Soil carbon sequestration
How soils capture and store carbon, and the role biochar can play in long-term carbon storage
What is soil carbon sequestration?
Soil carbon sequestration is the process of capturing carbon dioxide from the atmosphere and storing carbon in the soil.
Plants absorb CO₂ through photosynthesis and convert it into organic matter. Carbon then enters the soil through roots, plant residues and other organic material, where some of it can be stored as soil organic carbon.
How long that carbon remains in the soil depends on factors including climate, soil type, land management and the form in which the carbon is stored. Some soil carbon can cycle relatively quickly back to the atmosphere, while other forms can persist for much longer.
Increasing soil carbon can therefore contribute to climate mitigation while also supporting important soil functions such as soil structure, water retention and nutrient cycling.
The role of biochar in soil carbon sequestration
Biochar provides a different route for adding carbon to soils because the biomass is first converted through pyrolysis into a more persistent, carbon-rich material.
When suitable biochar is applied to soil, a proportion of the carbon originally captured by plants can remain stored for much longer than it would in the original biomass.
Biochar may also influence soil properties such as water retention, nutrient availability and microbial activity, although its effects vary considerably depending on the biochar, soil, crop and application conditions.
This combination of carbon storage and potential soil benefits is one reason biochar is being explored in agriculture, horticulture and land management.
Other natural carbon sinks
Soil is one of several important natural carbon stores. Carbon is continuously exchanged between the atmosphere, vegetation, soils, oceans and geological systems, with each storing carbon over very different timescales.
Understanding these natural carbon sinks helps put soil carbon sequestration and the role of biochar, into the wider carbon cycle.
Plants remove carbon dioxide from the atmosphere through photosynthesis and store carbon in their trunks, branches, roots and other biomass. Some of this carbon subsequently enters soils through roots, fallen leaves and other organic matter.
Vegetation therefore plays an important role in biological carbon sequestration, although much of the stored carbon can eventually return to the atmosphere through decomposition, fire or combustion.
The oceans are a major part of the global carbon cycle, absorbing carbon dioxide from the atmosphere through physical and biological processes. Carbon is stored in seawater, marine organisms and sediments over a wide range of timescales.
Ocean carbon storage operates very differently from terrestrial soil carbon, but both form important components of the Earth’s natural carbon cycle.
Peatlands accumulate large quantities of organic carbon because waterlogged conditions slow the decomposition of plant material. Over long periods this can create deep deposits of carbon-rich peat.
When peatlands are drained, damaged or extracted, however, stored carbon can be released. Protecting and restoring peatlands is therefore important for maintaining these existing carbon stores.
Soil carbon sequestration in agriculture
Agricultural soils can store carbon through practices that increase organic matter entering the soil or reduce the rate at which existing soil carbon is lost.
Approaches can include cover crops, reduced soil disturbance, incorporating crop residues, agroforestry, organic amendments and other forms of regenerative land management.
Increasing soil organic carbon can provide benefits beyond carbon storage, including improved soil structure, water retention, nutrient cycling and resilience. However, the amount of additional carbon stored varies between soils and farming systems, and some of this carbon can be lost again if land management changes.
This makes the durability of soil carbon storage an important consideration when assessing its contribution to climate mitigation.
Reduce disturbance of the top soil, make fine drills and plant directly into the ground.
This leads to more water storage, microbe growth and healthier more vigorous plants.
Plant cover crops between seasons, this provides diversity of root exudate, feeding the soil biology, building a healthy diverse soil.
This is the process of combining trees and agriculture together. This can either be planting trees directly into crop fields, growing trees next to agricultural fields or farming in a forest environment that already exists.
This optimises the beneficial interactions between trees and crops under ground, as well as bring in pollinators and biodiversity above ground.
Animals are a great way to improve soils. They eat the top layer of organic material, drop manure/urine full of microbes and nitrogen as well as break up the top layer of material with their hooves, stimulating growth. Its important to move them on quickly and allow that patch of land to rejuvenate. It is worth the extra planning and work involved.
Provide the ground with with material that will breakdown and provide the soil with nutrients and microorganisms.
All of these techniques can lead to more localised rainfall, pull carbon out of the atmosphere through photosynthesis and reduce the risk of desertification – turning the soil into dust, making it vulnerable to wind and water erosion.
How long can carbon remain stored in soil?
Soil is a major carbon store, but soil carbon is not necessarily permanent. Organic carbon can be broken down by microorganisms and returned to the atmosphere as carbon dioxide, while changes in land use or management can also reduce previously accumulated soil carbon.
The persistence of carbon depends on how it is stored and the environmental conditions within the soil. This means that increasing soil carbon stocks and keeping that carbon stored are related but distinct challenges.
Biochar is particularly interesting in this context because pyrolysis converts part of the carbon in biomass into forms that are more resistant to decomposition than the original organic material.
Where biochar production and storage result in a verified net removal of atmospheric CO₂, this can also be considered biochar carbon removal (BCR).