Biofertilizers – A Key to Sustaining Agricultural Productivity: A Review

Biofertilizers – A Key to Sustaining Agricultural Productivity: A Review

Asma Fayaz , Sushma Sharma , Bilal Lone^* , Sandeep Kumar , Nighat Mushtaq , Zahoor Ahmad Dar , Sameem Shafi Hafiz

University Institute of Agricultural Sciences, Chandigarh University, Mohali, India

Corresponding Author Email: alonebilal127@gmail.com

DOI : http://dx.doi.org/10.53709/ CHE.2021.v02i03.005

Abstract

Keywords

Download this article as:

Introduction

Conventional agriculture plays a significant role in meeting the food demands of a growing human population, which has also led to an increasing dependence on chemical fertilizers and pesticides [1; 44]. Chemical fertilizers are industrially manipulated substances composed of known quantities of Nitrogen, phosphorus and potassium, and their exploitation causes air and groundwater pollution by eutrophication of water bodies [37;56]. In this regard, recent efforts have been channelized more towards the production of ‘nutrient-rich high-quality food’ in sustainable comportment to ensure bio-safety. The innovative view of farm production attracts the growing demand of biological-based organic fertilizers exclusive of alternatives to agrochemicals. In agriculture, encourage alternate means of soil fertilization relies on organic inputs to improve nutrient supply and conserve the field management. Organic farming is one of such strategies that not only ensures food safety but also adds to the biodiversity of the soil. The additional advantages of biofertilizers include longer shelf life causing no adverse effects to the ecosystem.

Organic farming is mostly dependent on the natural microflora of the soil, which constitutes all kinds of Biofertilizers. Bio-fertilizers are micro-organisms that bring about nutrient enrichment of soil by enhancing the availability of nutrients to crops. The micro-organisms which act as bio-fertilizers are bacteria, cyanobacteria (blue-green algae), and mycorrhizal fungi. Bacteria and cyanobacteria have the property of nitrogen fixation, while mycorrhizal fungi preferentially withdraw minerals from organic matter for the plant with which they are associated. Nitrogen fixation is the process of conversion of molecular or dinitrogen into nitrogen compounds. Insoluble forms of soil phosphorus are converted into soluble forms by certain micro-organisms. This makes phosphorus available to the plants. Phosphate is also solubilized by some bacteria and by some fungi that form an association with plant roots. Biofertilizers keep the soil environment rich in all kinds of micro and macro-nutrients via nitrogen fixation, phosphate and potassium solubilization or mineralization, the release of plant growth regulating substances, production of antibiotics and biodegradation of organic matter in the soil [47]. When bio fertilizers are applied as seed or soil inoculants, they multiply and participate in nutrient cycling and benefit crop productivity. In general, 60% to 90% of the total applied fertilizer is lost and the remaining 10% to 40% is taken up by plants. In this regard, microbial inoculants have paramount significance in integrated nutrient management systems to sustain agricultural productivity and a healthy environment. The PGPR or co-inoculants of PGPR and AMF can advance the nutrient use efficiency of fertilizers. Synergistic interaction of PGPR and AMF was better suited to 70% fertilizer plus AMF and PGPR for P uptake. Similar trends were also reflected in N uptake on a whole-tissue basis which shows that 75%, 80%, or 90% fertilizer plus inoculants were significantly comparable to 100% fertilizer.

Brief Account of Beneficial Microorganisms

Rhizobium Belongs to family Rhizobiaceae, Symbiotic N2 fixation by Rhizobium in legumes contributes substantially to total biological nitrogen fixation. Different species of rhizobium are classified into two groups viz., (1) Slow growing rhizobia (under the genus Bradyrhizobium) and (2) Fast-growing groups (under the genus Rhizobium). Inoculation methods are necessary where seed treatment with fungicides and insecticides is needed or where seeds like groundnut and soya bean can be damaged when the inoculants is used with an adhesive. Direct contact with the acidic fertilizer can also be harmful for Rhizobium. Apart from the application with seeds, the normal carrier-based inocula can also be separately applied, fix nitrogen 50-100 kg/ ha in association with legumes only. It is useful for pulse legumes like chickpea, red-gram, pea, lentil, black gram, etc., oil-seed legumes like soybean and groundnut, and forage legumes like berseem and lucerne. Successful nodulation of leguminous crops by Rhizobium largely depends on the availability of compatible strain for a particular legume. It colonizes the roots of specific legumes to form tumor like growths called root nodules, which act as ammonia production factories. Rhizobium has ability to    

fix atmospheric Nitrogen in symbiotic association with legumes and certain non-legumes like Parasponia. , The rhizobium population in the soil depends on the presence of legume crops in the field. In the absence of legumes, the population decreases. Artificial seed inoculation is often needed to restore the population of effective strains of the Rhizobium near the rhizosphere to hasten N-fixation. Each legume requires a specific species of Rhizobium to form effective nodules [56].

