Microbial inoculants, also known as soil inoculants or bioinoculants, are agricultural amendments that use beneficial rhizosphericic or endophyticmicrobes to promote plant health. Many of the microbes involved form symbiotic relationships with the target crops where both parties benefit (mutualism). While microbial inoculants are applied to improve plant nutrition, they can also be used to promote plant growth by stimulating plant hormone production.[1][2] Although bacterial and fungal inoculants are common, inoculation with archaea to promote plant growth is being increasingly studied. [3]
However, it is increasingly recognized that microbial inoculants often modify the soil microbial community (Mawarda et al., 2020).
Bacterial
Rhizobacterial inoculants
The rhizobacteria commonly applied as inoculants include nitrogen-fixers, phosphate-solubilisers and other root-associated beneficial bacteria which enhance the availability of the macronutrients nitrogen and phosphorus to the host plant. Such bacteria are commonly referred to as plant growth promoting rhizobacteria (PGPR).
Nitrogen-fixing bacteria
The most commonly applied rhizobacteria are Rhizobium and closely related genera. Rhizobium are nitrogen-fixing bacteria that form symbiotic associations within nodules on the roots of legumes. This increases host nitrogen nutrition and is important to the cultivation of soybeans, chickpeas and many other leguminous crops. For non-leguminous crops, Azospirillum has been demonstrated to be beneficial in some cases for nitrogen fixation and plant nutrition.[1]
For cereal crops, diazotrophic rhizobacteria have increased plant growth,[4] grain yield (Caballero-Mellado et al., 1992), nitrogen and phosphorus uptake,[4] and nitrogen (Caballero-Mellado et al., 1992), phosphorus (Caballero-Mellado et al., 1992; Belimov et al., 1995) and potassium content (Caballero-Mellado et al., 1992). Rhizobacteria live in root nodes, and are associated with legumes.
Phosphate-solubilising bacteria
To improve phosphorus nutrition, the use of phosphate-solubilising bacteria (PSB) such as Agrobacterium radiobacter has also received attention (Belimov et al., 1995a; 1995b; Singh & Kapoor, 1999). As the name suggests, PSB are free-living bacteria that break down inorganic soil phosphates to simpler forms that enable uptake by plants.
Fungal inoculants
Symbiotic relationships between fungi and plant roots is referred to as a Mycorrhiza association.[5] This symbiotic relationships is present in nearly all land plants and give both the plant and fungi advantages to survival.[5] The plant can give upwards of 5-30% of its energy production to the fungi in exchange for increasing the root absorptive area with hyphae which gives the plant access to nutrients it would otherwise not be able to attain.[5][6] The two most common mycorrhizae are arbuscular mycorrhizae and ectomycorrhizae. Ectomycorrhizae associations are most commonly found in woody-species, and have less implications for agricultural systems.[7]
Arbuscular mycorrhiza
This diagram shows the beneficial symbiotic relationship between a plants roots and a fungus partner, which is referred to as a mycorrhiza association.[5] Plants can give upwards of 5-30% of their photosynthetic production to this relationship, represented by G, in exchange for enhanced nutrient uptake, via hyphae, which extend the plants root absorptive area, giving it access to nutrients it would otherwise not be able to attain, which is represented by N and P.[5]
Arbuscular mycorrhiza (AM) has received attention as a potential agriculture amendment for its ability to access and provide the host plant phosphorus.[7] Under a reduced fertilization greenhouse system that was inoculated with a mixture of AM fungi and rhizobacteria, tomato yields that were given from 100% fertility were attained at 70% fertility.[8] This 30% reduction in fertilizer application can aid in the reduction of nutrient pollution, and help prolong finite mineral resources such as phosphorus (Peak phosphorus). Other effects include increases in salinity tolerance,[9] drought tolerance,[10] and resistance to trace metal toxicity.[11]
Fungal partners
Fungal inoculation alone can benefit host
plants. Inoculation paired with other amendments can further improve
conditions. Arbuscular mycorrhizal inoculation combined with compost is a
common household amendment for personal gardens, agriculture, and nurseries. It
has been observed that this pairing can also promote microbial functions in
soils that have been affected by mining.[12]
Certain fungal partners do best in specific
ecotones or with certain crops. Arbuscular mycorrhizal inoculation paired with plant
growth promoting bacteria resulted in a higher yield and quicker maturation in
upland rice paddys.[13]
Fungal inoculants can be used with or without additional amendments in private gardens, homesteads, agricultural production, native nurseries, and land restoration projects.
