Acetylene Reduction Rates by Selected Leguminous and Non-Leguminous Plants of Groote Eylandt, Northern Territory.

Abstract

Acetylene (C2H2) reduction associated with the roots of various grasses and sedges and by nodules of selected legumes was investigated in a Eucalyptus tetrodonta open-forest and a Melaleuca leucadendron open-forest swamp complex. High rates of non-legume C2H2 reduction, as measured by the excised root assay, were associated only with plants found in the swamp complex: the sedge Fimbristylis dichotoma and the grasses Imperata cylindrica and Paspalum vaginatum (70 ± 30, 161 ± 69 and 23 ± 15 nmol C2H4 g dry wt -1h-1 respectively). An initial lag period of 8 h before C2H2 reduction was detected was not eliminated or reduced by the addition of sucrose. Nearly all the C2H2 reduction activity was associated with the roots with very little activity in topsoil or subsoil. The addition of inorganic nitrogen or aspartic or citric acids stopped the reduction of C2H2, but addition of sucrose stimulated it. Current potential photosynthesis appeared to be related to C2H2 reduction ability. Acetylene reduction activity by rooted nodules of various leguminous species was distinctly seasonal, being greatest in the wet season. Members of the Mimosaceae had twice the wet season activity of those of the Caesalpiniaceae and Fabaceae (20.4 ± 2.9 and 10.1 ± 4.0 nmol mg fresh wt-1h-1 respectively). The results indicate that the nitrogen accretion rate in the E. tetrodonta open-forest will be low (0.05-1 kg Nha-1yr-1) and that mineralization of soil organic matter could account for all the nitrogen utilized by the ecosystem over a wet season.