The ground beneath our feet is arguably our planet’s largest and most important bank. Flows of matter, energy, and information flow in and out of soils daily, helping to sustain ecosystems, the wider environment, and society.

These flows – the deposits and withdrawals of water, nutrients, biomass, and gases – are essential for creating a healthy soil. And a healthy soil is the currency on which we all depend for affording long-lasting plant production, biodiversity, and climate regulation.

Overdraft alert: the costs of soil mismanagement

As for any bank account, poor management of these below-ground assets can lead to a dreaded overdraft. Indeed, the mismanagement of soils, compounded by the exacerbating effects of climate change, has already left a third of the world’s soils moderately to highly degraded. The consequences of this are pervasive. Where soils have been stripped from the land surface by erosion, and a skeletal centimetre or two of soil remains, urgent action must be taken to curb further losses and build the soil account back up. 

Fortunately, soil scientists have demonstrated means by which to slow soil degradation and restore soil health. One of the most championed strategies is to invest organic carbon into the soil. Soils are among the largest carbon reservoirs on land, storing over 1,500 petagrams of organic carbon, which is more than twice the amount currently held in the atmosphere (Scharlemann et al., 2014). This organic carbon plays a vital role in maintaining soil health. It contributes to plant nutrition, improves water retention and quality, strengthens soil structure, and reduces erosion. But in lieu of an ATM to deposit tonnes of carbon belowground, how do we add carbon to the soil and how can we safeguard it from future threats? 

Making a deposit: the sequestration of carbon into soils 

Soil carbon sequestration is the process of capturing atmospheric carbon dioxide (CO₂) and storing it in the soil. This begins with photosynthesis, where plants absorb CO₂ to build biomass. When plants die and decompose, their organic matter is incorporated into the soil and further broken down by microbial communities. Approximately half of the organic compounds in soil organic matter consist of organic carbon, which becomes stabilized through physical aggregation and chemical interactions with minerals, making it more resistant to decomposition.

Soil carbon sequestration plays a crucial role in climate mitigation while offering numerous additional benefits. We can think of these as the ‘interest generated’ by investing carbon into the soil in the first place. By improving soil structure, soil carbon enhances fertility, boosts water retention, increases agricultural yields, and strengthens resilience against climate change. It also supports essential ecosystem services like water filtration and nutrient cycling, both of which are vital for sustainable land management.

Over the past decade, national and international policies have increasingly supported efforts to enhance soil organic carbon storage. This momentum has also driven a surge of research within soil science, particularly in optimizing soil carbon sequestration (Sykes et al., 2019). From financial managers, we are often advised to diversify our investments to mitigate risks; in other words, not to ‘put all the eggs into one basket’. This is also the case for building reserves of soil carbon. Our research has showcased a suite of different ways of adding carbon to soils. 

For example, conservation tillage, which minimizes ploughing and retains crop residues on the soil surface, helps to reduce soil erosion and preserve organic carbon. Cover cropping, practiced during fallow periods, increases carbon inputs through root biomass and organic matter, further strengthening soil structure and nutrient cycling. Similarly, crop rotation and diversification – especially with nitrogen-fixing plants like legumes – contribute to carbon sequestration while improving soil health.

Agroforestry, the integration of trees and shrubs into agricultural landscapes, enhances carbon storage by contributing both above-ground and below-ground biomass. Beyond sequestration, agroforestry also promotes biodiversity conservation and soil quality. Additionally, organic amendments such as compost, manure, and biochar enrich soil organic matter, increasing carbon inputs and improving fertility and water retention.

Protecting reserves from future shocks: how resilient is soil carbon? 

Just like Fort Knox, the soil has a finite cubic capacity. The most limiting aspect dictating this capacity is soil thickness. By ‘soil thickness’, I mean the distance between the ground surface and the parent material from which soils form. (In many cases, this parent material is the underlying bedrock). However, unlike the floors and ceilings of Fort Knox, soil thickness is not static.

Human-induced soil erosion is a major threat to soil thickness, often outpacing the natural generation of new soils. If this imbalance persists, it can lead to soil thinning and, ultimately, the loss of entire soil profiles. A global analysis of soil erosion data reveals that over 90% of conventionally managed agricultural soils – those that do not incorporate erosion mitigation strategies – are thinning, with many projected to last less than a century (Evans et al., 2020). As soils become thinner, their ability to sequester and store organic carbon declines, weakening their role in climate regulation.

Beyond erosion, climate change introduces additional risks to soil carbon storage. Rising temperatures accelerate the mineralization of organic carbon, releasing carbon back into the atmosphere as CO₂. Extreme weather events, including droughts and floods, can further destabilize soil carbon dynamics, disrupting the balance between carbon sequestration and release. Shifting precipitation patterns also affect plant growth and root systems, influencing the amount of organic carbon entering the soil in the first place.

