Discussions around soil carbon have traditionally revolved around organic matter, overlooking the substantial contribution of soil inorganic carbon. A recent study published in Science by an international team of researchers, however, addresses this oversight.

Soil carbon usually refers only to the organic matter component of soils, known as soil organic carbon. However, soil carbon also has an inorganic component, known as soil inorganic carbon. Solid soil inorganic carbon, often calcium carbonate, tends to accumulate more in arid regions with infertile soils, which has led many to believe it is not important.

In their study, Huang Yuanyuan from the Institute of Geographic Sciences and Natural Resources Research of the Chinese Academy of Sciences (CAS) and Zhang Ganlin from the Institute of Soil Science of CAS, lead colleagues from several other institutions including IIASA, to quantify the global store of soil inorganic carbon, challenging this long-held view.

Using a calculation method that is significantly more robust than earlier approaches, the researchers found a whopping 2,305 billion tons of carbon stored as soil inorganic carbon in the top two meters of soil worldwide, which is more than five times the carbon found in all of the world's vegetation combined. This hidden pool of soil carbon could be key to understanding how carbon moves around the globe.

"This huge carbon pool is vulnerable to changes in the environment, especially soil acidification. Acids dissolve calcium carbonate and remove it either as carbon dioxide gas or directly into the water," explains Huang. "Many regions in countries like China and India are experiencing soil acidification due to industrial activities and intense farming. Without remedial actions and better soil practices, the world is likely to face a disturbance of soil inorganic carbon in the next thirty years.”

Disturbances to soil inorganic carbon accumulated over Earth's history have a profound impact on soil health. This disruption compromises the soil's ability to neutralize acidity, regulate nutrient levels, foster plant growth, and stabilize organic carbon. Essentially, soil inorganic carbon plays a critical dual role in storing carbon and supporting ecosystem functions that depend on it.

The researchers revealed that approximately 1,13 billion tons of inorganic carbon are lost from soils to inland waters each year. This loss has profound but often overlooked implications for carbon transport between the land, atmosphere, freshwater, and ocean.

While society has recognized the importance of soils as a fundamental part of nature-based solutions to combat climate change, much of the focus has been on soil organic carbon. It is now clear that inorganic carbon deserves equal attention.

“While the amount of organic and inorganic carbon in the soil is approximately the same, significantly less research has been dedicated to inorganic carbon. It was believed that inorganic carbon is much more stable. However, with climate change, on the one hand it becomes more mobile and on the other, it may play a role in carbon sequestration technologies, for example, enhanced weathering,” notes study coauthor Dmitry Shchepashchenko, a senior research scholar at IIASA.

This study underscores the urgency of incorporating inorganic carbon into climate change mitigation strategies as an additional lever for maintaining and enhancing carbon sequestration. International programs such as the international "4 per 1000” Initiative, which aims to increase (mostly) soil organic carbon by 0.4% annually, should also consider the critical role of inorganic carbon in achieving sustainable soil management and climate mitigation goals.

By broadening the understanding of soil carbon dynamics to include both organic and inorganic carbon, the researchers hope to develop more effective strategies for maintaining soil health, enhancing ecosystem services, and mitigating climate change.

Adapted from an article prepared by the Institute of Geographic Sciences and Natural Resources Research of the Chinese Academy of Sciences (CAS). Read the original article here.

Reference
Huang, Y., Song, X., Wang, Y.-P., Canadell, J.G., Luo, Y., Ciais, P., Chen, A., Hong, S., et al. (2024). Size, distribution, and vulnerability of the global soil inorganic carbon. Science 384 (6692) 233-239. DOI: 10.1126/science.adi7918 [pure.iiasa.ac.at/19612]

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