Biology of Acacia pulchella R.br. With Special Reference to Symbiotic Nitrogen Fixation

Abstract

Growth, reproduction and longevity of the fire weed Acacia pulchella var. glaberrima were examined in natural populations of known age in coastal sands in and around Perth, W.A. Dense populations (10000 plantsiha) were established after a summer burn; plant density was 30% of its initial value at 4 years. less than 8% at 13 years. Plants accumulated dry matter, nitrogen and phosphorus throughout a 13-year growth period. Seed production commenced at 2 years, reached a maximum (12000 seeds per plant per year) at 3 or 4 years declining to 2000 seedsiplant in the 13th year. Only a small fraction of the shed seed accumulated in soil under the stands. Changes in total plant N, nodule weightlplant, and C₂H₂ reduction capacity of detached nodules were followed in populations in their first, second and fourth growing seasons. A new set of nodules formed with the autumn rains, peak nodule mass and C₂H₂ reduction activity were recorded in July-October, and virtually no nodules survived the summer into a second growing season. A glasshouse study of N accumulation and C₂H₂ reduction by nodules in minus N sand culture gave acalibration value of 2.26 mol C₂H₂ : mol N₂ fixed. Applying this value to data from nativepopulations, 8% of the N accumulated by first season plants, 45% of the N of second season plants and 68% of the N of fourth season plants were estimated to be derived from symbiosis. Average annual returns of N to the ecosystem were estimated at 3.9 kg/ha, probably more than half of this from N₂ fixation. Progressive death of plants in the populations gave the greatest return ( 1.9 kg N per ha per yr), the remainder from litter (1 kg N per ha per yr) and shed seed ( 1 kg N per ha per yr).