The "known unknowns" in our understanding of soil carbon sequestration as a GGR strategy

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Though the ground beneath our feet may feel reasonably solid, we live on the surface of a giant sponge. Soils are one of the largest storage reservoirs for carbon on land. They contain over 1,500 petagrams (Pg) of organic carbon, which is more than twice the amount currently stored in the atmosphere. This organic carbon is crucial because it facilitates soil functioning which, in turn, provides goods and services for the wider environment, its ecosystems and society. These include providing nutrients for plant growth, improving water availability and quality, helping soil particles to aggregate, and reducing the risk of soil erosion. Recent advances in greenhouse gas removal (GGR) technologies have highlighted the potential of soils to help mitigate climate change by capturing and storing carbon from the atmosphere. This potential is rooted in their ability to not only sequester organic carbon, but to safeguard it from the perturbations and pressures that confront soils globally.

Soil carbon sequestration

Soil carbon sequestration is the process of capturing atmospheric carbon dioxide (CO2) and storing it in the soil. The process begins with photosynthesis, where plants absorb CO2 and use it to form plant biomass. As plants die and decompose, the organic matter from this biomass is added to the soil, where it is further broken down by the soil’s microbial community. Approximately half of the organic compounds within soil organic matter are soil organic carbon (SOC). This SOC is stabilized in the soil through physical aggregation and chemical interactions with minerals, increasing its resistance to decomposition.

Soil carbon sequestration is not by itself a GGR technology. Soils have been sequestering carbon since the origin of Earth’s first land plants, and the accrual of carbon affords many benefits beyond climate mitigation. For example, it helps to improve soil structure, which enhances the fertility and water retention of the soil. These can lead to increased agricultural yields and a bolstered resilience to the impacts of climate change. SOC can also stimulate the provision of other key ecosystem services, such as water filtration and nutrient cycling, both of which are essential for sustainable land management.

As a GGR technique, soil carbon sequestration is receiving increased attention, and much of this is because we are continually learning about ways of accelerating or enhancing the carbon sequestration process. Conservation tillage, which reduces soil disturbance by minimizing ploughing and leaves crop residues on the soil surface, helps to protect soils from erosion. These residues increase soil organic carbon which preserves the soil’s structure; in turn, a more stable soil structure assists with reducing carbon losses from the system. Likewise, cover cropping, the practice of planting cover crops during fallow periods, increases carbon inputs to soils through root biomass and organic matter additions, and this also improves soil structure, reduces soil erosion, and enhances nutrient cycling. Elsewhere, crop rotation and diversification, particularly with legumes and other nitrogen-fixing plants, has similar benefits. Agroforestry, the integration of trees and shrubs into agricultural landscapes, enhances carbon sequestration by contributing both above-ground and below-ground biomass, and agroforestry schemes have also been shown to provide additional benefits such as biodiversity conservation and improved soil quality. Lastly, organic amendments, such as compost, manure, and biochar, supply organic matter to soils, increasing organic carbon inputs and improving soil properties like fertility and water retention.

The "known unknowns" of soil carbon sequestration as a GGR strategy

Over the past decade, legislation and guidance across national and international scales have significantly bolstered efforts to reduce carbon emissions from soil, boost the amount of organic carbon stored in soil, and improve the long-term stability of this carbon. Within the research arena, particularly within the Soil Sciences, this has stimulated research in enhancing soil carbon sequestration as a GGR strategy. As we dig deeper (often quite literally) into this space, we have happened upon some large "known unknowns", many of which can only be addressed with a fresh and transdisciplinary approach. 

1. The resilience of soil carbon in dynamic systems

Though only the surface is most visible and accessible to us, soils are three-dimensional and have a finite thickness. Soil thickness – the distance between the surface and the parent material from which soils form – determines the maximum capacity of soil organic carbon. Yet, soil thickness is not static. 

Globally, about one-third of soils are moderately or highly degraded, with up to 40 petagrams of soil lost to erosion each year. Human-induced soil erosion poses a significant threat to soil sustainability because it often occurs faster than new soils can form. If this imbalance between erosion and soil formation continues unchecked, it can lead to the thinning and eventual removal of the soil profile. A global analysis of soil erosion data indicates that over 90% of soils managed with conventional farming practices (e.g. those which do not implement or integrate erosion mitigation techniques) are thinning, with many having a lifespan of less than a century. As soils thin, their capacity to sequester and store organic carbon diminishes.  