Table 1: Quantity of biological N fixed by Liquid Rhizobium in different crops

Chenn (1999)

Azotobacter belongs to the family Azotobacteriaceae, aerobic, free-living, and heterotrophic in nature. Azotobacters are present in neutral or alkaline soils and A. chroococcum is the most commonly occurring species in arable soils. A. vinelandii, A. beijerinckii, A. insignis, and A. macrocytogenes are other reported species. The number of Azotobacter rarely exceeds of 104 to 105 g-1 of soil due to lack of organic matter and presence of antagonistic microorganisms in the soil. The bacterium produces anti-fungal antibiotics, which inhibits the growth of several pathogenic fungi in the root region, thereby preventing seedling mortality to a certain extent. The population of Azotobacter is generally low in the rhizosphere of the crop plants and in uncultivated soils. The occurrence of this organism has been reported from the rhizosphere of a number of crop plants such as rice, maize, sugarcane, bajra, vegetables, and plantation crops.

Azospirillum belongs to family Spirilaceae, heterotrophic and associative. In addition to their nitrogen-fixing ability of about 20-40 kg/ha, they also produce growth regulating substances. Although there are many species under this genus like A.amazonense, A.halopraeferens, A.brasilense, but, worldwide distribution and benefits of inoculation have been proved mainly with the A.lipoferum and A.brasilense. The Azospirillum form associative symbiosis with many plants, particularly with those having the C4-dicarboxyliac path way of photosynthesis (Hatch and Slack pathway), because they grow and fix Nitrogen on salts of organic acids such as malic, aspartic acid. Thus it is mainly recommended for maize, sugarcane, sorghum, pearl millet, etc. The Azotobacter colonizing the roots not only remains on the root surface but also a sizable proportion of them penetrates into the root tissues and lives in harmony with the plants. They do not, however, produce any visible nodules or outgrowth on root tissue.

Phosphate solubilizing microorganisms

 Phosphorus is also one of the significant elements required for plant growth and higher yields. This element is necessary for the modulation by Rhizobium and even to nitrogen fixers, Azolla and BGA. The phospho-microorganism mainly bacteria (Bacillus Megaterium, Pseudomonas Straiata) fungi (Aspergillus Awamori , Penicillium Digitatum) and Actinomycetes ( Streptomyces) make available insoluble phosphorus to the plants. The root fungus association or Mycorrhiza has high potential in accumulating phosphorus in the plants. The mixture of charcoal and soil is satisfactory material for these microorganisms in order to prepare commercial inoculants. It is reported that microphones cultures increase yield up to 200-500 kg ha^-1, and thus, 30-50 kg superphosphate can be saved.

Vesicular Arbuscular Mycorrhizae (VAM)

The symbiotic association between plant roots and fungal mycelia is termed as mycorrhiza (Fungal roots). These fungi are obligate symbionts and have not been cultured on nutrient media. VAM fungi infect and spread inside the root. They possess special structures known as vesicles and arbuscules. The arbuscules help in the transfer of nutrients from the fungus to the root system and the vesicles, which are saclike structures, store P as phospholipids. VAM has been associated with increased plant growth and with enhanced accumulation of plant nutrients, mainly P, Zn, Cu and S mainly through greater soil exploration by mycorrhizal hyphae.

Azolla

 Azolla is a floating fresh water fern inside which  grows the  nitrogen-fixing  BGA Anabaena. It contains 3.4% nitrogen and produces organic matter in the soil. This biofertilizer is used for rice cultivation in different countries such as Vietnam, China, Thailand and Philippines. This can be easily grown in cooler regions. There is need to develop tolerant strains to high temperature salinity and pests and disease-resistance for its wider adaption. Field trial indicated that rice yields are increased by 0.5- 2 t ha^-1 due to Azolla application. Recent studies have revealed the potentialities of Azolla as a nitrogenous fertilizer in carp culture ponds. Application of different doses of Azolla in fish culture ponds shown that a minimum of 25 kg N ha^-1 year^-1 could be provided through the application of 10-12 t of Azolla ha^-1 year^-1. There are six species of Azolla: A.caroliniana, A.nilotica, A.mexicana, A. filiculoides, A. microphylla and A. pinnata. It grows in ditches and stagnant water. This fern usually forms a green mat over water.

