Research Progress of Application of Microbial Inoculants in Agricultural Production

Yunyun ZHOU Yating XIE Xiu LIU Kaifa GUO Chenzhong JIN Shunli XIAO

Abstract Microbial inoculants have received increasing attention in strengthening plant biological barriers， antagonizing and inhibiting harmful microorganisms， and ensuring the safe production of agricultural products. This paper summarized the research status of agricultural microbial inoculants， the application of microbial inoculants in agriculture， and the trends and prospects of agricultural microbial research.

Key words Microbial inoculants; Agricultural production; Research progress

Chinas agricultural plantation has the characteristics of high intensity and high multi-cropping index， and relies on a large amount of chemical fertilizers and pesticides to acquire high output， especially for greenhouse vegetables. The input of pesticides and chemical fertilizers accounts for two-thirds of the entire production cost， but the fertilizer use efficiency is not high. The obstacle in sustainable soil productivity caused by long-term excessive use of chemical fertilizers， high cropping index and continuous cropping has caused soil quality health and agricultural product quality safety issues. Nowadays， soil compaction， soil acidification， soil salinization， soil fertility exhaustion， soil pollution， soil-borne diseases and other problems have appeared one after another[1].

With the rise and development of ecological agriculture and green food production， people pay more and more attention to the research and application of biological control methods. Great importance has been attached to the "three reductions" work in agriculture. For years， the "Central No.1 Document" has been an important agricultural development policy of China. For example， in 2013， the "Central No.1 Document" required the development of efficient slow-release fertilizers; in 2014， the "Central No.1 Document" put forward the issue of excessive use of chemical fertilizers and pesticides， which needs to be effectively improved urgently; and in 2015， the "Central No.1 Document" proposed to support the technological innovation of agricultural machinery， fertilizer， and pesticide companies. Microbial inoculants are preparations containing living organisms of specific microorganisms， which have the functions of improving soil， improving soil fertility， increasing roots， strengthening seedlings， increasing agricultural product yield， improving agricultural product quality and ecological environment， etc.[2]. They are safe and convenient to use， and can reduce the disadvantages in environmental quality， food safety and public health caused by the large-scale use of chemical fertilizers. They have economic， social and ecological effects， and are important for Chinas "three reductions" work and the sustainable development of agriculture. This paper summarized the research status of agricultural microorganisms， the application and development trends and prospects of microbial inoculants in agricultural production in recent years， aiming to provide a reference for the research of agricultural biological control methods.

Research Status of Agricultural Microbial Inoculants

Among the strains registered in microbial inoculant products， the most frequently used strains are Bacillus subtilis， Paenibacillus kribbensis， B. licheniformis， B. megaterium， B. amyloliquefaciens， Saccharomyces cerevisiae， and Brevibacillus laterosporu and other bacteria or fungi with functions of nitrogen fixation， phosphorus release， potassium release， etc.[1]. These beneficial microorganisms can effectively resist the attack of diseases such as sheath blight and root rot， promote plant growth， improve cold resistance and drought resistance， promote the synthesis of sugar and protein and improve product quality， and are safe， efficient and residue-free. The research and application of microbial fertilizers at home and abroad are all started from the application of inoculum of rhizobia on legumes， and the range of strains is continuously expanding[6]. Subsequently， countries such as the former Soviet Union applied silicate bacteria， phosphate-dissolving bacteria and nitrogen-fixing bacteria isolated from soil to agricultural production[7]. The research on effective microbes in inoculants is expanded to a variety of microbial species such as bacteria， actinomycetes and fungi， and the emphasis is on the composition of multiple strains and multiple functions[8]. At the end of the 20th century， associative nitrogen-fixer and biological potassium fertilizer were used as seed dressing agents successively in agricultural production， and nitrogen-fixing bacteria agents， silicate microbial agents， photosynthetic bacteria agents， plant rhizosphere growth promoter （PGPR） preparations and organic material （straw） decomposing agents that meet the needs of agricultural development appeared[5]. Many compound microbial inoculants have been used in agricultural production[7]， mainly composed of two or more microbes or a single microbe and other nutrients such as inorganic nutrients and organic matter， fully reflecting the comprehensive effects of microbial fertilizers， inorganic chemical fertilizers and oil fertilizers. At present， the main dosage forms of microbial inoculants are liquid and solid， of which the solid dosage forms are powders and granules， and the liquid dosage forms are aqueous agents. In addition， coating agents and microcapsules are under development and application.

