Soil carbon
Soil organic carbon (SOC) is the carbon associated with organic matter in soils, resulting from the decay of organic sources such as plants, animals and microbes. This is distinct from soil inorganic carbon which is held in soil minerals such as calcium carbonates (e.g. limestone).
Soil organic matter (SOM) broadly refers to any organic materials present in the soil, including plant material (living or dead, such as roots, leaves and humus), microbes (bacteria and fungi), plant and microbial residues and manure.
SOM plays a pivotal role in soil health, as an important source of soil nutrients and a strong influence on soil properties. It is important for stabilising soil structure, creating aggregates of soil particles, increasing water infiltration and overall water holding capacity and storage. It contributes to soil cation exchange capacity and pH buffering capacity. It has a critical role as a food source for soil organisms, increasing their diversity and activity so they can cycle the nutrients and compete with pests and pathogens.
SOC comprises about 50-58% of the organic matter in soils (depending on the age and type of organic matter) and is an indicator of soil health. A long-term decline in soil organic carbon has a negative impact on fertility, productivity, resilience and mitigation of climate change.
The amount of SOC is a balance between inputs (from plants and micro-organisms) and losses (from natural breakdown and erosion). Rainfall and soil texture are two key factors that determine the amount of carbon that can be grown and stored in soils. The various components of organic carbon have varying degrees of resistance to breakdown.
Measuring soil carbon
Soil tests for SOC normally report as a percentage, which translates directly as the weight of SOC (in grams) per 100 grams of oven-dried soil (g C/100g soil).
Therefore 1.5% SOC = 1.5g carbon per 100g soil = 15g carbon per kg soil.
Using a measure of bulk density, which is the weight of soil in a known volume, the amount of carbon in tonnes per hectare (t/ha), in a given depth of soil, can be calculated. This is known as the carbon stock which is required for carbon accounting schemes in Australia.
For example, if the soil sample depth is (0–30 cm); bulk density is 1.3 g/cm3 and organic carbon is 1.5%:
SOC (t/ha) = 10,000 x [soil depth in m] x [bulk density] x ([% SOC]/100)
SOC (t/ha) = 10,000 x 0.3 x 1.3 x (1.5/100) = 58.5 tonnes carbon per hectare.
Measuring soil carbon
SOM is comprised of various organic compounds (e.g. proteins, waxes, sugars and other complex substance) and nutrients (e.g., carbon, nitrogen, phosphorus, potassium, calcium, and magnesium), depending on its source.
SOM is difficult to measure so SOC is often used as a proxy. Generally, SOM is thought to be comprised of about 50-58% SOC, depending on the type (source) and age of SOM present. SOM content (%) of a soil sample can be approximated by multiplying SOC content (%) with a conversion factor.
For example: % SOM = [% SOC] x [conversion factor]
This conversion factor can vary between 1.4 and 2.5, depending on soil composition.
SOC is also often used as an indicator of soil biological health. But in some instances, such as highly acidic soils where biological activity is generally lower, measurements of soil carbon turnover and soil microbial biomass are better indicators of soil biological health.
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