Digging deeper: The overlooked carbon fortress beneath our feet

Global efforts to combat climate change often focus on what we can see: forests, renewable energy, and surface-level farming practices. A new analysis argues that one of the most significant battlegrounds in carbon sequestration lies hidden deep underground. A comprehensive review led by Professor Nanthi Bolan at The University of Western Australia consolidates global knowledge on deep soil carbon, the massive carbon reserve stored more than 30 centimeters below the surface, which has long been excluded from standard carbon accounting.

The paper provides a critical assessment of this enormous carbon sink, noting that deep soil layers (below 30 cm) contain over 850 petagrams of carbon worldwide. This accounts for approximately 50-60% of the total carbon stored in the top meter of soil. While surface soil carbon is closely tied to vegetation and is quick to respond to management changes, deep carbon dynamics are governed by different, more permanent factors.

The Hidden Stability Below

A key finding is that deep soil carbon is far more stable than its surface-level counterpart. Its persistence is attributed to strong interactions with clay minerals and iron oxides, which form protective organo-mineral bonds that shield the carbon from microbial decomposition. The subsoil environment, characterized by low oxygen and limited microbial activity, naturally slows the carbon cycle, allowing organic matter to remain locked away for thousands of years. This inherent stability makes deep soil an attractive target for long-term carbon removal strategies.

A Sleeping Giant's Weakness

Despite its stability, this deep carbon reservoir is not invincible. The review identifies significant vulnerabilities to global environmental changes. Rising temperatures can stimulate microbial activity even at depth, while altered precipitation patterns and land use practices like deep tillage can expose protected carbon to decomposition. The analysis also points to the priming effect, a phenomenon where fresh carbon inputs from deep-rooted plants can unexpectedly accelerate the breakdown of ancient, once-stable carbon, potentially turning a sink into a source.

"For decades, our carbon accounting has been surface-level, often stopping at 30 centimeters," states Professor Nanthi Bolan. "Our analysis shows that the majority of soil carbon is stored much deeper, acting as a vast, long-term reservoir. Understanding the unique dynamics of this deep soil carbon—how it's protected by minerals yet vulnerable to changes in climate and land use—is essential for developing effective climate mitigation strategies in agriculture."

From Knowledge to Action

The review synthesizes a range of management strategies aimed at enhancing deep carbon storage. These include breeding crops with deeper and more extensive root systems to directly deposit carbon into the subsoil, a practice that also builds drought resilience. Other promising approaches involve mechanical soil inversion to bury carbon-rich topsoil and the application of chemical amendments, such as clay or biochar, to improve the subsoil's capacity for carbon stabilization.

The authors identify critical knowledge gaps and call for globally coordinated deep soil surveys to properly map and characterize this resource. Long-term field experiments are needed to validate the effectiveness and economic feasibility of different sequestration technologies. Future innovations may lie in advanced materials like mineral-integrated biochars and polymer-clay hydrogels, or even in engineering plant-microbe systems specifically to enhance carbon stabilization at depth.

Ultimately, this work presents a clear message for policymakers, land managers, and scientists: to effectively leverage soils in the fight against climate change, we must look beyond the plough layer. Managing the entire soil profile is not just an option but a necessity for securing a sustainable, low-carbon future.

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).