In the global effort to combat climate change, soil has been recognized as the largest terrestrial carbon sink. Yet, most climate policies and carbon accounting systems focus only on the top 30 centimeters. A comprehensive new review, led by an international team of scientists from institutions including The University of Western Australia , Amity University , and Tsinghua University , explains why this surface-level view is dangerously incomplete and calls for a fundamental shift in how we manage the massive carbon reserves stored deep within the Earth.
This extensive work synthesizes global research to build a cohesive picture of deep soil carbon —the organic matter stored below the standard sampling depth. The analysis confirms that these subsoil layers contain a colossal amount of carbon, estimated at over 850 petagrams worldwide, which accounts for 50% to 60% of the total carbon stock in the top meter of soil. By examining the sources, distribution, and stability of this carbon, the review provides a critical framework for understanding its role in long-term climate mitigation and soil health.
A key finding is that carbon stored in deeper soil layers is significantly more stable than the carbon near the surface. This persistence is attributed to its strong interaction with clay minerals and the characteristically low microbial activity in subsoil environments. Organo-mineral bonds physically protect carbon from decomposition, giving it a residence time that can span thousands of years. This makes deep soil a potentially more secure and permanent storage vault for sequestered atmospheric CO₂.
However, this deep carbon bank is not immune to threats. The review details how this supposedly stable carbon can be vulnerable to global changes. The introduction of fresh carbon from deep-rooted plants can trigger a " positive priming " effect, where microbes are stimulated to break down and release the ancient, stored carbon. Furthermore, rising global temperatures, altered precipitation patterns, and land-use changes like deep tillage can all destabilize these vital subsoil reserves, potentially turning a long-term sink into a new carbon source.
The review outlines several agricultural strategies to protect and enhance deep soil carbon stocks. Planting deep-rooted perennial crops and pastures can directly deposit carbon into the subsoil, bypassing the more volatile surface layers. Other physical and chemical approaches include mechanical soil inversion (deep ploughing) to bury carbon-rich topsoil, though its effectiveness is context-dependent. Amending soils with clay, biochar, or iron minerals can also promote long-term stabilization by creating protective organo-mineral complexes far below the surface.
"For decades, our climate accounting has been scratching the surface, focusing on the top 30 centimeters of soil," states corresponding author Nanthi Bolan of The University of Western Australia . "Our analysis confirms that the real treasure, the long-term, stable carbon, lies much deeper. To make meaningful progress on climate mitigation, agricultural policies and carbon markets must adapt to manage and protect these vast subsoil reserves, turning our farmlands into more effective and permanent carbon sinks."
While providing a clear picture of the current state of knowledge, the authors emphasize the urgent need for more dedicated research. They call for globally coordinated deep soil surveys across different climates and soil types to better model carbon dynamics. Future work should also focus on developing novel composite materials for sequestration and combining advanced plant breeding with microbiome engineering to create agricultural systems optimized for deep carbon storage.
By looking beyond the plough layer, this work redefines the potential of agricultural land in the fight against climate change. It sends a clear message to policymakers, scientists, and farmers: the future of soil carbon sequestration depends on understanding and managing the world beneath our feet.
Corresponding Author: Nanthi Bolan
Original Source: https://doi.org/10.1007/s44246-026-00270-8
✍️ Contributions: Nanthi Bolan, Kadambot H.M. Siddique, and Shiv Bolan conceptualised the scope of the review. Nanthi Bolan and M.B. Kirkham provided thorough feedback and contributed to Section 1 (Introduction) and Section 8 (Conclusion). Manish Kumar and Juhi Gupta contributed to Section 7, which focuses on management strategies for promoting subsoil carbon storage. Cherukumalli Srinivasa Rao, Mani Chandana, and Jagadesh M contributed to Section 5, addressing factors affecting the distribution and stability of deep soil carbon. Deyi Hou and Cadie Huang contributed to Section 4 on the stability of deep soil carbon and Section 6 on the vulnerability of deep carbon to global changes. Shiv Bolan, Santanu Mukherjee, and Sreeni Chadalavada contributed to Section 2 (Methodology) and Section 3 (Sources of deep soil carbon).
Carbon Research
Literature review
Not applicable
Sources, distribution, stability and management of deep soil carbon in agricultural systems
13-May-2026
Deyi Hou is an editorial board member for Carbon Research and was not involved in the editorial review, or the decision to publish, this article. All authors declare that there are no competing interests.