While carbon in Earth’s atmosphere and within living organisms is well-documented, a substantial but often overlooked reservoir exists in the planet’s soil—specifically in the form of soil inorganic carbon. A new study published in Science highlights that this type of carbon is far more prevalent than previously understood, making it a significant component of the global carbon cycle.
Soil inorganic carbon exists in various forms, including trapped carbon dioxide gas, solutions in water, and solid carbonate minerals such as calcium carbonate, commonly found in limestone, marble, and chalk. These carbonates primarily arise from the weathering of rocks or the interaction of soil minerals with atmospheric carbon dioxide. They are notably prevalent in arid and semi-arid regions like Australia, where they contribute to the soil’s whitish appearance, unlike the darker hue imparted by organic carbon.
The research involved analyzing over 200,000 soil measurements globally, revealing that the top two meters of soil contain approximately 2.3 trillion tonnes of soil inorganic carbon, about five times more than the carbon in all terrestrial vegetation combined. It was also estimated that about 1% of this carbon might be released into the environment over the next 30 years, potentially impacting Earth’s lands, waters, and atmosphere in still poorly understood ways.
This inorganic carbon is mainly concentrated in high levels across the Middle East, North Africa, parts of Asia, and Australia, with these regions displaying varying levels of carbon due to their specific environmental conditions. For instance, in Australia, which holds the fifth-largest pool of this carbon, the top two meters of soil contain around 160 billion tonnes (7% of the world’s total). Such carbonates can also be found in alluvial deposits along rivers and in calcareous rocks around lakes and coastal areas, especially in karst regions characterized by limestone-rich terrains.
The stability of soil inorganic carbon has traditionally been viewed as relatively high, changing only gradually over time. However, environmental changes, particularly soil acidification driven by factors like acid rain and pollution from industrial and agricultural activities, are increasing the mobility of this carbon. Practices such as irrigation and fertilization accelerate its dissolution and leaching, thus affecting the carbon balance.
The mobilization of inorganic carbon from soil into inland waters is significant, with an estimated 1.13 billion tonnes lost annually. This movement has profound implications for carbon transport between terrestrial, aquatic, and atmospheric systems. Therefore, managing this carbon reserve is crucial, not only for mitigating climate change but also for maintaining soil health and ecosystem functions.
Given its importance, the study advocates for a greater focus on soil inorganic carbon in strategies for carbon sequestration. Current international efforts, like the 4 per mille initiative, aim to increase soil carbon storage annually and could benefit from integrating measures that consider both inorganic and organic components of soil carbon. Enhanced rock weathering, afforestation, and the incorporation of organic amendments such as compost are proposed as means to increase soil’s inorganic carbon content, thereby strengthening its role in global carbon management and helping achieve sustainability and climate objectives.