Sustainable Approach
Sustainable approach to improve soil health and fertility

Sustainable Approach to Improve Soil Health and Fertility

A sustainable approach for improving soil fertility and health is focused on the use of natural resources and soil biodiversity. There are different types of soil organisms playing a direct role in the overall growth and development of plants. Beneficial microbes are generally known as plant growth-promoting microbes or PGPMs. Usually, microbial-based bio formulations may be categorized into four different types such as nitrogen-fixing bacteria, Phosphorus solubilizing/mobilizing microorganisms, compost-producing microorganisms, and biopesticides. Notably, these microbial groups can all have other PGP traits, such as phytohormones and siderophores, amino acids, polysaccharides, and amino acids, and thereby conceivably contribute to crop improvement.

Silva et al., 2020 experimented to check the beneficial effects of soil microorganisms on growth and yield-related factors of soybean. This research was focused on the identification of the best soil microorganisms in a mixture or alone for checking the yield components, yield, nutrients uptake in the grains, roots, and shoots, positive changes related to gas exchange, and total production of biomass. This experiment was performed in well-managed greenhouse conditions in Brazil.

Bacterial suspensions for applications and seed treatments were prepared with the contaminant-free water from different cultures specifically grown in solid media for 24 hours. The treatments for this experiment consisted of Serratia sp., Azospirillum sp., Azospirillum brasilense, Burkholderia sp., Pseudomonas Sp., and fungus Trichoderma asperellum. Results of the study have shown that all treatments including the application of soil microorganisms as in isolation or combination greatly improved all growth and yield-related parameters.

Sustainable Approach to Use Plant Root Exudates to Support Soil Microorganisms

Because soil microbes grow in a carbon-restricted environment, the rich amounts of sugars and amino acids that plants accumulate into their rhizospheres provide a beneficial source of nutrition. Accumulation of this labile material does not encourage the proper use of beneficial microbes and soil bacteria. However, pathogenic strains may also use these molecules as valuable growth substrates. The possibility is that plants have developed perception systems to distinguish beneficial microorganisms and therefore can essentially improve overall growth and development by feeding the right microorganisms.

Plant Breeding Strategies for Targeted Secretion of Root Exudates

The particular molecules found in root exudates are likely to be targeted for plant breeding strategies. These molecules can help shape the microbial community structure. Flavonoids, strigolactones, and terpenoids are all components found in plant root exudates. Studying the microbiome of various plant species and successions led to strong differences, which suggests that exudates are critical in shaping plant-microbe interactions.

Root-derived signals are also known to attract beneficial interactions partners. Rhizodeposition refers to the total release of fixed carbon compounds (exudates and border cells) into the surrounding soil. These exudates differ in composition between species. Pea exudates have high levels of organic acids while oil radish exudates have high levels of sugars.

Plant Species Richness and Root Exudates

Eisenhauer et al., 2017 experimented to check the role of the richness of plant species on soil bacteria, soil fungi, and soil bacterial to fungal ratio via root exudates and root biomass. They used a microcosm experiment for investigating the effects of diverse plant species on the biomass of soil fungi, and soil bacteria.

Root biomass of different species was determined by using quantitative PCR and the quantity and diversity of root exudates were studied by high-pressure liquid chromatography (HPLC). Results of the findings have revealed a significant correlation between root and shoot biomass, plant production, species diversity, and root exudates.

Arbuscular Mycorrhizal Fungi (AMF) and Phosphate Solubilizing Biofertilizers 

AMF, in addition to PSM, is a key component of agricultural systems. These are the most common microbial components of agricultural systems (concerns over 80% of terrestrial and vascular plants). This is defined as a symbiotic relationship that involves a two-directional exchange between two organisms. They could contribute up to 80% to the total P uptake of phosphate in soil depending on the soil type and treatment.

AMF is morphologically a part of the rooting network and acts as an extension of it. This allows the plants to search for nutrients well beyond the boundaries of the rhizosphere. AMF acts as an extra pathway for P uptake (AMF pathway), with arbuscular acting as the symbiosis interconnect. This allows plants to scavenge nutrients and organic phosphate far beyond the boundaries of the rooting system.

