Despite the fact that grain production has doubled in the last four decades as a result of the widespread use of synthetic fertilisers and pesticides during the green revolution, this rate of increased agricultural output is unsustainable due to declining crop yields and the environmental impacts of modern agricultural practises. In many nations, improper agrochemical use is one of the most serious environmental challenges. In India, after the green revolution, the usage of synthetic chemicals in agriculture has skyrocketed, resulting in increased food production. On the other side, continued use of synthetic substances causes a slew of environmental problems, including harm to soil flora and wildlife as well as human health
(Meena et al., 2020). Zhang et al., (2018) conducted a review that clearly established the impact of pesticide use on the ecosystem over the last 30 years. Readers can consult
Meena et al., (2020) and
Tudi et al., (2021) for further information on the negative influence of agrochemicals on environmental issues. The loss of soil microflora is a major consequence of chemical use in agriculture. When the soil’s microbial population declines, soil fertility suffers and the soil becomes degraded
(Basu et al., 2021; Palia et al., 2021).
Soil microflora can be promoted in a sustainable manner by reducing the use of agrochemicals and increasing the use of bio inputs. Bio-inoculants (biofertilizers) are one of the numerous bio inputs used in agriculture and are one of the most essential products that aid in soil fertility and plant growth. Bio-inoculants are agricultural amendments that increase soil and plant health by using helpful microorganisms. Bio-inoculants are developing as a viable alternative to synthetic agrochemicals in terms of reducing their negative environmental impact
(Aruna and Jain, 2021; Basu et al., 2021). The microorganisms in biofertilizers deliver benefits to agricultural plants through a variety of direct and indirect ways. This could be in the areas of nitrogen fixation, phosphate solubilization, nutrient mobilisation, stress mitigation and plant growth promotion (PGP), or a combination of these features
(Bhardwaj et al., 2014).
The use of an effective bio-inoculant that performs well in the lab and in greenhouse settings does not always result in the expected effects on plant development in the field. This review includes information on the parameters that influence bio-inoculant adaption and performance in farmers’ fields (both biotic and abiotic factors), in addition constrains also discussed.
Factors influencing bio-inoculants
A thorough understanding of target soils and rhizospheres as habitats for imported microbes is required for the development of bio-inoculants that sustain beneficial microorganisms in the soil. The survival of imported bio-inoculants in soil is influenced by a variety of biotic and abiotic variables, as shown in Fig 1. The next sections go through the specifics of each aspect.
Soil temperature
The temperature of the soil is determined by heat flow in the soil and heat exchanges between the soil and the atmosphere. The longevity of applied bio-inoculants in soil is influenced by temperature. Microbial activity is stronger when the soil temperature is between 15 and 30 degrees Celsius
(Allison et al., 2010) and it is lowered as the soil temperature rises and falls. Solar radiation is the primary source of soil temperature, which is measured using a thermometer. Soil temperature fluctuates a lot and it varies seasonally, daily and even hourly depending on solar radiation. The majority of bio-inoculants used in the biofertilizer business operate better at ideal soil temperatures of 28°C to 35°C, however due to global warming, soil temperature fluctuations are becoming a big challenge for bio-inoculants. The symbiosis between
Bradyrhizobium japonicum and soybean is inhibited by temperatures near 15°C
(Antoun and Prevost, 2005). Furthermore, at 15°C,
B.
japonicum nod factor production and nodulation efficiency are impaired
(Duzan et al., 2006). On the other side, it was discovered that raising the soil temperature above 40°C causes microbial cell death
(Wu et al., 2010). (Chen et al., 2002), on the other hand, reported identified and screened Rhizobium strains that tolerate 40°C and nodulate soybean crops. Some strains may have evolved resistance mechanisms that allow them to perform better at higher or lower temperatures. As a result, all bio-inoculants must be chosen for their ability to survive and perform in a wide range of soil temperatures.
Soil pH
Rhizobium legume symbiosis is a well-studied and ancient symbiosis. Only adequate communication between
Rhizobium bacteria and plant roots can develop this relationship. Changes in soil pH, on the other hand, have a significant impact on signalling molecule synthesis, plant root attachment, nodule formation and soil longevity
(Lin et al., 2012). Fig 2 depicts the pH effect on nodulation activity and nitrogen fixation process. Soil acidity has an impact on the nodulation process and nitrogen fixation, from rhizobia survival and multiplication in the soil to infection and nodulation and finally nitrogen fixation
(Graham, 1992).
Rhizobium spp. are less acid tolerant than
Bradyrhizobium spp.
Lin et al., (2012) found that when soil pH is decreased below 5.5,
Bradyrhizobium japonicum nodulation in soybean is greatly influenced. Furthermore, the scientists provided genetic evidence that when soil pH is low, genes involved in forming a symbiotic interaction between soybean and rhizobium are down regulated. In contrast to this pH, when compared to neutral pH,
Rhizobium tropici CIAT899 tolerance was shown to grow efficient nodules at a pH of 5.5
(Moron et al., 2005). Because the results of the preceding study show that some rhizobium stains operate better in neutral pH soil and others in acidic pH soil, bio-inoculants should be tested in a variety of soil pH levels before being formulated. Furthermore, appropriate strains must be selected for various soil conditions and this information must be clearly stated on the product label.
