119 THE STATUS OF NITROGEN IN THE HAWKESBURY SANDSTONE SOILS AND THEIR PLANT COMMUNITIES IN THE SYDNEY DISTRICT. I. The Significance and Level of Nitrogen. By Nola J. Hannon, Linnean Macleay Fellow of the Society in Botany. (Plate x; one Text-figure.) [Read 27th June, 1956.] Synoj)sis. The soils derived from Hawkesbury Sandstone in the Sydney district are of low fertility. Phosphorus and nitrogen are the chief limiting nutrients. The level of nitrogen in the parent rock, the soils and the plant material of these communities is described. Low concentrations prevail throughout, but increase from approximately 180 p. p.m. in the rock to the order of 600 p.p.m. in the surface soils. The soils are acid, pH 4-5-5-0, with high carbon-nitrogen ratios (18-49). Consequently, the level of available nitrogen is extremely low. The nitrogen content of much of the fresh leaf material is only about 7,000 p.p.m., but members of the Leguminosae and Casuarinaceae average 12,000 p.p.m. Similarly, seed of legumes and Casuarina L. contains at least four times the amount of nitrogen found in other species. Freshly fallen leaf litter contains a large percentage of the nitrogen content of mature leaves. Introduction. The natural plant communities of the Sydney district attract attention because of the mosaic of formations which occur and the distinctive and sclerophyllous character of many of their members. The majority of the species belong to the endemic Australian element, with Indo-Malayan representatives occurring only in restricted areas of improved environmental conditions. Trees and woody shrubs predominate, herbs and grasses being less conspicuous. There is a great diversity of floral type and leaf form. Over a period of years several workers have been engaged in investigations of a number of aspects of the nature of these communities. These include floristic and structural descriptions, physiological studies, nutrient levels and requirements, anatomical, morphological and cytological investigations, and studies on the microflora and fauna involved in Casuarina litter decomposition. The present series of papers will make a further contribution to the knowledge already accumulated. Much detailed information, especially autecological studies, is still lacking; but when the communities have been characterized in sufficient detail, a synthesis of the various contributions should illustrate the basic chemical, physical and biological factors which are interacting to produce the resultant mechanism of the ecosystem. To date, reports of such syntheses are very few. Piatt's (1951) ecological study of the Mid-Appalachian shale barrens forms a notable exception. Since detailed descriptions of the associations and their habitat have been made by Pidgeon (1937, 1938, 1940, 1942), only a brief account will be included here. There are three geological formations within the district, all of which are Triassic sediments. They are the Narrabeen Group (shales and sandstones), the Hawkesbury Sandstone and the Wianamatta Group (shales and sandstones), as described by Hanlon, Joplin and Noakes (1952). Apart from several widely scattered outliers, the uppermost division of the Triassic, the Wianamatta Group, still remains in only the Sydney Plains, an area of approxi-mately 35 by 50 square miles. The Narrabeen Group outcrops cover a still smaller area near the coastal region to the north and south of Sydney. A deeply dissected plateau region composed chiefly of Hawkesbury Sandstone extends to the north, south and west of Sydney. The horizontal bedding and resistant nature of the sandstone are responsible for the typical physiographic characters of
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