Nutrient Cycling in a Stand of Acacia holosericea A. Cunn. Ex G. Don. I. Measurements of Precipitation, Interception, Seasonal Acetylene Reduction, Plant Growth and Nitrogen Requirement

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Abstract
Measurement of precipitation intercepted by a 3-year-old Acacia holosericea plantation (1111 trees ha-2) revealed a positive relation between tree stem diameter (2 cm above ground) and stemflow, and quadratic relations between throughfall; stemflow and rainfall interception with area precipitation. No relation was found between rainfall intensity and nitrogen concentration in any rainfall component and no seasonal trend was evident. Total nitrogen was significantly increased in throughfall and stemflow compared with that in area precipitation due to enrichment of the NH4+ and organic nitrogen components. The weight of the trees was estimated using allometric relations in which stem diameter was the indepen- dent variable. Total plant weight increased from 2 . 4 (s.d. = 0.1) to 5.5 (s.d. = 0.2) t dry wt ha-1 over the period January 1979-February 1980. Phyllode area index increased from 0.89 to 1.86 over the same period. Acetylene reduction by detached but rooted nodules was greatest in the wet season and almost undetectable in the dry season. Integration of the seasonal patter? of acetylene reduction allowed an estimate of the annual fixation of nitrogen of 6.4 kg N ha-1. Of the gross demand for nitrogen of 33.6 (s.d. = 1.3) kg N ha-1, 12% was supplied by mobilization from senescing tissues, 19% by nitrogen fixation, 7% by leaching of readily mineralized nitrogen from plant parts, 15% from area precipitation and the remainder (47%) by mineralization of soil organic materials. The view that nitrogen fixation provides legumes with an ecological advantage over other plants in the invasion of disturbed sites is discussed.