Seasonal variation in the nitrogenase activity of aPanicum maximum var.trichoglume pasture and identification of associated bacteria

Summary The influence of seasonal variation on nitrogenase (N₂-ase) activity of undisturbed soil-plant cores ofPanicum maximum var.trichoglume was measured using the C₂H₂ reduction assay. The largest N₂-ase activity in the field, 14.7 g N ha⁻¹ day⁻¹, occurred in spring when soil moisture was high, soil temperature was low and nitrogenous fertiliser influence was at a minimum. The potential N₂-ase activity of the cores, measured under controlled conditions, reached a maximum of 27.2 g N ha⁻¹ day⁻¹ and averaged 26.3 g N ha⁻¹ day⁻¹ over the 14 month sampling period. N₂-ase activity was positively correlated (P=0.05) with field soil moisture and negatively correlated with field soil temperature (r=0.59 and −0.78 respectively). Multiple regression showed that 69% of the variation of N₂-ase activity in the field was associated with the combined effects of soil moisture and soil temperature. Nitrogen fixing bacteria were isolated from the roots ofP. maximum and based upon morphology, biochemical tests and fluorescent antibody reaction, were found to be closely related toAzospirillum lipoferum.