Microbial fertilizers-promising partner for sustainable agriculture

Introduction

The term "biofertilizers" has been coined to the viable population of microbial inoculants which deliver or enhance the nutrient availability, further conserve and mobilize crop nutrients in the rhizosphere soil (Riaz et al., 2020) [25] . The process of improving the nutrient efficiency in the soil to enhance the nutrient needed for plant growth and improve plant yield (Bargaz et al., 2018) [3] . Beneficial soil microorganisms augment the management of the rhizosphere through different mechanisms that are multidimensional. These include the following: production of siderophore, nitrogen fixation, lytic acid production, production of hydrogen cyanide, phosphate solubilization, and production of indole acetic acid (Jacoby et al., 2017) [10] . The mechanisms of action of these beneficial microorganisms play a crucial role in improving soil fertility, plant growth, and yield. They act as an integral part of an agro-input system in sustainable agriculture (Mohamed et al., 2021; Koskey et al., 2021) [10,12] .

Role of biofertilizers

The diverse role of microorganisms in different agro ecological systems plays a significant and protracted role in sustainability of soil health and crop production by tripartite interactions (i.e plant-soil-microbe). Besides the genetic potential, the production potential attainment depends on ecological conditions such as soil health, soil water and on adoption of suitable technologies of crop production. For sustainable agriculture, soil health is more concerned, as crops take away nutrient reserves from the soil. The practice of chemical agriculture resulting in decline of soil microbial activity and soil organic matter coupled with decreased fertilizer use efficiency becomes a serious threat to sustainability Al Abboud et al., 2014; Nosheen et al., 2021 [1,21] . Soil is a biodynamic system as it supports a large population of diverse groups of beneficial microorganisms such as bacteria, actinomycetes, fungi, algae, protozoa etc (Shannon et al., 2006) [28] that decompose biomass, mediate in cycling of plant nutrients and protect plants from pests and diseases. All these functions of soil microorganisms are essential for maintaining soil fertility and productivity of agriculture soils.

The integration of growth enhancing factors that make a soil productive has often been referred to as 'soil quality'. The maintenance and restoration of quality is highly dependent on organic matter additions and an array of beneficial microorganisms it supports (Malusá et al., 2012; Jacoby et al., 2017) [15,10] .

Microorganisms as indicators of soil quality

Consortia of beneficial bio inoculants may act as biological indicators of soil quality that can be brought about by higher level of interactions among diverse species of soil microorganisms. The major attraction of using biofertilizers is their characteristic ability to convert the unavailable nutrient to available form, which is readily available and easily accessible by the plants. They improve soil properties and sustain soil fertility by providing aeration, biomassof organic carbon and nutrients. Use of biofertilizers not only makes atmospheric nitrogen available but also Solubilizes and mobilizes fixed phosphorus and potassium in soil and improve phosphorus and potassium uptake in plants (Nielsen and [19,29,27,20] . They enhance plant growth due to release of hormones, vitamins, auxins etc. They also help to proliferate other beneficial organisms, which, in turn, suppress soil borne plant pathogens. Besides helping to reduce deficit in fertilizer consumption, they are cheaper or without difficulty to available and eco-friendly.

The biofertilizers can be classified based on their functions and properties. Types of biofertilizers

Rhizobium

Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium species from an endosymbiotic nitrogenfixing association with roots of legumes. The bacteria colonize plant cells within root nodules, where they convert atmospheric nitrogen into ammonia using the enzyme nitrogenase and then provide organic nitrogenous compounds such as glutamine or ureides to the plant. The plant, in turn, provides the bacteria with organic compounds made by photosynthesis. This mutually beneficial relationship is true of all of the rhizobia, of which the genus Rhizobium is a typical example (Sawada et al., 2003) [26] . Rhizobium is also capable to solubilize phosphorus.

Rhizobium is the most important PGPR, which is able to develop a symbiotic association with its specific host plant and increase its growth and yield furthermore they support plant growth via synthesis of phytohormones or other growth promoting or protecting substances like siderophores and antibiotics (Harshitha and Goudar 2021) [9] .

Azotobacter

Azotobacter is a group of Gram negative, aerobic, heterotrophic free-living, nitrogen fixing bacteria inhabiting in the soil. They are oval or spherical in shape and form thick-walled cysts (dormant cells resistant to deleterious conditions) under unfavorable environmental conditions. Azotobacter improves seed germination and has beneficiary response on Crop Growth Rate (CGR) (Sumbul et al., 2020) [34] . It helps to increase nutrient availability and to restore soil fertility for better crop response. Azotobacter based inoculants are recommended for cereal crops like wheat, ail paddy vegetable crops like potato and tomato seed crops like mustard and sunflower and fibre crops like cotton and jute.