Composite inoculants
The combination of strains of Plant Growth Promoting Rhizobacteria (PGPR) has been shown to benefit rice and barley.[15][16] The main benefit from dual inoculation is increased plant nutrient uptake from both soil and fertilizer.[15] Multiple strains of inoculant have also been demonstrated to increase total nitrogenase activity compared to single strains of inoculants, even when only one strain is diazotrophic.[15][17][18]
PGPR and arbuscular mycorrhizae in combination can be useful in increasing wheat growth in nutrient poor soil[19] and improving nitrogen-extraction from fertilised soils.[20]
Galal, Y. G. M., El-Ghandour, I. A., Osman, M. E. & Abdel Raouf, A. M. N. (2003), The effect of inoculation by mycorrhizae and rhizobium on the growth and yield of wheat in relation to nitrogen and phosphorus fertilization as assessed by 15n techniques, Symbiosis, 34(2), 171-183.
Brady, Nyle C. (2010). Elements of the nature and properties of soils. Weil, Ray R. (Thirded.). Upper Saddle River, N.J. pp.343–346. ISBN9780135014332. OCLC276340542.{{cite book}}: CS1 maint: location missing publisher (link)
Chapin, F. Stuart; Matson, Pamela A.; Vitousek, Peter M. (2011). Principles of Terrestrial Ecosystem Ecology. New York, NY: Springer New York. pp.243–244. doi:10.1007/978-1-4419-9504-9. ISBN9781441995032.
Hirrel, M.C. and Gerdemann, J.W., 1980. Improved Growth of Onion and Bell Pepper in Saline Soils by Two Vesicular-Arbuscular Mycorrhizal Fungi 1. Soil Science Society of America Journal, 44(3), pp.654-655.
Ferrazzano, S. and Williamson, P. (2013). Benefits of mycorrhizal inoculation in reintroduction of endangered plant species under drought conditions. Journal of Arid Environments, 98, pp.123-125.
Firmin, S., Labidi, S., Fontaine, J., Laruelle, F., Tisserant, B., Nsanganwimana, F., Pourrut, B., Dalpé, Y., Grandmougin, A., Douay, F., Shirali, P., Verdin, A. and Lounès-Hadj Sahraoui, A. (2015). Arbuscular mycorrhizal fungal inoculation protects Miscanthus×giganteus against trace element toxicity in a highly metal-contaminated site. Science of the Total Environment, 527-528, pp.91-99.
Kohler, J., Caravaca, F., Azcón, R., Díaz, G. and Roldán, A. (2015). The combination of compost addition and arbuscular mycorrhizal inoculation produced positive and synergistic effects on the phytomanagement of a semiarid mine tailing. Science of the Total Environment, 514, pp.42-48.
Diedhiou, A., Mbaye, F., Mbodj, D., Faye, M., Pignoly, S., Ndoye, I., Djaman, K., Gaye, S., Kane, A., Laplaze, L., Manneh, B. and Champion, A. (2016). Field Trials Reveal Ecotype-Specific Responses to Mycorrhizal Inoculation in Rice. PLOS ONE, 11(12), p.e0167014.
Liu, L., Li, J., Yue, F., Yan, X., Wang, F., Bloszies, S. and Wang, Y. (2018). Effects of arbuscular mycorrhizal inoculation and biochar amendment on maize growth, cadmium uptake and soil cadmium speciation in Cd-contaminated soil. Chemosphere, 194, pp.495-503.
Belimov, A. A., Kojemiakov, A. P. & Chuvarliyeva, C. V. (1995a) Interaction between barley and mixed cultures of nitrogen fixing and phosphate-solubilising bacteria. Plant and Soil, 173, 29-37.
Khammas, K. M.; Kaiser, P. (August 1992). "Pectin decomposition and associated nitrogen fixation by mixed cultures of Azospirillum and Bacillus species". Canadian Journal of Microbiology. 38 (8): 794–797. doi:10.1139/m92-129. ISSN0008-4166. PMID1458371.
