The hidden giant of the soil carbon

Soil is made up of four main things: minerals, air, water, and organic matter. By soil carbon, people usually only refer to the organic matter component of soil (SOC). Yet, there are also inorganic carbon in soils, known as SIC.

SIC is found in minerals, air, and water of soils. Solid SIC, often calcium carbonate (a common substance found in materials such as limestone, marble, and chalk.), gives soil a whitish look, while SOC makes it dark. SIC tends to build up more in dry areas with infertile soils, which has led many to think it is not very important. A recent AI-guided study published in Science challenges this long-holding viewpoint.

The new study found a whopping 2305 billion tonnes of carbon stored as SIC in the top 2 meters of soil. To put it in perspective, that's over five times the carbon found in all the world's vegetation combined. This hidden pool of soil carbon could be a big deal for how carbon moves around globe.

But here's the thing: this huge carbon pool is easily affected by changes in the environment, especially soil acidification. Acids dissolve calcium carbonate, removing it as either carbon dioxide gas or into water. Countries like China and India are seeing a lot of this soil acidification due to industrial activities and intense farming like using nitrogen fertilizers. Without action, it's likely they'll face the biggest disturbances of SIC in the next thirty years.

Disturbances to SIC, accumulated over Earth's history, had a profound impact on soil health. This disruption compromises the soil's capacity to neutralize acidity, regulate nutrient levels, foster plant growth, and stabilize organic carbon. Essentially, SIC serves a critical dual role: not only in carbon storage but also in the maintenance of healthy soils and the support of various ecosystem functions.

The latest research, conducted by an international team from China, Australia, United States, France, Germany, Egypt, Russia and Austria reveals that annually, approximately 1.13 billion tonnes of inorganic carbon are lost from soils to inland waters. This loss has profound yet often overlooked effects on both atmospheric and hydrospheric carbon cycles.

While the society has recognized the importance of soils as a nature-based solution to combat climate change, much of the focus has been on SOC. It is now evident that inorganic carbon warrants equal attention.

This new study underscores the urgency of incorporating inorganic carbon into climate change mitigation strategies. International programs like the 4 per mille initiative , which aims to increase (mostly) soil organic carbon by 0.4% annually,  or initiatives to scale enhance weathering, a carbon dioxide removal technology grounded on artificial alteration of the SIC cycle, should consider the critical role of inorganic carbon in achieving sustainable soil management and mitigating climate change impacts.  

By broadening our understanding of soil carbon dynamics to include both organic and inorganic carbon, we can develop more effective strategies to preserve soil health, enhance ecosystem services, and mitigate climate change.

Text by Yuanyuan Huang.