PLANT AND SOIL RESPONSES TO MYCORRHIZAL FUNGI AND RHIZOBACTERIA IN NODULATED OR NITRATE-FERTILIZED PEAS (PISUM-SATIVUM L)

Rhizosphere organisms affect plant development and soil stability. Thi s study was conducted to determine the effects of a vesicular-arbuscul ar mycorrhizal (VAM) fungus [Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe] and a rhizobacterium (Bacillus sp.) on nitrate-fertilized or nodulated pea (Pisum sativum L.) plants and on the status of water-sta ble soil aggregates. The plants were grown in pots in a yellow clay-lo am soil, and inoculated with the VAM fungus and the rhizobacterium, wi th one of the two, or with neither. The Bacillus sp. and G. mosseae di d not affect shoot dry mass in nodulated plants. Under N fertilization , the VAM fungus enhanced plant growth, while the rhizobacterium inhib ited shoot growth, VAM root colonization, and nodule formation, but en hanced the root:shoot and the seed:shoot ratios. The inhibition of sho ot growth and of root colonization appeared to be related. The water s tability and pH of the VAM soils were higher than those of the non-VAM soils. The rhizobacterium enhanced the water-stable aggregate status in the non-VAM soils only. Under both N-nutrition regimes, the soils h ad the greatest proportion of the water-stable aggregates when inocula ted with both rhizo-organisms and the lowest when colonized by neither . The two rhizo-organisms affected both plants and soil, and these eff ects were modified by the source of N input through N-2 fixation or fe rtilization.