Interaction of Natural Biofertilizers

Different biofertilizers have shown nitrogen-fixing, phos-phorus-solubilizing, and phytohormone-producing abilities and are used as for increasing agricultural productivity, for e.g., (Brady)rhizobium for legumes (grain, fodder), plant growth promoting bacteria (PGPR) for cereals (wheat, rice, grasses, etc.), Azolla for the rice ecosystem, and actino-mycetes (Frankia spp.) for forest trees. The AM biofertilizer is known to increase the nitrogen-fixing potential of the legumes when given together with Rhizobium (Chaturvedi and Kumar 1991) and Bradyrhizobium (Werner et al. 1994; Xie et al. 1995). The mycorrhiza first stimulates the nodule bacteria in a sequential process by increasing the tissue phosphorus content; this results in improved nodulation. There are also reports of positive interaction between Azotobacter/Azospirillum, and AM fungi (Alnahidh and Gomah 1991). The AM colonization favorably affects the population of these free-living N-fixing bacteria and thus stimulates better growth of plants. The AM colonization also has a stimulatory effect with different nonlegume nitrogen-fixing plant species. In Casuarina sp., the double inoculation of AM and Frankia improves plant growth and nodulation (Sempavalan et al. 1995).

Two groups of bacteria, chemo-organotrophs like some Pseudomonas and Bacillus sp., and chemo-lithotrophs, such as Thiobacillus sp., are able to solublize insoluble phosphates.

The AM when given in addition to these bacteria, improve the plant performance. The AM favors the early establishment and efficacy of these bacteria. The synergistic effect of these fungi should thus be exploited on a large scale in the form of biofertilizers to increase the nitrogen-fixing potential of legumes and nonlegume plant species as well as with different phosphate solubilizers. Mycorrhizal fungi interact with a wide assortment of organisms in the rhizosphere. The result can be positive, neutral, or negative on the mycorrhizal association or a particular component of the rhizosphere. For example, specific bacteria stimulate EM formation in conifer nurseries and are called mycorrhization helper bacteria. In certain cases these bacteria eliminate the need for soil fumigation (Garbaye 1994).

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