Azospirillum

Azospirillum is considered to be an associative symbiotic bacterium because it lives in close association with root system of plants. However, they do not produce any visible nodules or out growth on the roots (Raffi and Charyulu, 2021) [23] . They are microaerophillic bacteria and are known to fix atmospheric nitrogen in grasses, sugarcane, paddy, sorghum, maize, millet and wheat. Some spiral shaped bacteria are known to fix nitrogen in maize and other grasses. They do not form nodules but fixes the atmospheric nitrogen inside the roots.

Phosphate solubilizing microorganisms

Phosphate solubilizing microorganisms produce organic acids like citric acid, glutamic acid, succinic acid, lactic acid, oxalic acid, maleic acid, fumaric acid, tartaric acid and alphaketoglutaric acid which are responsible for solubilization of insoluble form of P in to available form of P (Kalayu, 2019) [11] . These organisms have a role in completing the P cycle in nature. Some promising phosphate solubilizing bacteria like Pseudomonas striata, Bacillus polymyxa and fungi Aspergillus awamori, Penicillium digitatum are being multiplied and prepared as carrier based cultures and recommended for all kinds of crops.

Plant Growth promoting Rhizobacteria

Plant growth-promoting rhizobacteria (PGPR) are the rhizosphere bacteria that can enhance plant growth by a wide variety of mechanisms like phosphate solubilization, siderophore production, biological nitrogen fixation, rhizosphere engineering, production of 1-Aminocyclopropane-1-carboxylate deaminase (ACC), quorum sensing (QS) signal interference and inhibition of biofilm formation, phytohormone production, exhibiting antifungal activity, production of volatile organic compounds (VOCs), induction of systemic resistance, promoting beneficial plant-microbe symbioses, interference with pathogen toxin production etc. The potentiality of PGPR in agriculture is steadily increased as it offers an attractive way to replace the use of chemical fertilizers, pesticides and other supplements. Growth promoting substances are likely to be produced in large quantities by these rhizosphere microorganisms that influence indirectly on the overall morphology of the plants (Bhattacharyya and Jha 2011) [4] . Ex: Pseudomonas. Arthrobacter, Enterobacter, Flavobacterium, Azotobacter and Acetobacter.

Composting cultures

The crop residues are mainly comprised of cellulose hemicelluloses and lignin. The cellulolytic and lignolytic microorganisms are isolated, screened and multiplied and used as composting cultures Greff et al., 2022 [8] . The fungal cultures like Trichodema, Aspergillus, Phenerochaete, Trichurus and bacterial cultures of Bacillus, Cellulomonas, Thiobacillus are used as composting cultures.

Acetobacter

Acetobacter is an obligatory aerobic, nitrogen-fixing bacteria that is known for producing acid as a result of metabolic processes. More recently, a new species of nitrogen fixing bacterium Acetobacter diazotrophicus has been isolated from sugarcane in different parts of the world, A.diazotrophicus has been isolated from the root, leaf, bud and stem samples of sugarcane cultivars (Reis et al., 2015; Michavila et al., 2022) [24,17] . This is an acid tolerant and sucrose loving bacterium recommended for sugarcane. Acetobacter is a genus of acetic acid bacteria. Acetic acid bacteria are characterized by the ability to convert ethanol to acetic acid in the presence of oxygen.

Blue Green Algae (BGA)

These are free-living, photoautotrophic micro-organisms, many of which also fix atmospheric nitrogen and are found in abundance in rice fields. Cyanobacteria (blue green algae) like Nostoc, Anabaena, Tolypothrixetc. Generally forming nonfibrous slimy growth on the surface of soil and water. They are photoautotrophic microbes, which utilize solar energy to reduce atmospheric dinitrogen to ammonia Lee and Ryu 2021 [13] . They are suitable biofertilizers for paddy.

Azolla

Azolla is a floating, fast growing, aquatic fern belonging to Pteridophyta, which lives in symbiotic association with the filamentous nitrogen fixing cyanobacterium Anabaena and is ubiquitous in distribution. The Azolla-Anabaena association has a tremendous potential for providing photosynthetic production of nitrogen fertilizer (Seleiman et al., 2021; Thapa and Poundel 2021) [27,35] . The six species known are Azolla pinnata, A. microphaylla, A. mexicana, A. caroliniana, A. filliculoides and A. rubra.