As climate change intensifies, these stressors highlight the urgent need for adaptive land management strategies that enhance soil resilience and promote carbon retention (Jansson and Hofmockel, 2020). Addressing these challenges presents a key opportunity for transdisciplinary collaboration, integrating soil science, biogeoscience, climate science, and hydrology to develop innovative solutions for sustainable soil management.

Shallow soil overlying bedrock in pine forest. (SOURCE: Antonio Jordán, distributed via imaggeo.egu.eu).

Digging deeper: carbon transfers to subsoils and parent materials

It is commonly assumed that the uppermost six inches of soil are the most influenced by plant roots and land management practices. For this reason, the topsoil has been imagined, in many ways, like a ‘current account’ for carbon – the zone where frequent deposits (e.g. decomposing biomass) and withdrawals (e.g. CO₂ emissions) take place. As a result, soil scientists have traditionally concentrated their research on this topsoil horizon. A recent review of studies examining soil management strategies for increasing carbon storage found that the median depth studied was just 30 cm (Minasny et al., 2017). This focus is surprising, given that a significant portion – between 45% and 60% – of soil carbon is stored below this depth.

Research has demonstrated that the organic carbon found below one metre is actively cycled, emphasizing that deeper carbon pools are just as important as those near the surface. As a result, there has been considerable attention on how the carbon is transferred to these depths in the first place. Soil scientists have shown that some carbon dissolves in water and is transferred via infiltration processes down the soil. Others have examined how plant roots, growing into the subsoil, secrete organic compounds in a process known as rhizodeposition. 

There has been a growing effort to understand the persistence of organic carbon in deeper soil layers. These layers not only have the capacity to store larger amounts of carbon, but the carbon they contain is also more likely to remain stable over long periods. Organic carbon can become chemically bound to mineral surfaces or physically protected within soil aggregates, reducing its susceptibility to decomposition. In deeper zones where microbial activity is minimal, this carbon can remain preserved for decades or even centuries, highlighting the importance of subsoil processes for long-term carbon storage.

Furthermore, emerging studies are revealing how soil parent materials also play a role in carbon storage (Tye et al., 2022). My own research programme is investigating how organic carbon moves through the soil profile and becomes stored in weathered bedrock – a zone traditionally studied by geologists. Our findings suggest that a sizeable proportion of a soil’s carbon account is stored and managed in this weathered bedrock layer (Evans, 2025). This transition between intact rock and the soil profile presents an exciting space for us to develop new strategies for long-term carbon storage.

Hidden costs: the unbudgeted release of rock-derived carbon to soils

Much of the empirical research on soil carbon has been utilized by soil scientists to design models that are ultimately used to generate carbon budgets. Essentially, these use data about the inputs and outputs of carbon to forecast whether a soil is, or could become, a net carbon sink or source. An important aspect of this number-crunching work is to ensure that the data feeding the model are as complete as possible. 

However, ensuring that all the inputs and outputs are accounted for is easier said than done. One of the overlooked carbon inputs to soils is that which is ‘inherited’ from underlying rock during bedrock weathering and soil formation. One of my research foci is to study the petrogenic organic carbon (rock-derived carbon, or OCpetro) which originates from some carbon-rich bedrock such as shale. 

The story of how shale becomes carbon-rich in the first place is one that starts millions of years ago, in marine or lake environments, where plankton and microorganisms absorb CO₂. When they die, their remains settle to the floor and mix with fine mud and sand, transforming under heat and pressure into a waxy organic material called kerogen. Layer upon layer, these materials get compressed and ‘lithified’ (i.e. turned into rock, like shale). Tectonic forces slowly heave these lithified sediments into mountain ranges, where they become exposed to weathering (Hilton and West, 2020). Despite emerging work on OCpetro, no current soil carbon model fully accounts for the transfer of OCpetro from rock into soil. This represents a significant gap in our understanding of carbon dynamics across the bedrock-soil continuum.

Once the OCpetro has been released as a carbon input into the soil profile, it becomes more accessible to microbial activity and, therefore, more susceptible to being respired as CO₂.  This loss of OCpetro as CO₂ is currently absent from carbon models, despite recent studies indicating that OCpetro emissions (68 million tons of carbon per year) may equal or even exceed the global CO₂ uptake from silicate weathering (47–72 million tons of carbon per year) (Zondervan et al., 2023). 

Excluding OCpetro from soil carbon models introduces several challenges. One of these relates to how we manage soils that overlie carbon-rich rocks like shale. Strategies aimed at increasing soil carbon sequestration – such as promoting deeper-rooted plants to enhance belowground carbon deposits – may also deepen the distribution of microbial communities. Ordinarily, this is no bad thing: the soil microbiome is fundamental for the health and functioning of the soil system. However, as microbial communities dig deeper into the soil profile, it may enable them to access and feed on the OCpetro at the soil-bedrock interface. This represents a potential hidden (and, as yet, unquantified) cost of sequestering soil carbon deeper in soil profiles overlying stocks of OCpetro.  

