Carbon plays a vital role as a source of fuel and structural components for plants. Plants go through a process of generating, storing, and metabolizing carbohydrates, such as sugars, fructose, glucose, and starch, utilizing them to fulfill their energy needs. Moreover, carbon is a primary constituent of the building materials, including cellulose and lignin, which plants employ to construct new leaves, stems, and roots.

Plants also use part of this carbon to support microbial populations, through the secretion of exudates. Once utilized by microorganisms the carbon is either returned to the atmosphere as CO2 or stored in the soil as humic substances. This dynamic interaction between carbon, plants and soil microorganisms is a fundamental aspect of soil health and fertility, contributing to improved nutrient cycling and overall ecosystem resilience. 

Carbon, is abundant and vital for life on Earth, and can be found in rocks, the atmosphere, biomass, soils, fossil fuels, and the ocean. Like water, carbon has a geological and a biological cycle.

The geological cycle of carbon primarily involves the long-term storage of carbon in rocks and fossil fuels, affecting Earth’s geological processes. In contrast, the biological carbon cycle centers on the movement of carbon between living organisms and the environment, crucial for sustaining life and ecosystems.

This biological carbon cycle involves the continuous movement and recycling of carbon between living organisms, including plants and animals, and the environment. It’s the engine that drives agriculture, as carbon flows from the atmosphere into plants, then into animals, soils, and back into the atmosphere in a dynamic, interconnected process.

Understanding and effectively harnessing this biological carbon cycle is of utmost significance. It underpins the health of ecosystems, soil fertility, and the productivity of crops and livestock. In essence, focusing on the intricacies of the biological carbon cycle is essential for sustainable and productive ecosystems.

In conventional agriculture, several practices contribute to the decline in soil carbon levels. 

Intensive tilling, a common practice, disturbs the soil structure and accelerates the decomposition of organic matter, releasing carbon dioxide into the atmosphere. Continuous plowing and monoculture cropping reduce the input of organic materials into the soil, such as crop residues and organic amendments, which are essential for maintaining soil carbon. 

Soil erosion, another issue in conventional farming, not only transports fertile topsoil away but also carries away carbon-rich particles. 

The use of pesticides and synthetic chemicals can disrupt the delicate balance of soil microorganisms responsible for organic matter decomposition and carbon storage. Additionally, limited crop residue return to the soil and frequent disturbances, like harvesting and planting, further diminish soil carbon levels. The absence of cover crops, which can protect and enrich the soil during fallow periods, is another factor. 

These practices, often driven by a focus on short-term yields, can collectively deplete soil carbon and compromise long-term soil health.

Carbon enters the soil via two mechanisms:

(1) Through plant root exudates, were plants inject into the soil the sugars, protein/amino acids and lipids, produced during or as an immediate consequence of photosynthesis. 

(2) As a result of the decomposition of the biomass of plants and other organisms ( leaf litter, stubble, dead plant roots, manures, etc), that have been broken down and incorporated by microorganisms, earthworms and other creatures, into the soil.



Soil organic matter is made up of the countless carbon compounds that come from plant and organism biomass, as well as the byproducts of microbial decomposition. A subset of this matter, often called soluble or active carbon, serves as the primary energy and nutrient source for soil microorganisms.

The soluble or active portion of organic matter serves as a nutrient source in forms accessible to both microbes and often plants, primarily supplying carbon and nitrogen to the soil. Furthermore, organic phosphorus accounts for the majority of soil phosphorous, while soil sulfur is primarily found within the organic matter.[]

Soil organic matter (SOM) encompasses all the organic components within the soil. But its definition, can be simplified in terms of how these substances are more or less available to microorganisms and mainly consists of:

(1) Living organisms like bacteria, fungi, nematodes, protozoa, earthworms, arthropods, etc and the roots of living plants.

(2) Decomposing organic matter from deceased plants and organisms, including remnants, waste, and root exudates.

(3) Humic substances.

The first group, although not currently decomposable, participates in breaking down organic matter and serves as a source of soluble carbon, primarily through root exudates, mycorrhizal glomalin, bacterial secretions, and more. The second group houses the remaining soluble organic matter or active carbon and includes less easily degradable substances like lignin and cellulose. The third organic component in the soil represents the end product of microbial decomposition. While it contributes structurally and chemically to the soil ecosystem, it doesn’t provide nutritionally (these are often termed stable or insoluble forms of organic matter).

The carbon derived from the available sources is utilized by microorganisms to sustain their metabolic functions and reproduction. During this process, carbon is either emitted back into the atmosphere as CO2 through respiration or converted into intricate humic substances through successive breakdown, notably by microscopic fungi.

Its worth mentioning that the amount of total organic matter in the soil doesn’t necessarily correlate with the microbially available soluble organic matter, and these proportions can vary significantly among different soils. It’s entirely possible to encounter rich, dark soil with 7% total organic matter but very little soluble carbon available, mainly because the organic matter exists primarily as humic substances. This can lead to low crop yields. Conversely, sandy soils with as little as 0.5% total organic matter, much of it in soluble form, can be incredibly productive. For soils to function optimally, they require a mixture of fresh, partially degraded, and previously degraded organic matter, creating a balanced soil ecosystem. [1]

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