Blue green algae

These belong to eight different families, phototrophic and produce Auxin, Indole acetic acid, and Gibberellic acid, fix 20-30 kg N/ha in submerged rice fields as they are abundant in paddy, so also referred as , paddy organisms‟. N is the key input required in large quantities for low land rice production. Soil N and BNF by associated organisms are significant sources of N for low-land rice [54]. The 50- 60% N requirement is met through the combination of mineralization of soil organic N and BNF by free-living and rice plant-associated bacteria. To achieve food security through sustainable agriculture, the requirement for fixed Nitrogen must be increasingly met by BNF rather than by industrial nitrogen fixation. BGA forms a symbiotic association capable of fixing Nitrogen with fungi, liverworts, ferns, and flowering plants. Still, the most common symbiotic association has been found between a free-floating aquatic fern, the Azolla. Besides N-fixation, these bio fertilizers or bio manures also contribute significant amounts of P, K, S, Zn, Fe, Mb, and other micronutrients. The fern forms a green mat over water with a branched stem, deeply bilobed leaves, and roots.

Plant Growth-Promoting Rhizobacteria (PGPR)

A group of rhizosphere bacteria that exert a beneficial effect on plant growth is referred to as PGPR.They belong to several genera e.g. Actinoplanes, Agrobacterium, Alcaligenes, Amorphosporangium, Arthrobacter, Azotobacter, Bacillus, Cellulomonas, Enter obacter, Erwinia, Flavobacterium, Pseudomonas, Rhizobium and Bradyrhizobium, Streptomyces and Xanthomonas. The plant growth-promoting microorganisms improved potato growth and yield in short-but, not long-rotation soils, primarily by suppressing cyanide-producing deleterious rhizosphere microorganisms. Large populations of bacteria established on planting material and roots become a partial sink for nutrients in the rhizosphere, thus, reducing the amount of C and N available to stimulate spores of fungal pathogens or for subsequent colonization of the root. In field trials with wheat, potato, sugar beet, and zinnia conducted showed significant yield increases varying from 7-136%, with an average increase of 7-35% in different crops over the control. Seed treatment with B. subtilis increased the yield of carrot by 48%, oats by 33%, and groundnut up to 37%.

Table 2: Dosage of liquid Bio-fertilizers in different crops

Recommended Liquid Bio-fertilizers and its application method, quantity to be used for different crops are as follows:

Chenn (1999)

              Bio fertilizer exploitation and nutrient profile of crops

A key advantage of beneficial microorganisms is to assimilate phosphorus for their own requirement, which in turn available as its soluble form in sufficient quantities in soil. Pseudomonas, Bacillus, Micrococcus, Flavobacterium, Fusarium, Sclerotium, Aspergillus, and Penicillium have been reported to be active in the solubilisation process [36].  A phosphate-solubilizing bacterial strain   NII-0909   of Micrococcus sp. has polyvalent properties including phosphate solubilization and siderophore production [10].  Similarly,  two fungi Aspergillus fumigatus and A. Niger were isolated from decaying cassava peels were found to convert cassava  wastes  by the semi-solid   fermentation   technique   to   phosphate   biofertilizers. Burkholderia vietnamiensis, stress-tolerant bacteria, produces gluconic and 2-ketogluconic   acids,   which   involved in   phosphate solubilization [33]. Enterobacter and Burkholderia that were isolated from the rhizosphere of sunflower were found to produce siderophores and indolic compounds (ICs) which can solubilize   phosphate.   Potassium solubilising microorganisms   (KSM)   such as    genus Aspergillus, Bacillus, and Clostridium are found to be efficient in potassium solubilization in the soil and mobilization in different crops [28]. Mycorrhizal mutualistic symbiosis with plant roots satisfies the plant nutrients demand, which enhances plant growth and development and protects plants from pathogens attack and environmental stress. It leads to phosphate absorption by the hyphae from outside to internal cortical mycelia, which finally transfer phosphate to the cortical root cells [48]. Nitrogen-fixing cyanobacteria such as Aulosira, Tolypothrix, Scytonema, Nostoc, Anabaena and Plectonema are commonly used as biofertilizers [40]. Besides the contribution of Nitrogen, growth-promoting substances, and vitamins liberated by these algae Cylindrospermum musicola increase the root growth and yield of rice plants [58]. Interestingly, genetic engineering was used to improve the nitrogen-fixing potential of Anabaena sp. strain PCC7120. Constitutive expression of the hetR gene driven by a light-inducible promoter enhanced HetR protein expression, leading to higher nitrogenase activity in Anabaena sp. strain PCC7120 as compared with the wild-type strain. This caused better growth of paddy when applied to the fields [7]. In an experiment conducted on sodic soil [31] reported that grain and straw yield (6000 kgs and 7814 kg ha-¹) significantly increased over control as well as 100% N alone. (Table.4).The result of another field experiment showed that inoculation of A.diazotrophicus to sugar cane enhances the cane yield and sugar yield (Table.3) than Azospirillum inoculation.