Application of Microbial Inoculants in Agricultural Production

Improve soil quality and improve fertility

The microbial inoculants contain a large number of beneficial microorganisms. These beneficial microorganisms can adjust the soil environment， adjust the soil enzyme activity， and increase soil permeability and water permeability through their own activities in the soil， thereby improving the soil hardening problem and creating a beneficial rhizosphere environment for the growth of crop roots. Meanwhile， plant endogenous enzymes produced by the beneficial bacteria such as B. megaterium in the flora can significantly increase the absorption of nitrogen， phosphorus， potassium and other nutrients by crops， and a large amount of organic matter continuously releases the nutrient elements required for plant growth through beneficial microbial activities. Some microorganisms can release insoluble phosphorus and potassium fertilizer elements in the soil， increase soil fertility， improve fertilizer utilization， and reduce the use rate of chemical fertilizers[9-12]. Microbial agents such as Aspergillus oryzae， B. licheniformis and B. subtilis in the microflora can accelerate the decomposition of organic substances， produce rapidly available nutrients for crops， decompose toxic and harmful substances for continuous cropping， and relieve or prevent continuous cropping. Ma et al.[13] applied microbial inoculants to facility vegetables， and the results showed that the inoculants had the effects of improving soil nutrition， increasing the number of microbes that can be cultivated in the soil， and increasing soil microbial diversity; and the application of microbial inoculants could significantly promote the proliferation of bacteria and actinomycetes in the soil， inhibit the increase of the number of fungi， improve the soil properties after planting medicinal materials to a certain extent， and alleviate the continuous cropping obstacles after planting medicinal materials[14]. In addition， studies have shown that microbial inoculants could effectively reduce the toxic effects of pesticides， heavy metals， salts and alkalis， agricultural films and other harmful substances remaining in the soil， alleviate soil pollution， improve soil aggregate structure， and improve fertility[15-18].

Promote plant growth and protect the rhizosphere environment

The beneficial microorganisms in the microbial inoculants can secrete auxins such as cytokinin， indole acetic acid， vitamins and gibberellin and other active substances such as amino acids during the life activities in the soil， stimulate the growth of crops and regulate the metabolism of crops[20]. For example， beneficial microbes such as P. kribbensis， B. laterosporus and B. licheniformis promote the growth of the root system of crops， and increase the number of fibrous roots. Plant endogenous enzymes and plant growth regulators produced by the metabolism of beneficial microbial flora enter the plant body through the root system， promote leaf photosynthesis， regulate the flow of nutrients to the fruit， and have obvious effects on fruit expansion[21]. After the beneficial microorganisms such as B. licheniformis enter the soil， they quickly become the dominant bacterial group， control rhizosphere nutrition and resources depending on the inter-species competitive advantage， and make pathogens such as root rot， sheath blight and gray mold lose their living space and conditions. Li et al.[22] applied microbial inoculum FH-1 in rice field cultivation to change the structure and function of microbial community and promote rice growth and development. In addition， some microbial agents can stimulate the thickening， fibrosis and lignification of the cell walls of plant root cells and allow the formation of horny double silicon layer， forming a strong barrier for preventing pathogens[23].

Increase production and income and improve crop quality

The application of microbial inoculants can improve the quality and increase the yield of various crops. The application of microbial inoculants on various crops shows that the microbial inoculants have a positive effect on the improvement of crop biological properties， which is reflected by the increase in plant height， thickening of stalks， increase in leaf number， thickness， length and width， denser and greener leaves， the increases in seed setting rate per plant， single-fruit weight and solid content and the decrease in the number of deformed fruit， resulting in greatly improved crop yield and increased economic benefits[24-28]. Shi et al.[29] studied the effects of silicate microbial inoculants on the physiological characteristics and yield of rice. The results showed that silicate microbial inoculants increased the content of available silicon in the rice soil， promoted the absorption of silicon by rice plants， and increased the dry matter weight of rice plants， thereby increasing rice yield. The metabolites of beneficial microorganisms contain certain biostimulants， such as auxin， indoleacetic acid， vitamins， amino acids， etc.， which are the same as the products produced by vegetables during their growth. The root system can directly absorb and use these products to meet their own growth needs， and make the growth of the plant more robust.     The richer the beneficial metabolites in the soil， the more balanced the plant growth and therefore the higher the quality of the fruit produced. For crops to which microbial inoculants are applied， the nutritional contents of VC， sugar and amino acids in products can be effectively improved. Spraying microbial inoculants on sugar beet seedling beds and in the field promotes strong seedlings， improves sugar beet photosynthetic capacity， and can increase sugar beet root yield and sugar content[30].

