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How plants form organic matter and nourish the soil microbiota
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Life on earth intrinsically depends on the ability of plants to capture solar energy and convert it into organic matter, this fundamental process not only sustains the global food chain, but also establishes a symbiotic and complex relationship with the soil microbiome, a network of microorganisms that is vital for the health of the terrestrial ecosystem. This article explores the mechanisms by which plants synthesize biomass and how they share a significant part of this organic matter with the rhizospheric microbial community.
The synthesis of organic matter in plants begins with photosynthesis, a biochemical process that occurs mainly in the chloroplasts of leaf cells. The global equation of photosynthesis, 6CO₂ + 6H₂O + Sunlight → c₆h₁₂o₆ + 6O₂, in short is the conversion of carbon dioxide CO2 and water H2O into glucose C6H12O6 a simple carbohydrate, releasing oxygen as a by-product.
According to some authors, glucose is the fundamental monomer from which the plant builds structural and storage polymers. For example, glucose is polymerized into starch for short- and medium-term energy storage, and into cellulose, the main component of cell walls, which gives rigidity and shape to the plant. Other organic compounds, such as lipids, proteins and nucleic acids, are synthesized by secondary metabolic pathways that use the carbon skeletons derived from glucose, ensuring the formation of the total plant biomass.
Once synthesized in the aerial parts, the organic matter (mainly in the form of sucrose) is transported through the vascular system of the phloem to the carbon "sinks", among which are the roots, stems, fruits and, crucially, the rhizosphere. The rhizosphere is the thin layer of soil directly influenced by the secretions of the roots and harbors a significantly higher microbial concentration than bulk soil, it is at this soil-root interface where the most intense exchange of organic matter occurs.
The main way by which plants share their organic matter with soil microorganisms is through the release of root exudates. These exudates are a complex mixture of low molecular weight organic compounds that are actively (secretion) or passively (diffusion and cell lysis) released from the root tip.
The composition of the exudates is surprisingly varied and can account for up to 20% of all the carbon fixed by photosynthesis, especially in young or actively growing plants. The key components include:
- Simple sugars: Fructose, glucose and sucrose. They serve as a readily available source of energy and carbon.
- Organic acids: Citric, malic and oxalic. In addition to serving as microbial nutrients, they can chelate mineral nutrients such as iron and phosphorus, making them more bioavailable.
- Amino acids: Glycine, serine and glutamic acid. They are crucial as sources of organic nitrogen for microbes.
- Secondary compounds: Flavonoids, terpenes and phenols. These compounds act as chemical signals, attracting or repelling specific microbial groups or initiating symbiotic interactions, as in the case of Rhizobium nodulation.
By releasing these exudates, the plant selectively nourishes microorganisms that can provide it with a benefit. For example, the proliferation of plant growth-promoting bacteria (PGPR) and mycorrhizal fungi in the rhizosphere is largely due to the availability of exudates.
The microorganisms of the soil, when consuming the exudates, offer essential services to the plant as a contribution of nutrients the microorganisms decompose the complex organic matter of the soil (humus and dead residues) through mineralization, releasing inorganic nutrients such as nitrate NO3, ammonium NH4, among others, that the plant can absorb. A prominent example is biological nitrogen fixation, where bacteria such as those of the genus Azotobacter or Rhizobium convert atmospheric nitrogen N2 into ammonium, a usable form.
In short, the formation of organic matter by plants, driven by photosynthesis, is not an isolated biomass production event, but the first step in a finally tuned carbon and nutrient cycle. The release of root exudates represents a directed energy transfer that actively shapes the composition of the soil microbial community. This underground carbon economy is a pillar of ecological health, demonstrating that plant vitality and soil fertility are linked in one of nature's deepest and most essential biological associations.
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| Bibliographic references |
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- Bieto, J. and Talon, M. (2008). Fundamentals of plant physiology. McGraw-HILL, Madrid: Spain.
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