Potential for nitrogen fixation and biomass production in Panicum maximum

Abstract

SUMMARY Investigations were carried out to ascertain the relationship between the potential for biomass production and nitrogenfixation in the rhizosphere of Panicum maximum. With a dry matter production of 1.85 ±1.40 and 0.48 ± 033 kgm, a positive correlation (r=0.62) was found to exist between the numbers of nitrogen-fixing bacteria in the rhizosphere of this grass and total dry matter production. The influence of concentrations of organic carbon substrates in roots/ rhizomes on total bacterial populations was also investigated. Results showed that increasing concentrations of organic acids, reducing sugars and miscellaneous soluble carbohydrates at naturally-occurring levels were accompanied by reductions in bacterial population. Aerobic nitrogen-fixing bacteria appeared to play the most dominant role as viable counts of these bacteria were about four times higher than those of their anaerobic counterparts. Azospirillum sp. contributed approximately 48 per cent of total aerobic counts and was found to be capable of growth under strictly anaerobic conditions where it constituted about II per cent of total counts. Panicum maximum was found to be capable of growth in soils of clay-loam, sandyclay or purely loam types withpH 5.0-5.9. The ammoniumnitrogen levels in these soils were relatively low when compared to those of nitrite-nitrogen. Original scientific paper. Received 27 Jun 89; revised 17 Apr 90.