AMF hyphae are smaller than roots, they have a high affinity to inorganic phosphates and phosphorus chelate. They can also explore soil pores inaccessible to them and increase the translocation of that inorganic phosphorous. The polyphosphates are then transported through the vacuole to intra-radical and cleaved hyphae. Moreover, the use of phosphate solubilizing bacteria is also important to improve phosphorus uptake and accumulation in plants.

What are phosphates used for

It seems that plants have evolved specific P transporters, which have been recognized for different species such as tomato, potato, and barely. Although fungal hyphae are capable of exuding organic acids and acid phosphatase enzymes, it is not clear how AMF can increase P phosphate and P availability. AMF may be involved in P solubilization via a synergic relationship to PGPM, according to a consensus.

Hou et al., 2021 experimented to improve phosphorus uptake in maize plants under high plantation density conditions in China. They applied mesh barrier compartments for monitoring the hyphal phosphorus uptake distribution throughout growing periods in different soil depths and planting densities for two consecutive years. However, fungal colonization was different at different growth stages and an increase was observed in mycorrhizal colonization, AMF density, and shoot biomass with the growth stages.

The results of the study have revealed that AMF application significantly improved the phosphorus acquisition efficiency of the plants, especially during silking stages. Results have also suggested that AMF applications can eliminate the requirements for phosphorus fertilizers to improve phosphorus in soil for the maintenance of higher crop yields.

Sustainable Approach to Use Phosphate Solubilizing Microorganisms (PSM)

There are many genera of bacteria, including Pseudomonas and Bacillus, Azotobacter and Brady rhizobium, as well as fungi (e.g., Penicillium, Aspergillus, and Streptomyces), that can solubilize Phoshorus metal complexes for releasing bioavailable P. This is done all the way through certain mechanisms. These include organic acids, siderophore, and phosphatase enzymes which play a crucial role in the hydrolysis of organic P forms.

PSM could promote plant growth by increasing phosphorus use efficiencies through exudation and phosphorus hydrolyzing Phosphatase enzymes. This could also be done indirectly by producing phytohormones, antifungal chemicals, compounds having toxin resistance, and numerous high-value bioactive molecules that could aid in developing a robust shoot/rooting system. Results of various scientific studies have revealed that PSM is a significantly important bio fertilizer examples and helps to improve production on a sustainable basis.

Although many mechanisms can be involved in P solubilization the most important contributors are organic acids. Beneficial microorganisms are the main contributors to P solubilization. However, this process is mostly gene-dependent. Ecosystem environmental properties could influence organic acid production. N and C soil can create impacts on the quality of produced organic acids by the application of bacterial fertilizers.

Phosphates Uses

The nature of C sources could also affect the bio-solubilization process. A high ratio of C/P appears to increase organic acid production, while both C/N and N/P can affect the development of microorganisms. Important to remember that P solubilization effectiveness is dependent more on the quality than on the number of organic acids or P sources.

PSM can produce many organic acids, including acetic acid, gluconic, glucuronic, butyric, butyric, fumaric, and valeric acids. The most common are 2-ketoglutonic and gluconic acids in gram-negative bacteria. The reduction of pH and cations-chelating properties are often the reason for organic acid involvement in P solubilization. Acidification of the microbial cells’ boundary results in the release of phosphorus anion through substitution of H+ or Ca2+.

There may be other mechanisms behind this phenomenon such as the protons released from ammonium assimilation cells by microbial cells, and the production of certain inorganic acids (i.e., sulfuric and nitric acid), and the particular enzymes that act on amphiphilic fat substances are all possible.

Elhaissoufi et al., 2020 experimented to investigate the below ground and above ground responses of phosphorus solubilizing isolates in the wheat crop specifically fertilized with the rock phosphate under the controlled conditions. Researchers isolated the DNA for molecular investigations and taxonomic identification of phosphorus solubilizing isolates.

All the isolates were carefully studied for phosphorus solubility potential Additionally, they also studied plant growth-promoting traits of these isolates and the results of their study have reported that the application of phosphate solubilizing bacteria greatly improved the nutrient uptake, chlorophyll contents, protein contents, and the activity of acid phosphatase in the wheat roots.

