Global Store of Soil Inorganic Carbon Challenges Long-Held View Regarding Its Impact on Environmental Health

Soil is a complex body composed of four major components: 45% mineral, 20-30% water, 20-30% air, and 5% organic matter. Soil performs many vital functions like providing a growth medium for plants, providing habitat for organisms, acting as a modifier of Earth's atmosphere, and maintaining the overall health of the biosphere.

(Photo: Wikimedia Commons/ HolgerK)

A critical component in maintaining soil health is soil carbon, which is recognized as the largest storage of terrestrial carbon on Earth.

Understanding Soil Carbon

Soil carbon serves as a carbon sink in the global carbon cycle. Through photosynthesis, carbon gets into the soil as carbon dioxide molecules in the air are captured by plants, turning them into organic matter. The plants keep the carbon while giving off oxygen gas.

Soil carbon usually refers only to the organic matter component of the soil, also called soil organic carbon (SOC). However, soil carbon contains an inorganic component called soil inorganic carbon (SIC), like calcium calcium carbonate. Since soil inorganic carbon tends to accumulate more in arid regions with infertile soil, many believe it is unimportant.

A New Perspective on SIC

A study that analyzed a large database of field-based SIC measurements has challenged this long-held view about soil inorganic carbon. The study details are discussed in the paper "Size, distribution, and vulnerability of the global soil inorganic carbon."

Professor Huang Yuanyuan from the Institute of Geographic Science and Natural Resources Research of the Chinese Academy of Sciences (CAS) and Professor Zhang Ganlin from the Institute of Soil Science of CAS lead the research. Together with their collaborators, they have quantified the global store of soil inorganic carbon.

The team discovered 2,305 billion tons of carbon stores as SIC in the top 6.56 feet (two meters) of soil worldwide. This is over five times the carbon found in all of the vegetation in the world combined. The hidden soil carbon pool can be important in understanding how carbon is distributed worldwide.

The research team also found that the large carbon pool is vulnerable to environmental changes, particularly in soil acidification. As calcium carbonate is dissolved in acids, it is removed as carbon dioxide gas or directly into the water.

Many regions, such as China and India, experience soil acidification from intense farming and industrial activities. Without remedial actions and better soil practices, our planet will likely face a SIC disturbance in the next three decades.

Disturbances to SIC accumulated over Earth's history have a crucial impact on soil health. The disruption compromises the soil's ability to regulate nutrient levels, neutralize acidity, stabilize organic carbon, and foster plant growth. In other words, soil inorganic carbon plays a vital role in storing carbon and supporting the function of ecosystems that depend on it.

The study underscores the urgency of including inorganic carbon in climate change mitigation plans as an additional lever for enhancing and maintaining carbon sequestration. By understanding soil carbon dynamics, the experts hope to develop more effective strategies for maintaining soil health and improving ecosystem services.