Atmospheric carbon dioxide can be removed by sequestering it (that is, “locking it away”) in agricultural soil as soil organic carbon.

This natural biological process is widely recognised as an important and readily implementable, low-cost way to potentially draw down more than 5 billion tonnes of CO2-e globally per year.

Soil contains more of the Earth’s carbon stock than does the entire atmosphere.

Carbon occurs in the soil’s near-surface profile (typically at < 1 metre depth) mainly in organic form, termed soil organic carbon (SOC). Large stores of inorganic carbon also occur in some soils, mainly as carbonate minerals. 

SOC continuously cycles between various forms and fractions in response to various management actions at the surface (such as farm management). Two major forms of SOC are recognised: particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). Each form plays a role in the soil’s carbon flow, storage and persistence. 

POC is mainly produced when aboveground plant biomass dies and begins to decompose on the soil surface. The carbon in that detritus joins the soil subsystem as organic carbon particles (hence “POC”). Most POC is thus found in and around the soil’s microbial complex – e.g. fungi, bacteria, earthworms, etc.

The MAOC pool tends to be fed from living plant roots which, when functioning healthily, contribute carbon-rich substances to the soil. These comprise root exudates (from the living roots), root-associated carbon-rich mycorrhizal fungi, and aliphatic compounds derived from the decaying root biomass.

Farming and grazing practices that encourage plant root development are the most likely practices to build net SOC stocks over time. Of these, managed or “rotational” grazing that includes deep-rooted multi-species perennial pasture plants, is most effective at building SOC, particularly at depth.

On-farm studies in Australia and New Zealand highlight this potential, indicating it’s operationally feasible to achieve annual sequestration rates of between 1.3 and 4.8 tonnes of CO2-e per hectare through managed grazing in permanent pastures. 

Soil organic carbon build up is increasingly being viewed as an effective, low-cost climate policy action with multiple co-benefits (such as lower farm input costs, higher water-holding capacity, and biodiversity protection). An essential prerequisite for maintaining soil carbon accrual, and hence its permanent retention in the soil, is a commitment to maintaining the required farming and grazing practice changes over the long term. At the same time, new evidence is emerging of a more dynamic interplay between SOC stocks, microbial populations and clay-carbon interchange. This shifts the idea of permanence toward a more dynamic concept of persistence – i.e. the continued residence of carbon in the soil, but in different forms at different times.

Nevertheless, reversals in some of the gained SOC can also occur due to extrinsic extremes of climate, drought and other impacts such as fire. Such losses can be minimised by maintaining managed grazing principles, including taking timely destocking decisions. Also, drought impacts on carbon stores are likely to be lower in vegetation dominated by resprouting pasture species, providing a strong biophysical rationale for promoting these types of pastures. Also, SOC accrual can typically reach a plateau after 20 to 50 years, depending on the initial SOC status of a particular soil. Even with this caveat, the IPCC considers SOC sequestration to be a significant avenue for global CO2 drawdown in the few decades remaining for the world to achieve its committed climate change mitigation targets.