Adnan et al., 2020 experimented to check the effects of combined application of phosphorus supplements and phosphate solubilizing bacteria in the maize crop under specifically lime-induced saline conditions. This experiment was conducted in the pots and phosphorus contents in the soil were determined by the Olsen method.

Whereas phosphorus contents in plants were determined by using the acid-based digestion method. Results of this study have revealed that the application of phosphorus solubilizing bacteria greatly improved the soil phosphorus contents and plant growth and development.

Sustainable Approach to Use Soil Microorganisms for Biological Nitrogen Fixation

Biological Nitrogen Fixation (BNF) generally, refers to a microbially-facilitated process where atmospheric N2 is converted into ammonia (NH3) by nitrogenase. This enzymatic transformation is performed by a variety of diazotrophs, which are nitrogen fixing bacteria in soil. While some diazotrophs can fix N2 in their free-living status while others do it in association and with plants, together with endophytic bacteria (inside plant tissues) or symbiotic bacteria.

These include fundamental and functional modifications that both microbes and roots undergo in specialized structures called nodules. Legumes and leguminous plants, for example, are usually associated with soil rhizobia bacteria and offer excellent biological nitrogen fixation for improved plant growth, and developmental processes.

Sustainable Approach to Use Bacteria as biofertilizer

These bacteria can use root nodules for sequestering atmospheric N as ammonia. This form of N can later be used to make organic components such as proteins, nucleic acid, and nucleic acid. Symbiotic nitrogen fixation (symbiotic N) involves the net transfering of biologically fixed nitrogen from the bacteria directly to the host plant.

This is in addition to substantial photosynthetically fixed plant carbon being transferred to the NF bacteria. The ability to fix N2 in non-legumes like grasses has been extensively studied. This allows for remarkable progress from the plant’s cell to farms and can be used for different purposes. Several nitrogen-fixing bacteria that also have plant growth-promoting properties were identified as non-symbiotic nitrogen-fixing bacterial of grass species.

Non-symbiotic bacteria can proliferate due to nutrients and energy derived from plants’ roots, unlike rhizobia which causes root nodules to form with their legume hosts. Non-symbiotic (also known as associative N2 fixation or associative N fixation) is not a controlled exchange of N or C between bacteria and their plant hosts.

It has been difficult to accurately determine global N inputs. It is almost impossible to get data on the productivity and area of NF legumes, and non-legumes, so BNF is hard to measure. This challenge is also complicated by the methodological constraints that are used to estimate N2 fixation. Therefore, specific and target-oriented research is required to estimate biological nitrogen fixation all over the globe.

A study conducted by Shah, 2014 was focused on the measurement of the contribution of mineral nitrogen fertilizers and Bradyrhizobium Japonicum to promote emission of N2O. Gas sampling was done by using the closed chamber technique and measurements were performed after a short time of nodulation. Results of the study have revealed that cumulative fluxes were greater for the bacterial inoculated seeds than the mineral fertilization alone.

Horel et al., 2019 experimented to check nitrogen fixation rates and soil nutritional dynamic changes in temperate soils over pepper growth. Researchers collected samples from freshly tilled, silt loam arable soil in the temperate climatic zone. They also determined the physical characteristics of the soil before the experiment and ethylene production was measured to check nitrogen fixation.

The results of the study revealed that the pepper plant significantly facilitated the biological nitrogen fixation in specifically controlled conditions. While the addition of higher amounts of biochar in the soil greatly reduced nitrogen fixation rates.

Microbial Strains for Promoting Plant Growth through Augmenting N, P, and S Nutrition

To be actually useful in an agronomic setting, candidate growth-promoting strains must be shown to be able to be re-inoculated onto the plants and colonize the niche. Then, they will facilitate nutrient utilization that benefits plant health. Plant microbe interaction tests can help to verify this. For decades, researchers have sought to identify optimal inoculation methods, searching for the perfect combination of different plant genotypes, and rhizobial strains, to suit particular climatic conditions and soils.

