The new eco-friendly technologies for production of biofertilizers will overcome the shortcomings of the conventional chemical-based farming which dominates at present. The implementation of technologies shows positive influence on both soil sustainability and plant growth. They support and gradually improve soil fertility by fixing atmospheric nitrogen. They increase the phosphorous content of the soil by solubilizing and releasing unavailable phosphorous. They participate in restoring depleted nutrients in the soil. Growth-promoting substances released by biofertilizers improve plant root proliferation. They also guard the plant against some soil-borne diseases. To popularize and implement more biofertilizers, there is a need of development of new technologies as follows:
1.1 Correct soil treatment
The role of plant nutrients in crop production is well-established and 16 essential plant nutrients have to be available to the crops in required quantities to achieve the yield target. Many studies have also emphasized the importance of N, P and K in enhancing the natural ability of plants to resist stress from drought and cold, pests and diseases. The essential plant nutrients such as N, P, K, Ca, Mg and S are called macronutrients, while Fe, Zn, Cu, Mo, Mn, B and Cl are called micronutrients.
It is necessary to assess the capacity of a soil to supply the lacking amounts of needed plant nutrients (total crop requirement–soil supply). This is also important to produce a good biofertilizer formulation and to supply nutrients that can improve soil health and plant fertility. Several authors have focused their attention on the potential usage of nitrogen from animal manures. Nonetheless, the effort to find a source alternative to animal manure needs further study. Granite powder has also been studied as a good source of slow-release K fertilizer.
Generally, the addition of nitrogen to high C:N ratio residues is capable of accelerating the microbial activity during the fermentation process.
The number of microorganisms and the level of macro- and micronutrients obviously affect the growth of plants. One of the benefits of fertilizers is that they contribute to the availability of the microorganism population. Having a higher initial count of appropriate microbes in a ready biofertilizer right after the fermentation is essential. One of the ways to increase the number of selected microorganisms is by using the concept of an effective microorganism (EM) as introduced by Higa and Wididana (1991). Field experiments are needed to determine the nutrient availability and efficacy of most organic fertilizers. Such an experiment is important because the nutrient content of organic fertilizers varies widely. The quality is directly determined by the number of selected microorganisms in an active form per gram and their capability to promote plant growth and soil fertility.
1.2. Water-in-oil emulsions appear to be a good, yet underutilized, method for storing and delivering microorganisms through liquid formulations. The oil traps the water around the organism and, therefore, slows down water evaporation once applied. This is particularly beneficial for organisms that are sensitive to desiccation or in the case of use for horticultural crops where irrigation systems are in place. Water-in-oil emulsions allow the addition of substances to the oil and/or aqueous phases which could improve both the cell viability and the kinetics of release. However, cell sedimentation during storage is a major issue to be considered. Studies aimed at solving this problem with the help of nanomaterials are underway. Thickening the oil phase using hydrophobic silica nanoparticles can significantly reduce cell sedimentation and improve cell viability during storage.
1.3. Preparation of bacterial inoculants is supported by implementation of a new process based on the application of supercritical fluid properties which has been tested to encapsulate virus formulations. The process, named PGSS (Particles from Gas Saturated Solutions), is carried out at low temperatures and uses carbon dioxide as a supercritical fluid. Therefore, there should be no negative effects on the microbial viability, and the cost of production would be relatively low. The final product of the process is almost spherical particles that form a free-flowing powder which can be suspended in water. The possibilities of the PGSS process have already successfully been demonstrated for several solids and liquids.
1.4. Another interesting new technology is the exploitation of the natural production of bacterial biofilms as a possible carrier, and not only for the production of the inoculum, of defined bacterial or fungal–bacterial consortia. Biofilm production is already obtained for different industrial applications (e.g., wastewater treatment, production of chemical compounds). Two types of biofilms are employed in that case: biofilms growing onto inert supports (charcoal, resin, concrete, clay brick, and sand particles) and biofilms that are formed as a result of aggregate formation. In the first case, biofilms grow all around the particles, and the size of the biofilm particles grows with time usually to several millimeters in diameter. Biofilms formed by aggregation are called granular biofilms; granule formation may take from several weeks to several months.
There are four stages to the development of a mature biofilm: initial attachment, irreversible attachment by the production of EPS, early development, and maturation of biofilm architecture. What is particularly critical is the production of EPS, which serves to bind the cell to the surface and to protect it from the surrounding environment. EPS can be composed of polysaccharides, proteins, nucleic acids or phospholipids. A common EPS produced by bacterial cells in biofilms is the exopolysaccharide alginate. Beneficial biofilms developed in in vitro cultures containing both fungal and bacterial strains have been used as biofertilizers for non-legume species with good efficacy. Application of a biofilmed inoculant containing a fungal-rhizobia consortium significantly increased N2 fixation in soybean compared to a traditional rhizobium inoculant. Wheat seedlings inoculated with biofilm-producing bacteria exhibited an increased yield in moderate saline soils. Biofilms seem also to help the microorganisms to survive after inoculation even under stress conditions: this is a key aspect for the effectiveness of PGPM inoculation under agricultural conditions. Inoculants made with biofilms were shown to allow their rhizobia to survive at high salinity (400 mM NaCl) by 105-fold compared to rhizobial monocultures. Interestingly, beneficial endophytes were observed to produce higher acidity and plant growth-promoting hormones than their mono- or mixed cultures with no biofilm formation.
1.5. Technologies used for the production of living hybrid materials could be a new frontier in the development of carriers for PGPMs. Silica has appeared as a promising host for microorganism encapsulation: immobilization pathways are based on immobilization of a population of bacteria dispersed into a silica gel. Bacteria can be either entrapped into alginate microbeads coated with silica membranes or into macrocavities created inside the silica matrix. Such materials improve the mechanical properties of the alginate bead, the reduce cell leakage and enhance the cell viability.
1.6. The application of bio-nanotechnology could also provide new avenues for the development of carrier-based microbial inoculants. Nanotechnology employs nanoparticles which are made of inorganic or organic materials that are defined by having one or more dimensions in the order of 100 nm or less. The integration of whole cells with nanostructures leads to hybrid systems that have numerous applications in many fields, including agriculture. Indeed, even though nanoscale constructs are smaller than cells, macroscopic filters, made of radially aligned carbon nanotube walls, able to absorb Escherichia coli, were fabricated. The same technology could therefore be applied to collect bacterial cells from fermentation processes and deliver them to the plant. The physical stability and the high surface area of nanotubes, together with the ease and cost-effective fabrication of nanotube membranes may thus expand their use in the production of biofertilizer. The use of nanoformulations may enhance the stability of biofertilizers and biostimulators with respect to desiccation, heat and UV inactivation. The addition of hydrophobic silica nanoparticles of 7–14 nm to the water-in-oil emulsion formulation of the biopesticide fungus Lagenidium giganteum reduced the desiccation of the mycelium. The physical features of the formulation were improved and the microorganism was still effective after 12 weeks of storage at room temperature.
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