Table.03: Response of Sugarcane to Gluconacetobacter diazotrophicus inoculation

Table: 04 Effect of inoculating cyanobacteria in rice grain in sodic soils.

(Pandiyarajan, p.1999)

                 Biofertilizers relevance and plant tolerance to environmental stress

Abiotic and biotic stresses are the major constraints that are affecting the productivity of the crops. Many tools of modern science have been extensively applied for crop improvement under stress, of which PGPRs role as bio protectants has become paramount importance in this regard . Rhizobium trifolii inoculated with Trifolium alexandrinum showed higher biomass and an increased nodulation under salinity stress condition berseem. Pseudomonas aeruginosa has been shown to withstand biotic and abiotic stresses [35] found that P. fluorescens MSP-393 produces osmolytes and salt-stress induced proteins that overcome the negative effects of salt. P. putida Rs-198 enhanced germination rate and several growth parameters viz., plant height, fresh weight, and dry weight of cotton under the condition of alkaline and high salt via increasing the rate of uptake of K⁺, Mg²⁺ and Ca²⁺, and by decreasing the absorption of Na⁺. Few strains of Pseudomonas conferred plant tolerance via 2,4-diacetylphloroglucinol (DAPG). Interestingly, the systemic response was found to be induced against P.syringae in Arabidopsis thaliana by P. fluorescens DAPG [55] Calcisol produced by PGPRs viz., P. alcaligenes PsA15, Bacillus polymyxa BcP26, and Mycobacterium phlei MbP18 provide tolerance to high temperatures and salinity stress [12]It has been demonstrated that inoculation of plants with AM fungi also improves plant growth under salt stress Achromobacter piechaudii was also shown to increase the biomass of tomato and pepper plants under 172 mM NaCl and water stress. Interestingly, a root endophytic fungus Piriformospora indica was found to defend the host plant against salt stress in one of the studies, it was found that inoculation of PGPR alone or along with AM like Glomus intraradices or G. mosseae resulted in better nutrient uptake and improvement in normal physiological processes in Lactuca sativa under stress conditions. The same plant treated with P. mendocina increased shoot biomass under salt stress [22]. Mechanisms involved in osmotic stress tolerance employing transcriptomic and microscopic strategies revealed a considerable change in the transcriptome   of Stenotrophomonas   rhizophila DSM14405^T in   response   to   salt stress. The combination of AM fungi and N2-fixing bacteria helped the legume plants in overcoming drought stress [2] the effect of A.brasilensea AM can be seen in other crops such as tomato, maize, and cassava [14]. A. brasilense and AM combination improved plant tolerance to various abiotic stresses. The additive effect of Pseudomonas putida or Bacillus megaterium and AM fungi was effective in alleviating drought stress. Application of Pseudomonades sp. under water stress improved the antioxidant and photosynthetic pigments in basil plants. Interestingly, the combination of three bacterial species caused the highest CAT, GPX, and APX activity and chlorophyll content in leaves under water stress. Pseudomonas spp. was found to cause a positive effect on the seedling growth and seed germination of A. officinalis L. under water stress. Photosynthetic efficiency and the antioxidative response of rice plants subjected to drought stress were found to increase after inoculation of arbuscular mycorrhiza [41] the beneficial effects of mycorrhizae have also been reported under both drought and saline conditions. Heavy metals such as cadmium, lead, mercury from the hospital, and factory waste    accumulate in the soil and enter plants through roots [15] Azospirillium spp, Phosphobacteria spp, and Glucanacetobacter spp. isolated from the rhizosphere of rice field and mangroves were found to be more tolerant to heavy metal specially iron [43] P. potida strain 11 (P.p.11), P. potida strain 4 (P.p.4), and P. fluorescens strain 169 (P.f.169) can protect canola and barley plants from the inhibitory effects of cadmium via IAA, siderophore and 1-aminocyclopropane-1-carboxylate deaminase (ACCD) It was reported that rhizo remediation of petroleum contaminated soil can be expedited by adding microbes in the form of effective microbial agents (EMA) to the different plant species such as cotton, ryegrass, tall fescue, and alfalfa [50]. In pot culture experiment [23] reported that dual inoculation of both Glomus fasciculatum and Rhizobium japonicum produced higher dry weight, nodule weight, VAM colonization and nutrient uptake of N and P than individual inoculants.(Table.10). It was shown that the addition of arbuscular mycorrhizal fungi and Pseudomonas fluorescens to the soil can reduce the development of root-rot disease and enhance the yield of Phaseolus vulgaris L. [29] [46] reported that with regard to the effect on the subsequent crop of rap