The classification of nitrogen-fixing associations should be noted that there are many nitrogenase gene species in different bacterial taxa. Also, non-leguminous plants have been found to host N2-fixing strains of bacteria. This may suggest that other plant-microbe combinations could also be optimized to promote nitrogen fixation (not just Rhizobia or legumes). Some reports also suggest that plant growth can be promoted by microbial mobilization of other nitrogen sources.

Identifying Best Microbes for Soil

Future research could be focused on documenting and characterizing other fungal strains that have this capability. The literature is full of reports on fungal and bacterial strains that are capable of solubilizing inorganic P and many reports on strains that can mine organic P. The findings of scientific studies suggest that future research could be focused on identifying further fungal strains with such ability and determining the genes and mechanisms that underlie the growth promotion. Studies of a plant progress-boosting Pseudomonas spore have shown that organic sulfur mineralization contributes to some growth-promoting properties.

Sun et al., 2017 experimented to check the soil nutrient improvement potential by the application of microbial fertilizers in the degraded wetland conditions. Researchers have applied potassium and phosphorus strains isolated from the wetlands along with four kinds of fertilizers viz, inorganic fertilizers, organic fertilizers, chicken manure, and microbial-based biofertilizers.

Results of this study have shown that microbial fertilizers exert significantly beneficial effects on soil enzymatic activities, on plant growth, dry weight, and the fresh weight of biomass. The microbial treatment in comparison with non-fertilizer treatments greatly increased the contents of N, P, K, and urease activities in soil. Moreover, the bacterial communities were relatively stable predominant in the microbial fertilizer-based treatments.

Wang et al., 2020 experimented to check the effects of bacterial strains’ application for activation of nutrients and promoting the growth of wheat under reduced application of fertilizers. They isolated thirty-nine plant growth-promoting rhizobacteria and investigated their potential for growth promotion. Thirteen isolates were reported to promote nitrogen-fixing ability, and eleven isolates showed efficient phosphorus solubilizing ability, and fifteen isolates showed efficient potassium solubilization abilities. Application of these bacterial strains greatly reduced fertilizer application by 25% and greatly improved the NPK contents, dry weight, and plant height of wheat.

Sustainable Approach to Use Beneficial Soil Bacteria

Rhizospheric bacteria can increase the plant’s ability to absorb nutrients and/or promote plant growth. Through direct competitive effects, they produce antimicrobial substances and protect the root surfaces of plants from pathogenic microbes. These bacteria can affect plant growth either right away or incidentally. Plant growth may be promoted by both symbiotic and non-symbiotic bacteria through the production of phytohormones and other plant growth booting activities.

Plant growth-promoting rhizobacteria, (PGPR), integrate phytohormones and distribute them in their surroundings and rhizospheric environment. These phytohormones are known as plant growth regulators. These microorganisms can play a huge regulatory role in plant developmental and growth-related processes. PGRs are organic compounds having a profound effect on the functional processes of plants at exceptionally lower levels.

There are five main classes of PGRs including auxins, gibberellins, cytokinin, and ethylene. The role of the phytohormone auxin has received much attention. Indole-3-acetic Acid (IAA) is physiologically the most effective auxin for plants. It is well-known to encourage both short-term (e.g. cell elongation) and long-term (e.g. cell division and differentiation). IAA is the most well-studied phytohormone and it can be produced by up to 80% of soil bacteria that are specifically isolated from the rhizosphere.

These compounds have huge potential to indirectly improve plant growth on a sustainable basis. Other important phytohormones, such as cytokinin, are often found in low amounts in plant samples. It is therefore difficult to detect and quantify them for proper studies. Cytokinin has a noticeable effect on plants, with an increase in cell division.

However, it can also have an impact on root development and root hair growth. Over 30 growth-stimulating compounds belonging to the cytokinin family have been discovered in plants and microorganisms that are plant-associated. In vitro culture of microorganisms in the rhizosphere has shown that up to 90% can release cytokinin.

