Nutrient Solubilizing Microbes (NSMs): Its role in sustainable crop production

Abstract

Nowadays the land degradation, deteorating the soil health, is the major constraint, which led to the nutrient depletion and limited potential yield of crops toward the food security worldwide. To enhance the crop production, soil sustainability is one of the ideal and sustainable approaches to overcome depleting the soil fertility status and land degradation. The rhizospheric beneficial microorganisms (RBMs) may offers rate of plant growth, agronomically, pathogenically, and environmentally advantage for intensive agricultural production system. The nitrogen-fixing rhizobacteria (NFR) fix atmospheric nitrogen in the soil, while phosphate-solubilizing microorganisms solubilize the insoluble phosphorus (P) in the soil, potassium-solubilizing rhizobacteria (KSR) mobilizes the stable potassium in field, and similarly other microorganisms mobilize nutrients in soil and make it easily available to the crop plants. These NFR include symbiotic N2-fixing forms, in leguminous plants, viz., Rhizobium, and obligate symbionts in nonleguminous plant comprising species Alcaligenes, Azomonas, Beijerinckia, Achromobacter, Acetobacter, Arthrobacter, and Bacillus spp.; however, P is the primary essential macronutrient for vegetative and reproductive development of the plants. The majority of phosphorus in most soil is insoluble form and cannot be used by crops. Some important species are Aspergillus, Bacillus, Pseudomonas, Penicillium, etc., which secrete organic acids (products) that lower pH in their vicinity and help to bring the dissolution of fixed phosphates in soil. Additionally seed bio-priming can provide ~30 kg P2O5/ha. In soil system potassium (K) is associative to movement of water, nutrients, carbohydrates, and cellular and osmotic pressure in plant tissues. Supposing potassium is lacking or not provided in suitable amount, growth of the plants stunts and production reduces. Several bacterial species particularly rhizosphere-colonizing bacteria have been found, which solubilize insoluble inorganic phosphate like that tri-calcium phosphate (TCP), di-calcium phosphate (DCP), hydroxylapatite, and rock phosphate. It is already proved that application of Zn in the form of chemical fertilizer is inappropriate due to its unavailability to crop plants. In the recent past, rhizobacteria have exhibited terrific ability to improve zinc availability in root zone and enhance zinc in plants. In rice, silicate-solubilizing rhizobacteria (SSR) have gained importance in recent times because of their role in solubilization of silicate minerals, rendering potassium silicate (K2SiO3), and makes readily available potassium (K) and silicon (Si) for crop plants. Recent advancement in biotechnology and genetic engineering have provided new opportunity to find out the presence and abundance of particular microbes or to quantify the expression of target genes directly in soil or in rhizosphere with high levels of sensitivity. Genetically modified strains could be capable of solubilizing more available nutrient from the soil or rhizosphere. Development of genetically modified strains with enhanced mobilization by genetic engineering techniques and DNA technology is needed to maintain an eco-friendly and sustainable agriculture. This chapter focuses on diversifying of nutrient solubilizer/mobilizer microbes, mechanism of solubilization/mobilization, role of various enzymes/ auxins/acids effect of various factors on nutrient solubilization, the present and future scenario of their utilization, and potential for application of this knowledge in managing a sustainable environmental ecosystem.