Rhizosphere microbial communities: Isolation, characterization, and value addition for substrate development

Abstract

Rhizosphere, the soil region nearest to plant root system, inhabits various microorganisms varying in their community structure and diversity, still remains largely unexploited despite so much of breakthroughs in isolation and characterization, but lacks in addressing citrus specific rhizosphere properties. Exploiting microbial synergisms is one of the popular methods of substrate dynamics and associated changes in nutrient environment of rhizosphere. Soil microbial biomass dominates fungal mycelium accumulates and retains mineral nutrients, immobilizing as much as ∼20% of the total soil N and P. The extraradical mycorrhizal mycelial (attains as much as 3% of root weight) networks are recognized as the hidden nutrient/water-absorbing interfaces (glomalin, a soil-based glycoprotein secreted by AMF as a major seat of activity) in addition to safeguarding citrus against a number of other biotic and abiotic stresses. The microbial diversity existing within top 0-20-cm citrus rhizosphere soil was characterized and isolated by the promising microbes, namely, Bacillus polymyxa, Azoto-bacter chroococcum, Bacillus mycoides, Pseudomonas fluorescens, and Trichoderma harzianum. Pure culture of these microbes in value-added form (Bacillus polymyxa 33-44 × 10 6 -10 7 cfu mL -1, Azotobacter chroococcum as 10-38 × 10 6 -10 7 cfu mL -1, Bacillus mycoides 7-22 × 10 5 -10 6 cfu mL -1), Pseudomonas fluorescens 14-28 × 10 6 -10 7 cfu mL -1, and Trichoderma harzianum 30-32 × 10 6 -10 7 cfu mL -1 was developed and prepared a mixture called microbial consortium (MC). MC so developed displayed an excellent response under both nursery as well as grown-up plants in integrated nutrient management (INM) module. These results suggested two cardinal points, namely, (1) the developed microbial consortium holds a good promise as an independent use as well as use in format of integrated nutrient management and (2) deserves an application in providing resilience to rhizosphere against any possible depletion in either available supply of nutrients or loss of native soil microbial biomass as an indicator of rhizosphere health.