By Wong Ching Nam (Jimmy)
Biofertilizers are fertilizers containing microbes. When applied into the soil, plants, or seeds, the microbes in biofertilizers will colonize the rhizosphere soil around the plant, leading to increased plant growth and food yield. The growth-promoting effect of biofertilizers is usually achieved by accelerating natural microbial processes that either give plants the required nutrients to grow, or by impacting plant growth-hormones. In the process, biofertilizer microbes will improve soil fertility, water-retention ability, and help break down chemical pollutants (bioremediation). Further studies show biofertilizers can generally enhance crop yield by 10-40% by giving plants supplement in proteins, essential amino acids, vitamins, and nitrogen fixation. (Deepak Bhardwaj et al., 2014)
But given our current system of chemical fertilizers is very efficient and does yield good food production, why bother changing to biofertilizers? This is because the excessive use of chemical fertilizers in the past decades is affecting ecological balance in the environment, making crops around the globe more prone to disease while also depleting soil fertility. Moreover, too much dependence on chemical fertilizers pollute air, water, and soil. For example, the leaching of nitrogen and phosphorus chemical fertilizers in waterways can lead to eutrophication which damages local biodiversity (Rana Nauman Shabbir et al., 2019). In the board background of raising world population, rapid urbanization and industrialization our ecosystem and food security is under increasing threat. Therefore, biofertilizers are becoming a promising alternative to chemical fertilizers. However, it is important to note that introducing biofertilizers does not always mean totally abandoning the use of old chemical fertilizers; in a lot of scenarios they are be used simultaneously to get the best result. (G. Chandramohan Reddy, R. K. GoyalShriniketan & PuranikVijaykumar, 2020)
Generally, microbes in biofertilizers help plants to growth in two major ways, either directly promoting resources acquisition, or indirectly by affecting other ecological processes. This article will mainly be focused on how plants acquire the resources nitrogen, phosphorus and iron through the help of biofertilizer microbes.
Nitrogen is one of the most important nutrients for plant growth and food productivity. It is crucial for the production of hormones, proteins, DNA, etc. However, most nitrogen is unavailable to plant as it exists in the form of N2 in the air, making up about 78% of our atmosphere. Biofertilizer solutions to this is with microbes that have nitrogenase activity, which can undergo biological nitrogen fixation that converts atmospheric N2 into ammonia (NH3). Then the ammonia can be readily taken up by plants. These nitrogen-fixing bacteria can be symbiotic, such as the Rhizobiaceae family, or in free living endophytic forms like Azotobacter. Regardless of the family, these bacteria can be present in biofertilizers to promote plant nitrogen uptake.
Interestingly, within the symbiotic Rhizobiaceae nitrogen-fixing bacteria, the genus Rhizobium (form nodules with legume roots) can be further engineered by introducing bacterial hemoglobin(Hb) genes into them to assist respiration. This in turn will yield 68 % increase in nitrogenase activity and 16% increase in nitrogen content of seeds than wild types, potentially boosting food production. (Bernard R. Glick, 2015)
It is worth noting that, apart from fixing nitrogen, some other nitrogen-fixing microbes such as Cyanobacteria and Azolla promote plant growth by synthesizing and secreting auxin, indole acetic acid, and gibberellic acid. These secreted chemicals interact with plant biological pathways in a positive way that strengthen plant resilience and health. In addition the microbe Azolla itself contains a high amount of micronutrients (phosphorus, potassium, zinc, iron, molybdenum); when Azolla die and decompose in the soil these micronutrients become readily available to plant uptake, making “dead microbes” part of the fertilizer as well. (Rana Nauman Shabbir et al., 2019)