Control crop diseases and pests and improve crop resistance

Chemical fertilizers and pesticides are used in large amounts in crop fields， which leads to the deterioration of soil physical and chemical properties， the reduction of beneficial microorganisms in the soil， the increase of soil-borne diseases and the decline of crop quality. Bottom application of microbial agents not only improves plant biological properties， promotes its growth， increases yield， and improves quality， but also induces plants to secret various enzymes such as peroxidase， polyphenol oxidase， lipoxygenase and chitinase to participate in the defense reaction， or to produce antibiotics to kill microbes， inhibit the propagation of soil pathogens and reduce the occurrence of crop diseases[31]. The microorganisms in microbial agents grow and multiply in the roots of plants into the dominant bacterial group， forming a local advantage， which inhibits and reduces the invasion and reproduction opportunities of pathogenic bacteria and achieves the effect of reducing crop diseases. Among the inoculants， for example， B. laterosporus can strengthen the leaf protective film， resist the infection of pathogens， and enhance the plants disease resistance and pest resistance. Studies have shown that the application of microbial inoculants on soils with high incidence of rhizome diseases can effectively reduce the incidence of soil-borne diseases on various economic crops such as peppers， potatoes， peanuts and grapes[32-35]. After the application of multifunctional microbial inoculants to wheat， the control effects against such three diseases as wheat take-all， root rot and sheath blight reached about 70%[36]. Li et al.[37] studied the effect of "Fukangqi" microbial inoculant on rice， and the results showed that the microbial inoculant improved the disease resistance of rice and reduced the symptoms of rice leaf blast， sheath blight， and stripe virus disease. The beneficial microorganisms such as B. licheniformis and B. megaterium in the flora can enhance soil buffering capacity， retain water and moisture， and enhance the ability of crops to resist drought， cold and waterlogging.

Yunyun ZHOU et al. Research Progress of Application of Microbial Inoculants in Agricultural Production

Research Trends and Prospects of Agricultural Microbial Inoculants

Microorganisms play an important role in the conversion and flow of materials and energy in the soil. The diversity of their functions determines their irreplaceability in soil and agriculture. As modern agriculture strongly advocates environmental protection concepts such as green ecology， microbial inoculants will play a more important role in environmental protection and sustainable agricultural development. At present， there are problems in the production of domestic microbial inoculants such as low number of effective microbes， many miscellaneous microbes， unstable effects， and high prices[7]. With the continuous development of biotechnology， these problems will surely be solved slowly. The multifunctional and stability development of microbial inoculants will be the research direction and development trend. In terms of agricultural applications， future research trends may be on three aspects. The first is to breed good strains with strong adaptability and develop microbial agents for application in adverse environments， so as to increase the yield and quality of crops in unhealthy environments and restore the ecological environment， such as heavy metal-polluted farmland. Worldwide heavy metal pollution of farmland has become an important obstacle affecting the safe production of agriculture[38]. According to the 2014 Report on the national general survey of soil contamination， the over-standard rate of farmland soil in China was 19.4%， involving the pollution caused by metal elements including cadmium， nickel， copper， arsenic， mercury and lead[39]. The second is to research and develop compound biological inoculants to improve the stability of microbial agents. We can screen a variety of microorganisms for combination or mixing with other nutrients and choose appropriate carriers and dosage forms to maintain biological activity， so as to increase the stability of microbial inoculants in the field and exert better biocontrol effects. The third is to study the comprehensive application mode based on microbial agents， explore the nutritional needs of crops， and screen the best carriers and dosage forms， so as to maximize the benefits of crops by cooperating with other pesticides and fertilizers and applying at the best application time in the best combination. The ultimate goal is to improve yield and quality while keep land healthy.

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