Sustainable Approach of Using Bacteria for Soil

Many bacteria species can produce ethylene. Ethylene can be used to regulate plant growth and development and it also affects senescence. Ethylene is recognized as a ripening hormonal. It encourages the formation of adventitious roots and root hairs, accelerates germination, and breaks down dormancy in seeds.

Many plants’ growth-promoting bacteria could stimulate plant growth by lessening ethylene levels in plants, according to some theories. This is credited to the action of enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, causing hydrolyzation of ACC, the important biosynthesis precursor of ethylene in plants.

Feeding Soil Microbes for Removing Soil Contaminants

PGPR can also be used to treat contaminated soils as it mineralizes organic compounds in conjunction with plants. The combined use of PGPR and particular contaminant-damaging bacteria can efficaciously remove multifaceted pollutants. Certain rhizobacteria may increase Ni uptake from soils by changing the phase of the bacteria and Bacillus and Pseudomonads are most important in this regard.

They are used in bioremediation and can further be used for cleaning up pollutants from iron, copper, and silver mines. Some bacteria can remove carbon, nitrogen, and phosphorous compounds. Others can remove toxic metals as well as pesticides and herbicides. These multi-step processes involve both anaerobic and aerobic metabolisms.

Beneficial Soil Microorganisms

Indirect plant growth promotion is related to the inhibition of harmful impacts from phytopathogenic organisms. The use of siderophores is essentially an important way to achieve this growth improvement and development. Iron is found in soils predominately in the form of ferric ions. This form cannot be immediately absorbed by microorganisms.

The production of siderophores allows bacteria to fight against pathogens by eradicating iron from their surroundings or the environment. Pseudomonads and Frankia are very familiar with siderophore production. They also produce iron-chelating compounds. Many bacterial species are also capable of synthesizing antibiotics and soil-borne pathogens. Rhizobacteria also prevent phytopathogens through the production of hydrogen cyanide (HCN), and/or fungal wall-destroying enzymes, such as chitinase, ss-1, and 3-glucanase.

Pectinolytic ability is often associated with phytopathogenic bacteria and pectinolytic enzymes are important in root infiltration by bacteria. Although PGPR can be found in many bacterial taxa, the most common commercially available PGPR is derived from Bacillus species. These endospores confer stability to the population during product formulation and storage. Because of their antibiotic-producing and disease-reducing capabilities Bacillus subliminal is the most commonly used PGPR among bacilli. PGPR suppresses pathogens through a variety of mechanisms.

These include specific substrate competition, manufacturing of antibiotics, and specifically induced systemic immunity in the host. Fluorescent pseudomonads have been shown to suppress soil-borne fungal diseases by the production of antifungal compounds and sequestering iron in rhizospheres through releasing iron-chelating siderophores.

Microbial Soil Amendments A Significant Sustainable Approach

Research has demonstrated the feasibility of introducing beneficial soil microbes to marketable peat-based substrates used for vegetable and fruits production. This is done to increase plant vitality, decrease root diseases, and improve yields. Trials on muskmelon and watermelon showed that a variety of PGPR formulations reduced the severity of root-knot nematode diseases.

Beneficial Soil Organisms

Soil microorganisms and soil biota have a direct role to improve plant growth, development, and nutritional contents of produce on a sustainable basis. However, the beneficial roles of soil microorganisms to improve food security and crop production have not been fully explored. To develop targeted products and meet specific consumer demands, strategies that focus on understanding the potential actions of microbes are crucial.

It is important to foster closeness to growers as farmers’ acceptance must be a priority. This can only be accomplished by in-field experiments, creating reports and data that are personalized to growers’ needs. It is also important to note that the success of subsequent generation agro-inputs using microbial inoculants will depend on regulatory clarity and a cooperative mindset. This is where farmers, scientists, advisors, and policymakers can all work together. This will allow us to move towards integrated, profitable ecosystems that use all inputs following healthy principles.

It also helps us to optimize nutrient utilization efficiency in an environment where climate changeability is constantly endangering our food productivity. Therefore, there is a dire urgent need for strong and efficient collaboration between researchers, leading scientists, governments, international organizations, and policymakers to promote the use of beneficial microbial strains all over the globe.

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