seed, the treatment containing Azolla was found better in yield and the highest was recorded with the higher dose of N in integration with Azolla.(Table 5).In another experiment [42]reported that combined inoculation of Azolla with 60kgN/ha gave maximum grain yields with an increase of 56 percent over the uninoculated control.(Table.7) also, a field trial conducted on soyabean over six years showed increased yield because of rhizobial inoculations(Table.8)

Table: 05 Effect of BGA and Azolla with graded levels of Nitrogen on the yield of rapseed Crop (Pooled data of 1997-98 and 1998-99)

Table 06: Effect of Azolla with different levels of Nitrogen on the number of Tillers, grain yield and straw yield of Rice

Table :07 Front line demonstration trial on the use of Rhizobium in Soyabean conducted by Tamil Nadu Agriculture University

Table: 08 Residual effect of Vermicompost and Biofertilizers applied to Chickpea     on green and dry fodder yield of Maize

Table: 09 Effect of treatments on the economics of rice

(Patra, S.K.1989)

The potential significance of beneficial microbes in sustainable Agriculture

The rhizosphere, which is the narrow zone of soil surrounding plant roots, can comprise up to 10¹¹ microbial cells per gram of root [12] and above 30,000 prokaryotic species [27] that in general, improve plant productivity [27]. The collective genome of rhizosphere microbial community enveloping plant roots is more significant compared to that of plants and is referred to as microbiome [5] whose interactions determine crop health in natural agro-ecosystem by providing numerous services to crop plants viz., organic matter decomposition, nutrient acquisition, water absorption, nutrient recycling, weed control and bio-control [4]. The metagenomic study provides the individual the core rhizosphere and endophytic microbiomes activity in Arabidopsis thaliana using 454 sequencings (Roche) of 16S rRNA gene amplicons 18]. It has been proposed that exploiting tailor-made core microbiome transfer therapy in agriculture can be a potential approach in managing plant diseases for different crops [16]. Rhizosphere microbial communities an alternative for chemical fertilizers has become a subject of great interest in sustainable agriculture and bio-safety program.

A major focus in the coming decades would be on safe and eco-friendly methods by exploiting the beneficial micro-organisms in sustainable crop production [30]. Such microorganisms, in general, consist of diverse naturally occurring microbes whose inoculation to the soil ecosystem advances soil physicochemical properties, soil microbes biodiversity, soil health, plant growth, and development and crop productivity. The agriculturally useful microbial populations cover plant growth promoting rhizobacteria, N2-fixing cyanobacteria, mycorrhiza, plant disease beneficial suppressive bacteria, stress tolerance endophytes, and bio-degrading microbes [46]. Biofertilizers are a supplementary component to soil and crop management traditions viz., crop rotation, organic adjustments, tillage maintenance, recycling of crop residue, soil fertility renovation, and the biocontrol of pathogens and insect pests, which operation can significantly be useful in maintaining the sustainability of various crop productions. Azotobacter, Azospirillum, Rhizobium, cyanobacteria, phosphorus, and potassium solubilizing microorganisms, and mycorrhizae are some PGPRs that were found to increase in the soil under no-tillage or minimum tillage treatment [3]. Efficient strains of Azotobacter, Azospirillum, Phosphobacter, and Rhizobacter can provide a significant amount of Nitrogen to Helianthus annus and  increase the plant height, the number of leaves, stem diameter percentage of seed filling, and seed dry weight [11]. Similarly, in rice, the addition of Azotobacter, Azospirillium, and Rhizobium promotes the physiology and improves the root morphology [9][46] found that Azolla can be integrated with 80kg N ha-¹in low land rice cultivation in order to obtain a higher yield in rice and the subsequent crop rapseed is also found economically feasible as it gave the highest return per rupee investment in the rice rapseed sequence.(Table.14)

 Table :10 Effect of BGA and Azolla with graded levels of Nitrogen on the economics of Rice-Rapseed –crop     sequence

