Soil contains a significant amount of carbon, both in organic matter and inorganic form. A new study published in Science reveals that the inorganic carbon in soil may play a larger role in Earth’s carbon cycle than previously thought. The researchers analyzed over 200,000 soil measurements from around the world and estimated that the top two meters of soil globally holds approximately 2.3 trillion tonnes of inorganic carbon. They also predict that about 23 billion tonnes of this carbon may be released over the next 30 years, with unknown consequences for the environment.
Inorganic carbon exists in various forms in soil, including trapped carbon dioxide gas, dissolved in water, or as solid carbonate minerals. Solid carbonates, such as calcium carbonate, are the most abundant form of inorganic carbon in soil. In arid and semi-arid environments like Australia, inorganic carbon tends to accumulate more due to water carrying away carbonates when it runs through the soil.
The study found that Australia has the fifth-largest pool of soil inorganic carbon in the world, with an estimated 160 billion tonnes in the top two meters of soil. In wetter regions, soil carbonates can also be found along rivers, lakes, and coastal areas. Soil acidification, caused by factors like acid rain and pollution from industrial activities and intense farming, can dissolve calcium carbonate and release carbon into water or as carbon dioxide gas.
Disruptions to soil inorganic carbon can have profound effects on soil health and its ability to neutralize acidity, regulate nutrient levels, support plant growth, and stabilize organic carbon. The loss of inorganic carbon from soils to inland waters is estimated to be 1.13 billion tonnes per year, impacting carbon transport between land, freshwater bodies, the atmosphere, and oceans.
To mitigate climate change and protect soil health, it is important to consider both organic and inorganic carbon in soil management practices. Improved land practices, such as better irrigation and fertilization techniques, can reduce the disturbance to soil inorganic carbon. Organic amendments like compost and manure can also protect against acidification and increase soil inorganic carbon levels. Efforts to sequester carbon in soil should incorporate both forms of carbon and consider strategies like enhanced rock weathering, afforestation, and trapping organic carbon in soil minerals.
Existing soil carbon programs, such as the 4 per mille initiative, could increase their focus on inorganic carbon to achieve sustainable soil management and climate targets. Recognizing the critical role of inorganic carbon in soil can contribute to nature-based solutions for combating climate change.