Singh, S. & Kapoor, K. K. (1999) Inoculation with phosphate-solubilising microorganisms and a vesicular-arbuscular mycorrhizal fungus improves dry matter yield and nutrient uptake by wheat grown in sandy soil. Biology and Fertility of Soils, 28, 139-144.
Galal, Y. G. M., El-Ghandour, I. A., Osman, M. E. & Abdel Raouf, A. M. N. (2003), The effect of inoculation by mycorrhizae and rhizobium on the growth and yield of wheat in relation to nitrogen and phosphorus fertilization as assessed by 15n techniques, Symbiosis, 34(2), 171-183.
Bashan, Y. & Holguin, G. (1997), Azospirillum-plant relationships: environmental and physiological advances (1990-1996), Canadian Journal of Microbiology 43, 103-121.
Bashan, Y., Holguin, G. & E., D.-B. L. (2004) Azospirillum-plant relationships: physiological, molecular, agricultural, and environmental advances (1997-2003). Canadian Journal of Microbiology, 50, 521-577.
Belimov, A. A., Kunakova, A. M., Vasilyeva, N. D., Gruzdeva, E. V., Vorobiev, N. I., Kojemiakov, A. P., Khamova, O. F., Postavskaya, S. M. & Sokova, S. A. (1995b) Relationship between survival rates of associative nitrogen-fixers on roots and yield response of plants to inoculation. FEMS Microbiology Ecology, 17, 187-196.
Caballero-Mellado, J., Carcano-Montiel, M. G. & Mascarua-Esparza, M. A. (1992), Field inoculation of wheat (triticum aestivum) with azospirillum brasilense under temperate climate, Symbiosis, 13, 243-253.
Gutierrez Manero, F. J. (2008) Systemic disease protection elicited by plant growth promoting rhizobacteria strains: relationship between metabolic responses, systemic disease protection, and biotic elicitors. Phytopathology, 98 (4), 451-457.
Heitefuss, R. (2001) Defence reactions of plants to fungal pathogens: principles and perspectives, using powdery mildew on cereals as an example. Naturwissenschaften, 88, 273-283.
Khammas, K. M. & Kaiser, P. (1992) Pectin decomposition and associated nitrogen fixation by mixed cultures of Azospirillum and Bacillus species. Canadian Journal of Microbiology, 38, 794-797.
Khaosaad, T., Garcia-Garrido, J. M., Steinkellner, S. & Vierheilig, H. (2007) Take-all disease is systemically reduced in roots of mycorrhizal barley plants. Soil Biology and Biochemistry, 39, 727-734.
Lippi, D., Cacciari, I., Pietrosanti, T. & Pietrosanti, W. (1992) Interactions between Azospirillum and Arthrobacter in diazotrophic mixed culture. Symbiosis, 13, 107-114.
Mawarda, P.C., Le Roux, X., van Elsas, J.D. & Falcao Salles J. (2020) Deliberate introduction of invisible invaders: A critical appraisal of the impact of microbial inoculants on soil microbial communities. Soil Biology and Biochemistry, 148, 107874.
Nguyen, T. H., Kennedy, I. R. & Roughley, R. J. (2002) The response of field-grown rice to inoculation with a multi-strain biofertiliser in the Hanoi district, Vietnam. IN I. R. Kennedy & A. T. M. A. Choudhury (Eds.) Biofertilisers in Action. Barton, ACT, Rural Indrustries Research & Development Corporation.
Rabie, G. H. & Almadini, A. M. (2005) Role of bioinoculants in development of salt-tolerance of Vicia faba plants under salinity stress. African Journal of Biotechnology, 4 (3), 210-222.
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Waller, F., Achatz, B., Baltruschat, H., Fodor, J., Becker, K., Fischer, M., Heier, T., Huckelhoven, R., Neumann, C., Von Wettstein, D., Franken, P. & Kogel, K.-H. (2005) The endophytic fungus Piriformis indica reprograms barley to salt-stress tolerance, disease resistance, and higher yield. Proceedings of the National Academy of Sciences, 102 (38), 13386-13391.
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