Similar to Nitrogen, phosphorus is also crucial for plant development and growth, but most phosphorus exists in insoluble form and is thus unavailable for uptake by plants. The modern chemical solution to this is to apply huge amount of phosphate fertilizers that contain soluble inorganic phosphorus, but this is rather inefficient, as a huge proportion of this chemical phosphorus will naturally become immobilized and unavailable to plants after a short period of time, wasting most of the soluble inorganic phosphorus. (FENG Ke et al., 2004)
In comparison, biofertilizers use microbes that have the ability to solubilize phosphates to make them readily available to plants. This is much more cost efficient and ecofriendly than chemical fertilizers. These phosphates solubilizing microbes do their jobs in a variety of ways. Generally these microbes will produce low molecular weight organic acids with hydroxyl and carboxyl groups, which can form chelates with cations underground that lock phosphates, freeing up the phosphates making them soluble in soil, in turn benefiting plants around them. Phosphate-solubilizing bacteria include Burkholderia, Agrobacterium, and Flavobacterium. Interestingly these phosphate-solubilizing bacteria also increase efficiency of biological nitrogen fixation by nitrogen fixing microbes through bacterial interactions, which make them ideal to be applied as biofertilizers together. (Mahdi et al., 2012)
Under aerobic conditions, the majority of iron exists in the form Fe3+, which usually binds to hydroxides, making the iron unavailable for plant uptake. However, microbes in biofertilizers such as Rhizobacteria obtain iron by secreting iron chelators called siderophores, which have high affinity to metals ions. Siderophores assist microbe uptake of Fe3+, the microbes then reduce it to Fe2+. The bacteria can then release Fe2+ through gated channels to benefit plants. Not limited to iron binding, siderophores secreted by many microbes can also bind to heavy metals, in the process reduced heavy metal pollution stress and improved soil quality. (Deepak Bhardwaj et al., 2014).
Overall, biofertilizer seems to be an environmentally-friendly and cost efficient way to make use of the natural abilities of microbes to enhance food production. However, this technology is still in its nascent stages, and can be improved in many ways. Hopefully, future advancement in molecular biology, plant pathogen interactions, and genetic engineering can provide the tools to shed more light in understanding and exploiting the use of these microbes.
References:
Bernard R. Glick. (2015) Beneficial Plant-Bacterial Interactions . Available from: https://link.springer.com/book/10.1007/978-3-319-13921-0#about.
Deepak Bhardwaj, Mohammad Wahid Ansari, Ranjan Kumar Sahoo & Narendra Tuteja. (2014) Biofertilizers function as key player in sustainable agriculture by improving soil fertility, plant tolerance and crop productivity. Available from: https://microbialcellfactories.biomedcentral.com/articles/10.1186/1475-2859-13-66.
FENG Ke, LU Hai-Ming, SHENG Hai-Jun, WANG Xiao-Li & MAO Jian. (2004) Effect of organic ligands on biological availability of inorganic phosphorus in soils . Available from: http://pedosphere.issas.ac.cn/trqcn/ch/reader/view_abstract.aspx?file_no=20040111&flag=1.
G. Chandramohan Reddy, R. K. GoyalShriniketan & PuranikVijaykumar. (2020) Biofertilizers Toward Sustainable Agricultural Development. Available from: https://link.springer.com/chapter/10.1007%2F978-3-030-36248-5_7.
Mahdi, S. S., Talat, M. A., Dar, M. H., Aflaq Hamid & Latief Ahmad. (2012) Soil phosphorus fixation chemistry and role of phosphate solubilizing bacteria in enhancing its efficiency for sustainable cropping – a review. Available from: https://www.cabdirect.org/cabdirect/abstract/20133096711.
Rana Nauman Shabbir, Hakoomat Ali, Fahim Nawaz, Shabir Hussain, Ahsan Areeb, Naeem Sarwar & Shakeel Ahmad. (2019) Use of Biofertilizers for Sustainable Crop Production . Available from: http://link-springer-com-443.webvpn.fjmu.edu.cn/chapter/10.1007%2F978-981-32-9783-8_9.
Imperial Bioscience Review 