A RHIZOBACTERIUM MODIFIES PLANT AND SOIL RESPONSES TO THE MYCORRHIZAL FUNGUS GLOMUS-MOSSEAE

The effects of a rhizobacterium (Bacillus sp, strain BH-II) on plant (Pisum sativum L.) parameters (biomass, root/shoot ratio, seed/plant ratio), on root colonization by a vesicular-arbuscular mycorrhizal (VAM) fungus (Glomus mosseae) and on the formation of water-stable soil aggregates were investigated in two soils in a greenhouse pot study. The soils were a grey silt-loam of high P and a yellow clay-loam of low P content. The rhizobacterium had no significant effect on plant biomass production in either soil in the absence of the VAM fungus, but decreased plant growth by more than 30% in its presence. It enhanced the root/shoot and seed/plant ratios in plants of both VAM and non-VAM treatments. Without the VAM fungus, both soils disaggregated. This slaking of water-stable aggregates (over 0.5 mm) was significantly less severe when the rhizobacterium was present. With the VAM fungus, aggregation increased up to 27% during the experiment, but the rhizobacterium did not affect this process significantly. It is concluded that rhizo-organisms acting in concert may have both positive and negative Effects on the plant and soil components of an agrosystem.