which a definite statement cannot yet be made, may nodulate. Included in these twelve and of particular significance with regard to nitrogen economy (because they are community-dominants or abundant species) are Acacia pendula A. Cunn. (Myall), A. harpophylla F. Muell. (Brigalow), A. cambagei R. T. Baker (Gidgee), A. loderi Maiden, and A. excelsa Benth. (Ironwood); on all of these five species what appear to be nodule-scars have been recorded. The remaining inland species, including those which have been examined and those not, are rare in the field so that, even if Proceedings of the Linnean Society of New South Wales, 1964, Vol. Ixxxix, Part 2.