Microbes in Human Welfare · Lesson 7 of 8
Biofertilisers, Symbiosis and Soil Nutrient Cycling
“Nitrogen-fixing bacteria and cyanobacteria, mycorrhizal fungi, and how microbial partnerships improve soil nutrient availability.”
• Define biofertilisers and distinguish nutrient enrichment from direct addition of a chemical fertiliser. • Explain symbiotic and free-living microbial nitrogen fixation. • Describe how mycorrhizal fungi improve mineral uptake and what they receive in return. • Relate microbial activity to long-term soil fertility and sustainable agriculture.
Fertile soil depends on biologically available nutrients
Plants need water, light, carbon dioxide and mineral nutrients to build their tissues. Nitrogen is required for amino acids and nucleic acids; phosphorus is needed for ATP, nucleic acids and membranes. Although molecular nitrogen makes up most of the atmosphere, plants generally cannot use N₂ directly. Their roots absorb nitrogen in suitable combined forms, such as ammonium and nitrate, supplied by soil processes or fertilisation.
Biofertilisers are preparations of organisms that can increase nutrient availability or support nutrient uptake. They act through biological processes such as nitrogen fixation, nutrient solubilisation or symbiotic exchange. They do not necessarily replace every amount of fertiliser needed in every field; soil conditions, crops and nutrient balances determine their contribution.
A preparation containing beneficial microorganisms that helps make plant nutrients available or improves nutrient uptake through biological activity.
Nitrogen fixation by Rhizobium and legume roots
Rhizobium and related bacteria can form mutualistic associations with many leguminous plants. Signals exchanged between compatible partners initiate formation of nodules on the roots. Inside suitable nodules, bacterial cells carry out nitrogen fixation, converting atmospheric N₂ into ammonia by using the enzyme nitrogenase and substantial cellular energy. The plant uses nitrogen derived from this process to make organic compounds, while the bacterial symbiont receives carbon-containing nutrients and a protected habitat.
Nitrogenase is sensitive to oxygen, although nitrogen fixation is energy-intensive and requires respiration in many symbioses. Nodules therefore regulate oxygen availability, including through proteins such as leghemoglobin, helping protect the enzyme while supporting energy supply. Not every bacterium in soil can make a nodule, and the symbiosis depends on compatible host species.
Biological conversion of atmospheric N₂ into reduced nitrogen compounds, typically initiated by the nitrogenase enzyme.
Free-living nitrogen-fixing bacteria
Some nitrogen-fixing bacteria do not require a permanent symbiotic nodule in order to function. Azotobacter species can fix nitrogen while free-living in suitable soils. Azospirillum species often associate closely with plant roots and can contribute to the plant-growth environment, including in certain conditions through nitrogen fixation or other growth-promoting activities. Their relationships with plants differ from a Rhizobium nodule association.
These organisms still require nutrients, energy and suitable environmental conditions. Nitrogen fixation consumes substantial energy because the N₂ triple bond is difficult to break. The existence of a nitrogen-fixing species in a soil sample does not guarantee that it contributes an identical amount of nitrogen under all field conditions.
Cyanobacteria in wet soils and paddy fields
Cyanobacteria are photosynthetic prokaryotes found in water and on land. Some species, including representatives of Anabaena and Nostoc, can fix atmospheric nitrogen, often using specialised cells or physiological strategies to protect nitrogenase from oxygen. Others vary in their capacity; being a cyanobacterium does not automatically mean a strain fixes nitrogen. Some non-heterocystous cyanobacteria, including strains classified within Oscillatoria, have also been studied for nitrogen fixation under appropriate conditions. Cyanobacterial genera vary considerably, so the ability must be attributed to suitable strains rather than assumed for every member.
In flooded rice-growing environments, certain cyanobacteria can enrich the ecosystem through nitrogen fixation and by adding organic material when biomass is turned over. Their role links photosynthetic carbon capture to nutrient cycling. The biological benefits depend on light, water availability, suitable strains and local ecological conditions.
Mycorrhizal fungi exchange nutrients for carbon
Mycorrhiza is a mutualistic association between certain fungi and plant roots. Many species belonging to the fungal group that includes Glomus form arbuscular mycorrhizal associations. Fungal hyphae extend through soil beyond the immediate root surface, reaching zones from which the plant may otherwise take up nutrients less efficiently. Fungal networks can supply phosphorus and in some circumstances other nutrients or water to the plant.
In return, the fungus receives carbon-containing compounds from the host plant, obtained ultimately through photosynthesis. This exchange explains why mycorrhizal fungi are described as partners rather than simply as substances added to soil. Some associations can also alter tolerance to drought, salinity or root pathogens, though the size and direction of effects vary among species and environments.
A close association between a fungus and plant roots that commonly involves an exchange of plant-derived carbon for fungal-assisted nutrient acquisition.
Microbes link soil, plants and environment
A soil ecosystem contains decomposers, nutrient-transforming bacteria and fungi, plant roots and many organisms that influence physical structure. Biofertilisers can support these processes, particularly where a compatible plant–microbe interaction is likely to function. Their contribution is different from applying soluble mineral nutrients directly: the beneficial organism must establish or remain metabolically active.
Thoughtful nutrient management uses soil conditions and crop requirements to decide when microbial inoculants are likely to help. In healthy agriculture, microbiology and chemistry work together. Reducing unnecessary chemical inputs may lower some environmental burdens, but a biofertiliser should be judged by verified nutrient effects rather than by a claim that anything biological is automatically sufficient.
Quick Quiz
Quiz
Why do plants commonly depend on nitrogen fixation or soil nitrogen compounds?
What is a major benefit to a legume in a compatible Rhizobium symbiosis?
Which pair includes free-living or root-associated nitrogen-fixing bacteria rather than obligate legume-nodule symbionts?
What is a common contribution of arbuscular mycorrhizal fungi?
What usually does the fungal partner receive from a mycorrhizal plant?
Practice Problems
- Explain how a Rhizobium–legume association benefits both partners and why oxygen regulation matters.
- Distinguish Rhizobium, Azotobacter and Azospirillum by their associations with plants.
- How can cyanobacteria contribute to the fertility of a rice-growing ecosystem?
- Why is mycorrhiza considered a mutualism and how might it improve nutrient uptake?
Compatible bacteria induce and inhabit root nodules, where nitrogenase catalyses reduction of atmospheric N₂ into biologically available nitrogen. The plant uses nitrogen compounds to construct molecules such as amino acids and nucleic acids. The bacteria gain a protected environment and carbon compounds from the plant. Nitrogenase is oxygen-sensitive, so nodules regulate oxygen conditions to protect the enzyme while permitting energy production.
Key Takeaways
• Biofertilisers use living microorganisms to support nutrient availability or plant uptake. • Plants normally require nitrogen in chemically available forms rather than atmospheric N₂. • Rhizobium–legume nodules enable mutually beneficial nitrogen fixation. • Azotobacter and Azospirillum provide different free-living or root-associated microbial strategies. • Selected cyanobacteria can supply nitrogen and organic material in wet agricultural soils. • Mycorrhizal fungi help roots obtain phosphorus and receive carbon from plants. • Biofertiliser effectiveness depends on compatible organisms, plant species and soil conditions.