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Soil carbon

Learn about what soil carbon is, how it is sequestered and measured

Image source: NIAB

What is soil carbon?

Forms of carbon

Carbon exists in two main forms in the soil, the relative proportions of which vary with soil type and mineralogy. Soil organic carbon (SOC) comprises organic matter associated with plant roots, decomposing plant or animal residues, fungi, and other micro-organisms. This accounts for about 60% of the carbon present in UK soils. Soil inorganic carbon (SIC), found in rock minerals such as chalk or limestone (calcium carbonate), makes up the remaining 40% of the carbon in UK soils. Through the natural carbon cycle, carbon is continuously being exchanged between SOC and the atmosphere, whereas SIC is more stable.

Soil properties and soil amendments

SOC affects many soil properties including structure, stability, aeration, water infiltration and retention, and nutrient cycling and availability. This in turn affects crop resilience to drought or waterlogging and therefore to extreme weather resulting from climate change. Agricultural soils that have been intensively-cultivated over many decades typically contain less SOC than if they had remained undisturbed. Sustained addition of organic matter in the form of crop residues, cover crops or organic amendments can slowly increase SOC over many years.

Particulate vs. Microbial-associated organic matter

The soil organic matter (SOM) that contains the SOC is complex but can broadly be separated into two components that differ in their formation, properties, and persistence. Particulate organic matter (POM) is less well protected from degradation and may only last for a few years. Mineral-associated organic matter (MAOM) is associated with soil minerals that slow its further decomposition, and it can remain in the soil for many decades. POM has usually experienced less microbial breakdown than MAOM, and its ratio of carbon to nitrogen is also typically lower. Crop types may differ in their relative impact on the two SOM components. Legumes for example, may enhance MAOM due to interactions with broader microbial species. In practice, however, a diversity of crop types may be best for achieving the benefits associated with both short- and long-term soil carbon.


Measurement of soil carbon

There are two main methods of testing SOC or SOM content. It is important that a consistent approach is used to monitor change from year to year, or to compare fields. In both cases, it is vital that soils are sampled representatively.

Dumas is the standard method for analysing SOCWhilst a more accurate and direct measure of carbon content, the testing laboratory must appropriately account for SIC, especially in chalk or limestone soils.

Loss on Ignition is widely used for measuring SOM content and may be more useful in the context of assessing soil health and the impact of management practices. This test can vary more between laboratories, so changing from year to year should be avoided. SOC is derived from the SOM measurement.

Sampling depth is also an important consideration. Soils are often sampled to only 15cm depth, or a maximum of 30cm, when assessing SOC/SOM. Ideally, the depth sampled should be to 60-75cm to account for the 30-60% of the carbon that can exist deeper in the root zone.

As changes in SOC take place very slowly, they are difficult to detect and quantify except over a long time (often decades rather than years). A ‘pre-treatment’ baseline measurement is vital when seeking to understand the impact of changes to cropping or management.

Sequestration of carbon in the soil

Where SOC levels are substantially depleted, appropriate management practices can lead to an increase in SOC and therefore increased carbon capture. The rate at which this occurs is likely to decrease as SOC increases, and it will eventually reach an equilibrium level whereby the additions and losses of carbon each year are the same. How long this takes, and at what level, will depend on factors including land use, climate, soil type and management. Failure to maintain appropriate management practices can lead to SOC levels starting to decline, with a net loss of captured carbon. With the continual turnover of SOC in the soil, long-term carbon capture, or sequestration, is hard to prove. Nevertheless, whilst this is not a single solution to achieving Net Zero, when considered alongside its other potential benefits, in many situations raising SOC remains an important objective.

More information

Farm Carbon Toolkit

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