Conclusion

Environmental stresses are becoming a major problem, and productivity is declining at an unprecedented rate. Our dependence on chemical fertilizers and pesticides has encouraged the thriving of industries that are producing life-threatening chemicals and which are not only hazardous for human consumption but can also disturb the ecological balance. Biofertilizers can help solve the problem of feeding an increasing global population at a time when agriculture is facing various environmental stresses. It is essential to realize the valuable aspects of bio fertilizers and implement their application to modern agricultural practices. The new technology developed using the powerful tool of molecular biotechnology can enhance the biological pathways of the production of phytohormones. If identified and transferred to the useful PGPRs, these technologies can help provide relief from environmental stresses.

 References

1. Adesmoye A.O, Kloepper J.W. (2009) Plant-microbes interactions in enhanced fertilizer use efficiency. Applied Microbiology & Biotechnology.
2. Aliasgharzad, N., Reza, M., Neyshabouri, and Salimi, G. 2006. Effects of arbuscular mycorrhizal fungi and Bradyrhizobium japonicum on drought stress of soybean. Biologia. 19:324–328.
3. Aziz, G., Bajsa, N., Haghjou, T., Taule, C., Valverde, A., Mariano, J. and Arias A. (2012). Abundance, diversity and prospecting of culturable phosphate solubilizing bacteria on soils under crop–pasture rotations in a no-tillage regime in Uruguay. Applied   Soil   Ecology. 61:320–326.
4. Berg, G., Zachow, C., Muller, H., Phillips, J. and Tilcher, R. Next-generation bio-products sowing the seeds of success for sustainable agriculture. Agronomy. 2013; 3:648–656.
5. Bulgarelli, D., Schlaeppi, K., Spaepen, S., Loren, V., Van, T. E., Schulze,L. P. 2013. Structure and functions of the bacterial microbiota of plants. Annual Review of Plant Biology. 64:807–838.
6. Carrapico, F., G. Teixeira and M.A. Diniz, 2000. Azolla as a biofertiliser in Africa: A challenge for the future. Revesta Cincies Agraries, 23: 120-138.
7. Chaurasia, A.K. and Apte, S.K. 2011. Improved eco-friendly recombinant Anabaena sp. strain PCC7120 with enhanced nitrogen biofertilizer potential. Applied Environment Microbiology. 77:395–399.
8. Chenn, P., Micro-organisms in Agriculture. Biological Sciences Review, May 1999, Vol. 11, pp2-4
9. Choudhury, M.A. and Kennedy, I.R. 2004. Prospects and potentials for system of biological nitrogen fixation in sustainable rice production. Biology and Fertility of Soils. 39:219–227.
10. Dastager, S.G., Deepa, C.K. and Pandey, A. 2010. Isolation and characterization of novel plant growth promoting Micrococcus sp NII-0909 and its interaction with cowpea. Plant Physiology and Biochemistry.48:987–992.
11. Dhanasekarm R. and Dhandapani, R. 2012. Effect of biofertilizers on the growth of Helianthus annuus. International Journal of plant, Ani Environ Sci.2:143–147.
12. Egamberdieva, D., Kamilova, F., Validov, S., Gafurova, L., Kucharova, Z. and Lugtenberg, 2008. High incidence of plant growth stimulating bacteria associated with the rhizosphere of wheat grown on salinated soil in Uzbekistan. Environment Microbiology. 10:1–9.
13. Egamberdiyeva, D. 2007. The effect of plant growth promoting bacteria on growth and nutrient uptake of maize in two different soils. Applied. Soil Ecology. 36:184–189.
14. German, M.A., Burdman, S., Okon, Y. and Kigel, J. 2000. Effects of Azospirillum brasilense on root morphology of common bean (Phaseolus vulgaris) under different water regimes. Biology and Fertility of Soils. 32:259–264.
15. Gill, S.S., Khan, N.A. and Tuteja, N. 2012. Cadmium at high dose perturbs growth, photosynthesis and Nitrogen metabolism while at low dose it up regulates sulphur assimilation and antioxidant machinery in garden cress  (  Lepidium  sativum  ) Plant  Science. 182:112–120.
16. Gopal, M., Gupta, A. and Thomas, G.V. 2013. Bespoke microbiome therapy to manage plant diseases. Frontiers in Microbiology. 5:15.
