Nitrogen fixation (acetylene reduction) by Acacia pellita on areas restored after mining at Groote Eylandt, Northern Territory

Abstract

Factors affecting the reduction of acetylene to ethylene by nodules of Acacia pellita O. Schwarz were studied on 3-year-old trees in the field. The specific activity of rooted nodules (22.8 ± 3.3 nmol mg^-1 fresh wt h^-1) was greater than that of detached nodules alone (13.5 ± 2.1 nmol mg^-1 h^-1). Acetylene reduction started without a lag phase and remained linear for about 4 h. The apparent Km value for acetylene was derived to be 11.2 x 10^-3 atm. Nodule material could be kept successfully without loss of activity when moist and shaded from direct sunlight for periods of up to 1 h. Acetylene reduction ceased when the assay temperature was 0°C, activity increasing to 20.3 ± 2.3 nmol mg^-1 h^-1 at 22°. The Acacia plants exhibited a diurnal cycle of nitrogen (acetylene) fixation; the greatest specific activity at midday was 27.7 ± 4.8 nmol mg^-1 h^-1 when the shaded soil temperature was 32°C and photosynthetically active radiation was 2550 μE m^-2 s^-1. By using a regression of nodule number per tree on stem diameter to obtain the total number of nodules per hectare, the total nitrogen fixation per wet season was estimated to be 12 ± 4 kg ha^-1 where the planting density in the study area was 1110 trees ha^-1.

The work is discussed with reference to its implications for management of the restored areas and the role of acacias as nitrogen-fixing plants in the pre-mining native Eucalyptus tetrodonta open-forest.