Measurements of CO₂ from OCpetro taken in front of a retreating glacier at La Fouly, Switzerland (Photo credit: Dan Evans). 

Going forwards: the importance of sustained and holistic soil carbon research 

If soils weren’t already critical enough, their ability to sequester and store carbon makes them an essential component of any greenhouse gas removal (GGR) strategy. Yet, as is often the case in scientific research, each discovery brings more questions than answers. How resilient is soil carbon in the face of a rapidly changing climate? What hidden reservoirs of carbon lie beneath the surface, and how can we ensure they remain secure? How do carbon inputs from underlying bedrock influence soil carbon dynamics? These questions challenge us to rethink how we manage and interact with the ground beneath our feet.

References

Evans, D. L., Quinton, J. N., Davies, J. A. C., Zhao, J. and Govers, G. (2020) ‘Soil lifespans and how they can be extended by land use and management change’, Environmental Research Letters, 15(9), doi: 10.1088/1748-9326/aba2fd.  

Evans, D. L. (2025) ‘Organic carbon stocks in weathered bedrock—Establishing the soil parent material as a new horizon in soil carbon research’, Vadose Zone Journal, 24(2), doi: 10.1002/vzj2.70007.

Hilton, R. G. and West, A. J. (2020) ‘Mountains, erosion and the carbon cycle’, Nature Reviews Earth and Environment, 1, doi: 10.1038/s43017-020-0058-6. 

Jansson, J. K. and Hofmockel, K. S. (2020) ‘Soil microbiomes and climate change’, Nature Reviews Microbiology, 18, doi: 10.1038/s41579-019-0265-7

Minasny, B., Malone, B. P., McBratney, A. B., Angers, D. A., Arrouays, D., Chambers, A., Chaplot, V., Chen, Z. S., Cheng, K., Das, B. S., Field, D. J., Gimona, A., Hedley, C. B., Hong, S. Y., Mandal, B., Marchant, B. P., Martin, M., McConkey, B. G., Mulder, V. L., O’Rourke, S., Richer-de-Forges, A. C., Odeh, I., Padarian, J., Paustian, K., Pan, G., Poggio, L., Savin, I., Stolbovoy, V., Stockmann, U., Sulaeman, Y., Tsui, C. C., Vågen, T., can Wesemael, B. and Winowiecki, L. (2017) ‘Soil carbon 4 per mille’, Geoderma, 292, doi: 10.1016/j.geoderma.2017.01.002. 

Scharlemann, J. P. W., Tanner, E. V. J., Hiederer, R. and Kapos, V. (2014) ‘Global soil carbon: understanding and managing the largest terrestrial carbon pool’, Carbon Management, 5(1), doi: 10.4155/cmt.13.77. 

Sykes, A.J., Macleod, M., Eory, V., Rees, R. M., Payen, F., Myrgiotis, V., Williams, M., Sohi, S., Hillier, J., Moran, D., Manning, D. A. C., Goglio, P., Seghetta, M., Williams, A., Harris, J., Dondini, M., Walton, J., House, J. and Smith, P. (2019) ‘Characterising the biophysical, economic and social impacts of soil carbon sequestration as a greenhouse gas removal technology’, Global Change Biology, 26(3), doi: 10.1111/gcb.14844. 

Tye, A. M., Evans, D. L., Lee, J. R. and Robinson, D. A. (2022) ‘The role of post UK-LGM erosion processes in the long-term storage of buried organic C across Great Britain – A ‘first order’ assessment’, Earth-Science Reviews, 232, doi: 10.1016/j.earscirev.2022.104126

Zondervan, J. R., Hilton, R. G., Dellinger, M., Clubb, F. J., Roylands, T. and Ogrič, M. (2023) ‘Rock organic carbon oxidation CO₂ release offsets silicate weathering sink’, Nature, 623, doi: 10.1038/s41586-023-06581-9.

Author

Dr Dan Evans is a Lecturer in Soil Formation at Cranfield University. He is the Course Director for the UK’s first Level 7 Soil Scientist Apprenticeship and MSc in Soil Science. Arriving at Cranfield with a 75th Anniversary Fellowship, he leads research on soil formation and the parent materials from which soil is formed. Dan is the Early Career Scientist representative for the European Geoscience Union, and the Chair of the ECR Editorial Board for the European Journal of Soil Science. In 2024, his research was internationally recognized with the Arne Richter Award for Outstanding Early Career Science. Dan's also passionate about science communication. He was the inaugural speaker at the first Royal Holloway TEDx conference, and has spoken at the Royal Geographical Society, the UK Climate Emergency network, and the Science Futures stage at Glastonbury Festival.