17. Gyaneshwar P, Kumar G.N., Parekh L.J, Poole P.S. 2002 Role of soil microorganisms in improving P nutrition of plants. Plant Soil.245:83–93
18. Hirsch, P.R. and Mauchline, T.H. 2012. Who’s who in the plant root microbiome? Nature Biotechnology. 30:961–962
19. Jat, N.2002. Residual effect of Vermicompost and Biofertilizers applied to Chickpea on green and dry fodder yield of Maize. Biofertilizer Technology p.32-37.
20. Jayakumar,P. and Thangaraju, M.1996.Assessment of nitrogen fixation by Acetobacter diazotrophicus in sugarcane. TANU Newsletter.,26:1-2.
21. Kennedy, I.R, Choudhury, M.A, Kecskes, M.L. 2004. Non-symbiotic bacteria diazotrophs in crop-farming systems. Soil Biology & Biochemistry.36:1229–1244
22. Kohler, J. and Caravaca, F. 2010. An AM fungus and a PGPR intensify the adverse effects of salinity on the stability of rhizosphere soil aggregates of Lactuca sativa Roldan. Soil Biology and Biochemistry. 42:429–434.
23. Kumutha,K., Sempavalan, J. And Santhanakrishnan.2006. Effect of Insoluble phosphate and dual inoculation on soybean. Biofertilizer Technology p.354-358.
24. Liu, J.Y., Maldonado,M. I., Lopez,M. M., Cheung, F., Town, C.D. and Harrison, M.J. 2007. Arbuscular mycorrhizal symbiosis is accompanied by local and systemic alterations in gene expression and an increase in disease resistance in the shoots. Plant Journal. 50:529–544.
25. Medeiros, F.H.V., Souza, R.M., Medeiros, F.C.L., Zhang, H., Wheeler, T., Payton, P., Ferro,
26. H.M. and Pare, P.W. 2011. Transcriptional profiling in cotton associated with Bacillus subtilis (UFLA285) induced biotic-stress tolerance. Plant Soil. 347:327–337.
27. Mendes, R., Garbeva, P. and Raaijmakers, J.M. 2013. The rhizosphere microbiome: significance  of  plant  beneficial  plant  pathogenic  and  human  pathogenic  microorganisms. FEMS Microbiology Reviews. 37:634–663.
28. Mohammadi, K. and Yousef, S. Y. 2012. Bacterial Biofertilizers for sustainable crop production: A review. Journal of Agriculture and Biological Science. 7:307–316.
29. Neeraj, K.S. 2011. Organic amendments to soil inoculated arbuscular mycorrhizal fungi and Pseudomonas fluorescens treatments reduce the development of root-rot disease and enhance the yield of Phaseolus vulgaris. European Journal of Soil Biology. 47:288–295
30. Nina, K., Thomas, W.K. and Prem, S.B. 2014. Beneficial organisms for nutrient uptake. VFRC report 2014/1, virtual fertilizer research center. Washington, DC: Wageningen Academic Publishers. p. 63.
31. Pandiyarajan, P. and Nagarajan, P.1999.Inoculating cyanobacteria for rice cultivated in sodic soils. Biofertilizers Technology. P141-145.
32. Panwar, J.D.S. and O. Singh, 2000. Response of Azospirillum and Bacillus on growth and yield of wheat under field conditions. Indian Journal Plant Physiology 5: 108-11
33. Park. J., Bolan, N., Megharaj, M. and Naidu, R. 2010. Isolation of Phosphate-Solubilizing Bacteria and characterization of their Effects on Lead Immobilization. Pedologist. 53:67–75.
34. Patra, S.K., Padhi, A.K. and Mishra, S.N.1989.Effect of BGA, PSB, at varying levels of Nitrogen on paddy in north eastern ghat of Orissa. Environment Ecology, 7(2):271-274.
35. Paul D, Nair S. 2008. Stress adaptations in a plant growth promoting Rhizobacterium (PGPR) with increasing salinity in the coastal agricultural soils. Journal of Basic Microbiology. 48:1–7.
36. Pindi, P.K. and Satyanarayana, S.D.V. 2012. Liquid microbial consortium- a potential tool for sustainable soil health. Journal of Biofertilizers and Biopesticides. 3:4.
37. Rajendra, P., S. Singh and S.N. Sharma, 1998. Interrelationship of fertilizers use and other agricultural inputs for higher crop yields. Fertilizers News, 43: 35-40.
38. Rao, D.L.N., 1999. Biofertilizer situation in India and future thrusts. Indian Council of Agricultural Research (ICAR), pp: 1-3.