Agrochemical residues in soil
The number and quality of soil micro and macro flora are the most important factors in determining soil fertility. Traditional farming practises, on the other hand, have an impact on soil flora. Agrochemicals have affected soil biodiversity, agricultural sustainability and food safety, resulting in long-term detrimental effects for nutritional security and human health
(Meena et al., 2020). Table 1 shows a list of key agrochemicals and their impact on bio-inoculants. Many of the chemical pesticides used in agriculture have a direct effect on the bio-inoculants that have been introduced. It primarily influences the survival of bio-inoculants, nodulation efficiency, nitrogen fixation, mineral solubilization and the synthesis of plant growth hormones, among other things.
Native microbial population
When biofertilizer strains are introduced to the soil, they compete with the soil’s natural microbes. On the other hand, ecological interactions between soil native and introduced microorganisms are still unknown. Strains expressing features that aid the colonisation process, as well as the “fight” for the roots’ environment, must be chosen to assure the efficacy of any biofertilizer
(Malusa et al., 2016). The release of quorum sensing molecules, the development of biofilm and swarming movement all help bio-inoculants colonise roots more effectively. Bio-inoculants should be used in big quantities to achieve a favourable effect. In addition, the imported bio-inoculant must compete with the soil’s native microflora or form a mutualistic relationship with it. Introduced microorganisms may interact with native soil organisms in a beneficial or negative way
(Chapin et al., 2002; Mosupiemang et al., 2021). When
Pseudomonas corrugate was used as a bio-inoculant, a positive interaction was seen in the maize rhizosphere
(Kumar et al., 2007). Contrary to popular belief,
Conn and Franco, (2004) found that an applied bio-inoculant suppressed numerous native soil microorganisms.
Predatory organism
Protozoan grazing, especially that of naked amoeba, the most common bacterial grazer in soil, appears to play a role in biofertilizer efficiency. Protozoans feed on specific bacterial strains in the soil
(Rosenberg, 2008). Three nematode species (
Caenorhabditis elegans,
Acrobeloides thornei and
Cruznema sp.) dramatically reduced
Pseudomonas species and
Bacillus subtilis colonisation in the wheat rhizosphere
(Knox et al., 2003). One of the predatory bacteria against gram negative cells has been identified as
Bdellovibrio naceae (Morgan et al., 2010). In addition, soil fungus and bacteria are preyed upon by Myxobacteria present in terrestrial soil
(Dawid, 2000). To attack prey cells,
Myxococcus xanthus uses chemotaxis-like genes and predation is successful when it comes into intimate contact with pray cells
(Berleman et al., 2008).
Issues related to microbial strain development
Bio-inoculants are tested and developed as a biofertilizer after being isolated from a specific crop rhizosphere region. Isolated species/strains may perform better for a single crop and have a limited host range
(Vejan et al., 2016). Unlike bio-inoculants, chemical fertilisers are not crop specific and can help any crop. Bio-inoculants must also be selected to work better in a variety of crops, soil types and environmental circumstances
(Meena et al., 2020). The following are some of the essential characteristics of an efficient bio-inoculant strain:
• Ability to compete with native microbial population.
• It should not antagonistic to beneficial microorganism.
• Should have higher survival rate in soil.
• Must tolerant to drought, salinity and heavy metal stress.
• Must grow in soil with low organic matter content.
• Should establish positive relationship with the host plant.
• Must express its PGP characters at maximum rate.
Quality control
The Indian government has established the basic requirements for bio-inoculant production and marketing, however there is no continual monitoring of the goods on the market. The Government of India, Ministry of Agriculture and Rural Development, enforces the Fertilizer Control Order - FCO (1985). Since 2006, biofertilizers and organic fertilisers have been incorporated in FCO. Quality specifications for both the carrier and the liquid biofertilizer were developed and implemented. Many requirements for production and quality control for biofertilizer manufacture have been established
(Pandey and Chandra, 2016). However, India has a very small number of quality control laboratories dedicated to the bio-inoculants. It is critical to enhance the number of quality control laboratories staffed by technical personnel in order to provide farmers with high-quality bio-inoculants.
Constrains in bio-inoculants production and usage
Basu et al., (2021) review many constraints linked to bio-inoculants in detail and a list of relevant constraints is provided in Fig 3. The ten major constraints in the field of bio-inoculant production and use include biological, technological, regulatory, infrastructural, financial, marketing, field-level, quality control, carrier and biosafety. Biological restrictions associated with the microbial strain and their competition with soil micro and macro flora and fauna. Because biofertilizers contain living microbial strains, product shelf life and effectiveness reduction owing to mutation are regarded as technological constraints.
New product registration takes a lengthy time and there are no uniform regulatory policies in India for biofertilizer production. Infrastructure for bio-inoculant production and quality control necessitates sophisticated technology, trained personnel and a considerable financial investment to construct a large-scale production plant. Farmers’ interest in purchasing bio-inoculants is very low and a lack of information about their use creates marketing constraints. At the field level, the efficacy of bio-inoculants is not immediate, because the applied microbe must adapt to a new environment and colonise the root system before providing benefits to the plant. At field-level constraints, farmers are hesitant to use bio-inoculants because they want immediate results. Continuous availability of carrier matter is a big issue for a manufacturing company and quality testing of the product necessitates sophisticated technology that most farmers do not have access to. The biosafety of the bio-inoculants is the most significant constraint, because microorganisms evolve naturally and some mutations might lead to the development of dangerous characteristics.
