Abstract
The relationships between underground tissue biomass, carbon substrates and various diazotrophic bacterial groups associated with five Nigerian forage grasses on lateritic soils were studied. Biomass (kg m-2) was < 1.0 except for Pennisetum purpureum (1.65 ± 0.60). Sugars and organic acids were higher in rhizomes than roots, except in Andropogon tectorum where the reverse was noted; the root/rhizome ratios for these substrates varied between 31 and 87%. Organic acids were < 1.0 mg g-1 freshweight in all cases. Most Probable Number estimates of aerobes, microaerophiles and anaerobes showed the highest population in Panicum maximum and Andropogon tectorum. The range of counts (x 106 g-1 fresh wt) were 8.40-58.11 (aerobes), 6.15-16.75 (microaerophiles), 0—6.20 (Azospirillum) and 0.06-115 (anaerobes). The diazotrophic genera in decreasing frequency of occurrence were Enterobacter — Pseudomonas — Azospirillum —Klebsiella. Correlation matrices indicate that the root system biomass/diazotroph relationship was significant only with Azospirillum in P. maximum (P = 0.02; r = 0.861; n = 9). Microaerophiles as subsurface pellicle formers correlated with anaerobes in A. gayanus (P = 0.1; r = 0.904; n = 3) and with Azospirillum only in Hyparrhenia rufa (P = 0.001; r - 0.999; n = 3).