39. Roberts, N.J., Morieri, G., Kalsi, G., Rose, A., Stiller, J., Edwards, A., Xie, F., Gresshoff, P.M., Oldroyd, G.E., Downie, J.A. and Etzler, M.E. 2013. Rhizobial and mycorrhizal symbioses in Lotus japonicus require lectin nucleotide phosphohydrolase, which acts upstream of calcium signaling. Plant Physiology. 161:556–567.
40. Roy, M. and Srivastava, R.C. 2013. Assembling BNF system in rice plant: frontier areas of research. Current Sciences. 104:3–10.
41. Ruiz, S . M., Aroca, R., Munoz, Y., Polon, R. and Ruiz,L. J.M. 2010. The arbuscular mycorrhizal symbiosis enhances the photosynthetic efficiency and the antioxidative response of rice plants subjected to drought stress. Journal of Plant Physiology. 167:862–869.
42. Samal, K.C.1999.Impact of Azolla bio fertilizer on rice yields in Hirakud Command area. Biofertilizer Technology. p.193-197.
43. Samuel, S. and Muthukkaruppan, S.M. 2011. Characterization of plant growth promoting rhizobacteria and fungi associated with rice, mangrove and effluent contaminated soil. Current Botany.2:22–25.
44. Santos, V.B., Araujo, S.F., Leite, L.F., Nunes, L.A. and Melo, J.W. 2012. Soil microbial biomass and organic matter fractions during transition from conventional to organic farming systems. Geoderma. 170:227–231.
45. Sevilla, M., Burris, R.H., Gunapala, N. and Kennedy, C. ( 2001) Comparison of benefit to sugarcane plant growth and 15n2 incorporation following inoculation of sterile plants with Acetobacter diazotrophicus wild-type and Nif–mutant strains. Molecular Plant-Microbe Interactions. 14:358–366.
46. Singh,L.N.(1999).Effect on Bio fertilizers with graded levels of Nitrogen on Low land rice and its carryover effect on Succeeding crop of Rapseed . Biofertilizer Technology.p.182-186.
47. Sinha, R.K., Valani, D., Chauhan, K. and Agarwal, S. 2014. Embarking on a second green revolution for sustainable agriculture by vermiculture biotechnology using earthworms: reviving the dreams of Sir Charles Darwin. International Journal of Agricultural health and safety .1:50–64.
48. Smith, S., Lakobsen, I., Gronlund, M.and Smith, F.A. 2011. Roles of arbuscular mycorrhizas in plant phosphorus nutrition: interactions between pathways of phosphorus uptake in arbuscular mycorrhizal roots have important implications for understanding and manipulating plant phosphorus acquisition. Plant Physiology. 156:1050–1057.
49. Sundara Rao, W.V.B and Sinha, M.K 1963. Phosphate dissolving organisms in Soil and rhizosphere. Indian Journal of Agricultural Science .33..272-278
50. Tang, J., Wang, R., Niu, X., Wang, M. and Zhou, Q. 2010.Characterization on the rhizoremediation of petroleum contaminated soil as affected by different influencing factors. Bio geosciences Discuss. 7:4665–4688.
51. Tilman D, Cassman KG, Matson PA, Naylor R, Polasky S. 2002. Agricultural sustainability and intensive production practices. Nature. 418:671–677.
52. Tilman D. 1998. The greening of the green revolution. Nature. 396:211–221
53. Venkataraman, G.S. and Neelakantan, S. 1967. Effect of cellular constituents of the Nitrogen fixing blue-green algae.  Cylindrospermum nusciola on the root growth of rice seedlings. Journal of General Applied Microbiology.13:53–61.
54. Venkatashwarlu, B. (2008). Role of bio-fertilizers in organic farming: Organic farming in rain fed agriculture: Central institute for dry land agriculture, Hyderabad, 85-95.
55. Wani, S.P. and Lee, K.K. (1995). Microorganisms as biological inputs for sustainable agriculture in Organic Agriculture (Thampan, P.K.ed.) Peekay Tree Crops Development Foundation, Cochin, India, 39-76.
56. Weller, D.M., Mavrodi, D.V., Van, P. J.A., Pieterse, C.M., Van L.C. and Bakker, P.A.  2012. Induced systemic resistance in Arabidopsis thaliana against Pseudomonas syringae pv. tomato by 2,4-diacetylphloroglucinol-producing Pseudomonas fluorescens. Phytopathology. 102:403–412
57. Youssef, M.M.A. and Eissa, M.F.M. 2014. Biofertilizers and their role in management of plant parasitic nematodes. A review. E3 Journal of  Biotechnology. Pharmaceutical Research. 5:1–6.