Mycorrhizae

The symbiotic association between fungi and higher plants is known as mycorrhizal association. The term mycorrhiza was coined by a German scientist, Frank. Mycorrhiza means fungus root 'Myco' means fungus 'rhiza' means root Mycorrhizal fungi is associated with almost all the terrestrial plants. Many graminaceous plants and legumes are highly, susceptible to VAM colonization. The transfer of nutrients mainly phosphorous, Zinc and Sulphur from the soil to the cells of the root cortex is mediated by intracellular obligate fungal endosymbiont of the genera Glomus, Gigaspora, Acaulospora, Sclerocystis and Endogone, which possess vesicles for storage of nutrients and arbuscules for funneling these nutrients into the root system. AM fungi are known to secrete a sticky and highly resistant substance known as glomalin a glycoprotein. This chemical has been shown to be very important in soil aggregation (Wright and Upadhyaya 1998) [41] which helps to maintain the soil aggregate stability and prevent the soil runoff. Mycorrhizal fungi improve the plant growth through the increased uptake of nutrients especially phosphorus, Zn, N, Cu, Mn and Soil enzyme activities like dehydrogenase, phosphates, urease activity. Due to exploration of higher soil volume by the external hype even beyond the root zone of nutrient absorption and better soil enzyme activity also influenced by AM fungi Waratadar et al., 2022 [39] . Better tolerance to water stress to the extent of 7-9 days in the annual crops without affecting yield Suma et al., 2019 [33] . Recent studies have revealed that the Mycorrhizal precolonization persuades resistance to plant parasitism by converting strigolactones and orobanchol biosynthetic pathway into mycorradicin, which is assembled in the root zone and thereby condensed accessibility of strigolactones and orobanchol for parasitic weed germination Madhura et al., (2017) [14] ; Manjunath et al., (2018) [16] ; Shubha et al. (2018) [30] ; Chandrashekhara gowda et al., (2018) [6] ; Waratadar et al., 2021a/b [38,40] . They are also known to enhance the activitie of beneficial microflora in the rhizosphere (free living N2 fixess, Psolubilisers and PGPR) while reducing the disease severity caused by root pathogens (Chamkhi et al., 2022) [5] .

Pink pigmented facultative methylotrophs (PPFMs)

A group of bacteria that possess the ability to utilize methyl compounds as a carbon and energy source. They are known for their plant growth-promoting properties and have been studied extensively for their potential applications in agriculture. Here's how PPFMs can promote plant growth:  Methylotrophic Activity: PPFMs have the ability to metabolize methanol, methylamine, and other methyl compounds. By utilizing these compounds, PPFMs can release growth-promoting substances such as cytokinins, auxins, and gibberellins, which positively influence plant growth and development (Nysanth et al., 2023) [22] .  Nutrient Availability: PPFMs can solubilize phosphate, making it more accessible to plants. They can also fix atmospheric nitrogen, converting it into a usable form for plant uptake. These activities contribute to increased nutrient availability and uptake, leading to enhanced plant growth.  Disease Suppression: PPFMs have been found to possess biocontrol properties against various plant pathogens. They can produce antifungal compounds, inhibit pathogen growth, and induce systemic resistance in plants. By protecting plants from diseases, PPFMs contribute to healthier and more robust plant growth (Srivastava et al., 2022) [32] .  Stress Tolerance: PPFMs can enhance plant tolerance to various abiotic stresses such as drought, salinity, and heavy metal toxicity. They do so by producing stress-related enzymes, metabolites, and antioxidants, which help plants, cope with adverse environmental conditions and improve their overall growth and survival (Bajpai et al., 2022) [2] .

Frankia

Species of the Frankia Genus are Gram positive bacteria. Frankia sp. are filamentous nitrogen-fixing bacterium that grow by branching and tip extension and thus resemble the antibioticproducing Streptomyces sp. They live in the soil and have a symbiotic relationship with certain woody angiosperms, called actinorhizal plants. During growth, the Frankia sp. produces three cell types: sporangiospores, hyphae, and diazo-vesicles (spherical, thick walled, lipid-enveloped cellular structures). Actinomycetes of the genus Frankia forms N 2 fixing nodules on several tree species and on shrubs. They are very improved N 2 fixing groups of the environments which are very poor in soil fertility. It has been estimated around 300 kg of N 2 is fixed per ha per year in these environment. Frankia forms nodules in at least 25 genera of plant species. These plants are woody shrubs & tree species found in temperate climate and also from the arctic to tropics and from semi-desert to rain-forest ecosystem. Their population is higher in slightly alkaline soil. The area surrounding plant rhizosphere, is richly colonized by actinomycetes. Their high synthesis of antimicrobial compounds and other metabolites gives them a competitive advantage, besides helping with the improvement of soil fertility by promoting plant growth, providing protection against phytopathogens and resisting environmental stresses (Silva et al., 2022) [31] .

Conclusion

Overreliance on the use of chemical fertilizers has encouraged industries to produce chemicals that arenoxious to human health. Thus, impact on ecological imbalances. These pitfalls are combined with a high cost of production that is beyond the means of many farmers in the developing world. The application of biofertilizers is eco-friendly, relatively inexpensive, nontoxic, and possesses the significant potential to increase plant yield. Thus, the function of plant growth-promoting microorganisms and the application of biofertilizer made from viable microbial strains to the field will be promising approach for the successful management of nutrient in the rhizosphere for sustainable agriculture.

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