Soils are the largest terrestrial pool of organic carbon on the planet, holding as much as 3,000 Gt of carbon in the upper 2 meters of soil. To put that into perspective, the total amount of carbon in the atmosphere is currently about 870 Gt and annual CO2-C emissions from fossil fuels equal about 9.7 Gt per year. Furthermore, it is estimated that human activity such as converting land from native vegetation to grazing and cropland, has transferred 133 Gt of carbon from the upper 2 m of soil to the atmosphere. Sequestering carbon in the soil is the largest potential carbon sink, however…
… this is about more than climate change!
The soil beneath our feet provides innumerable ecosystem services that directly affect our lives. They are a warehouse, storing the critical plant nutrients needed to grow the food we eat and the forests we enjoy. They manage hydrological cycles, regulating drainage, flow and storage of water; playing a critical role in the recharge of groundwater. They support biodiversity by providing a habitat for and supporting the growth of a variety of plants, animals and microbes. They protect water and air quality by filtering toxic compounds and excess nutrients. They provide physical stability and support for anchoring plants, withstanding erosive forces, allowing the passage of air and water and serving as the foundation for human structures.
Soil organic carbon (SOC), which is the fraction of soil carbon derived from historical vegetative cover and biological activity, is critical to soils ability to provide these ecosystem services. Physically, SOC acts as a glue, binding soil particles together for the best structure to retain water and resist compaction. Chemically, SOC retains critical nutrients and makes them available to plants. Biologically, SOC sustains soil microorganisms critical to nutrient cycling and protecting roots from diseases and parasites.
Currently, land managers can receive carbon offsets for not converting grasslands and woodlands under native vegetation into cropland, preventing potential future carbon emissions. However, there are many ‘carbon negative’ agricultural management practices that also have the potential to sequester atmospheric carbon (see table below). Detailed analysis of these land management strategies suggest that any one of these methods could sequester between 0.1 and 1 ton C ha-1 yr-1. At the farm level, this may seem like an insignificant amount of carbon. Taken globally, however, there is the potential to sequester as much as 8 Gt of carbon per year in agricultural lands, enough to offset as much as 80% of fossil fuel emissions each year.
Using soils as a sink for atmospheric carbon will require a dramatic shift in how we view and manage the world’s soils. The process of terrestrial sequestration is a slow, long term proposition. Stimulating the national or global action required to achieve the sequestration outlined above is a challenge that will require the coordination of policy makers, institutions, farmers groups and carbon emitters.
Providing the incentive to increase adoption of carbon negative management practices can follow multiple tracks. One is a policy of subsidies to reward those farmers who adopt carbon negative practices. However, the government must update its subsidies program based on innovations in new sequestration strategies. Historically, the government cannot successfully pick winners in a space with lots of innovation.
Another option is a “Carbon Tax”: make carbon emitters pay a tax for each ton of carbon produced. While simple and efficient, a carbon tax is a political nonstarter. In either of these cases, the government effectively sets the price for carbon sequestration and bears the burden of additional administrative costs to manage subsidy or taxation systems.
As such, carbon markets have become political and economic middle ground. They provide financial incentives for land managers who sequester carbon in soils, are “market based”, and are relatively flexible to changing technology. Carbon markets include ‘compliance markets’ and ‘voluntary markets.’ Compliance markets are formal cap-and-trade markets where government agencies decide what carbon offsets are allowed, and they often set the carbon pricing as well. Similar to subsidies and taxation above, formal cap-and-trade markets place a large administrative burden on public institutions. Conversely, voluntary markets generally adhere to the standards developed by a number of voluntary standard-setting bodies.
Any attempt to develop meaningful subsidies and taxation policies would require significant buy-in from politicians in Washington D.C., something that does not seem possible under the current political climate. Likewise, the development of national level compliance markets within the USA would appear to be years away at best, although some regional compliance markets do exist within the USA. This leaves voluntary markets as the most promising entry point for carbon negative land management to provide carbon offsets.
Voluntary carbon markets are much smaller than compliance markets, trading $278 million worth of carbon compared to over $50 billion on compliance markets.4,5 Likewise, the volume of carbon traded is much less than on compliance markets, 84 Mt compared to 6 Gt in 2015.
Despite their smaller size, voluntary markets can be a good space to test carbon negative farming offsets. Since they are governed by voluntary standard bodies, and not by government agencies, they are more willing to develop and test new pathways. Purchasers of carbon offsets in these markets tend to be large companies looking to reduce their carbon footprint and prepared for future carbon regulations. These companies tend to be forward thinking and willing to experiment.
Developing pathways for carbon offsets via soil sequestration will require a concerted effort to overcome technical challenges. These include the lack of existing pathways to soil carbon sequestration credits, the difficulty of model based SOC predictions and the lack of affordable methods for accurately measuring SOC.
There are no accepted pathways for sequestering new soil carbon
Under the current regulatory framework guiding both compliance and voluntary markets, there are no accepted pathways to sequester ‘new’ carbon in soil. There is a pathway to prevent the release of existing SOC into the atmosphere by stopping the conversion of native grassland or forest into agricultural production. While this is an important step in reducing greenhouse gas emissions, it does not lead to new, carbon negative innovations nor is its potential impact very high. The amount of US land that could benefit from this type of carbon credit is very low with only 400,000 acres of US grasslands and woodlands being converted to agriculture in 2011/12.
There are two critical factors which prevent the establishment of soil carbon sequestration pathways. First, there are concerns about the potential reversibility of carbon sequestered in soils. For example, a manager builds up soil carbon using no-till management, sells carbon offsets based on that SOC build-up, and then plows his land, releasing a portion of that carbon back into the atmosphere. This is an issue that can be resolved by maintaining financial incentives. The second factor is that the process of verifying soil carbon levels, either through predictive models or direct soil measurements, is very expensive, leading to very high transaction costs. Without technologies to overcome this challenge, the transactions costs of soil carbon offsets would be prohibitively expensive.
The Problems with Model Based Verification
Soil organic carbon dynamics at the landscape level are very complex, with spatial variability causing significant verification challenges. The current approach to overcome this challenge is with ever-increasingly complex SOC models. Often, these models require a combination of direct measurements of SOC, peer-reviewed and project specific parameterization, and frequent auditing to develop accurate predictions. The result is an overly complex methodology, such as the 80 page handbook for verifying carbon credits from not converting grass and woodlands to agricultural lands. This approach leads to high transaction costs which, when combined with the relatively low carbon price, fails to provide incentive for land managers and does not support an innovate atmosphere for new sequestration methods.
Direct Measurement of Soil Organic C
Directly measuring SOC stocks currently requires laboratory-based methods, such as gas chromatography and elemental analysis. While they are highly accurate, they are also time consuming and expensive. High costs limit both the frequency and area of sampling, making it harder to quantify stocks of SOC on a farm or ranch scale. As a result, current soil inventory methods lack the spatial and temporal resolution needed to accurately quantify SOC stocks across large scales and to adequately detect change over time. Some companies and academics are attempting to build libraries to address this variation, their efforts are usually not public or not collaborative by nature.
We are building open software and hardware so anyone, anywhere can predict soil organic carbon.