In addition to the changes in soil thickness, a warming climate and recurring extreme weather events that bring droughts and flooding pose significant threats to current levels of soil carbon. Rising temperatures accelerate the mineralisation of organic carbon in the soil, releasing it back to the atmosphere as CO2. These events can disrupt the balance of SOC accrual and its release, making it more challenging to maintain stable levels of soil carbon. Compounding these impacts, changes in precipitation patterns can alter plant growth and root dynamics, affecting the amount of organic matter that is added to the soil in the first place. As climate change continues to intensify, the vulnerability of soil carbon to these stressors underscores the need for adaptive land management practices that enhance soil resilience and promote carbon retention. This is a prime opportunity for transdisciplinary work, integrating the soil and biogeosciences with climate science and hydrologists. 

2. How much carbon is stored within global subsoils? 

It is commonly assumed the uppermost six inches of soil is the layer most affected by plant roots and land management practices. Consequently, soil scientists have traditionally focused their research on this topsoil horizon. A recent review of studies exploring the opportunities for soil management to increase soil carbon found that the median depth studied was 30 cm. This focus is surprising given that a substantial portion (between about 45% and 60%) of soil carbon is stored below 30 cm. 

Two decades ago, it was mistakenly suggested that soil carbon located below one meter was inactive. Since then, studies have demonstrated that organic carbon at these depths is actively cycled, indicating deeper carbon pools are as crucial as those near the surface. Moreover, new interdisciplinary research is shedding light on how soil parent materials (the materials from which soils are formed, such as consolidated bedrock or unconsolidated glacial deposits) can also store carbon. One of my own research focuses on investigating the mechanisms whereby soil organic carbon is transported through the soil profile and stored within the underlying weathered parent material, a zone of interest traditionally reserved for geologists. Yet, as our research has found, soil organic carbon does not respect the neat delineations we use to demarcate different fields of study. We, too, must strive to transcend our own disciplinary boundaries, and the weathered zone between intact rock and the soil profile is a promising space in which to target new soil carbon sequestration and storage strategies. 

3. Unaccounted release of petrogenic (rock-derived) organic carbon to soil systems 

Soil carbon models often overlook the contribution of organic carbon inherited from underlying rock during soil formation. This refers specifically to petrogenic (rock-derived) organic carbon (often referred to as OCpetro) which originates from carbon-rich bedrock such as shale. Although the transport of OCpetro from bedrock to soil has been studied, no existing soil carbon model fully accounts for the contribution of OCpetro, highlighting a significant gap in our understanding of carbon dynamics across the bedrock-soil continuum.

OCpetro located in the bedrock represents a major organic carbon reservoir, and thus a large player in the terrestrial carbon cycle. Whilst past research has mainly focused on the role of silicate mineral weathering in removing CO2 from the atmosphere and stabilizing the Earth's climate, these weathering processes can also mobilize OCpetro into the soil, where it becomes more accessible to microbes, and therefore more at threat of mineralisation. This particular mechanism resulting in CO2 losses to the atmosphere is often missing from carbon models, even though recent studies have shown that the release of CO2 from OCpetro (68 million tons of carbon per year) is comparable to, and possibly greater than, the global CO2 uptake from silicate weathering (47–72 million tons of carbon per year). Despite its importance, this geological CO2 release has not been well quantified.

Failing to account for OCpetro contributions in soil systems can lead to several issues. First, without partitioning soil carbon inputs of the biosphere from those that are petrogenic, we may overestimate the carbon sequestration arising from plant inputs. In addition, ignoring the roles that OCpetro plays in soil carbon cycling means that we also neglect how OCpetro influences subsoil microbial communities and enhances microbial respiration. Moreover, we are yet to fully understand how land management affects the stability of OCpetro in soils. After all, we may choose to deepen the belowground root inputs of carbon through deeper-rooted plants – a much-discussed option of enhancing soil carbon sequestration – but this may also deepen the distribution of soil microbial communities, priming them to assimilate and mineralize the petrogenic organic carbon emerging into soils at the soil-bedrock transition. Combined, these knowledge gaps lessen our ability to create accurate models for SOC accounting and impede the effective design of land-use policies aimed at achieving GGR through soil carbon sequestration. 

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