The value of monitoring soil
Finally, a soil MRV review paper we didn’t have to write.
by Jane Zelikova and Elizabeth Reali
After the widespread climate protests of the past month and hundreds of conversations centered on New York climate week, climate change — as a phenomenon, as a crisis — seems to be center stage. As we address the impacts and potential solutions, food systems are finally getting serious attention. Today, we use more than a third of the planet’s land to grow food, making food production a clear opportunity for both reducing emissions and carbon sequestration.
This is especially true if we focus on agricultural soils. There’s more carbon sitting in the earth’s soils than in the atmosphere or all the plants in the world. But over the last few hundred years, we’ve been losing that carbon as a result of how we grow our food and manage our land. Shifting agricultural practices could help reverse this trend, turning agriculture into a net-sink for atmospheric CO2. The challenge is getting an accurate read on how much carbon soils can actually sequester.
Coming up with that number is not easy, but doing so reliably and consistently is critical for the success of future climate initiatives focused on agriculture. To fully harness the power of agricultural soils to draw down carbon, we must be able to predict with confidence how our management of agricultural, forested, and grazing land will affect future soil carbon content. Building that confidence depends on having trusted monitoring, reporting, and verification, or “MRV” protocols, that are both cheap and accessible enough to be used widely.
As interest in soil carbon grows, so does confusion about what it is and how to measure it. Here we’ll explore some of the opportunities and challenges associated with restoring the carbon we’ve lost and present some key findings from Pete Smith and his colleagues’ recent paper reviewing prominent MRV protocols in use today.
How we think about and measure soil carbon
Though there is renewed interest in soil health and carbon today, the concept of soil “quality” or “health” has been around for a long time. Fifty years ago, “soil health” was shorthand for maximizing crop yields. Over time, our objectives have shifted, along with the methods we use to assess soil health and how we turn those data around into management decisions. Today, the definition of soil health has expanded to consider the role that soils can play in supporting ecosystem services and mitigating climate change.
One of the greatest revolutions in how we think about soil came from under a microscope. Microbes, the tiny organisms that make up the bulk of life in soils, are the movers and shakers of carbon sequestration — they transform organic matter into soil organic carbon (“SOC”) and other nutrients and, in doing so, drive nutrient-water dynamics, carbon sequestration, and ultimately crop productivity.
While rapid innovation in molecular and computational tools has given us a peek into how microbes influence soil health, that information is not fully incorporated into today’s soil carbon MRV protocols. A key next frontier is to apply this microbial knowledge to informing sustainable soil management and building models that better represent the fundamental roles microbes play in sequestering carbon in soils, both today and under future climates.
A number of other factors influence soil carbon stocks, including plant inputs, land use and management activities, climate, and soil types. Whether in field experiments, using chronosequences as proxies for time, or in established monitoring networks, we need to be able to measure or estimate SOC change.
Pete Smith and his colleagues summarize common approaches to soil carbon quantification and identify some immediate opportunities to improve how we measure and track soil SOC, detailed below.
A lot comes down to taking a good soil sample — even remote sensing and modeling approaches rely on collecting physical samples. Good sampling design calls for samples that reflect the unique attributes of a particular site, including the soil type, land use, vegetation, and climate present.
Today, there’s no universal standard for doing this. Approaches to sampling and analysis differ by country and platform, making it a challenge to draw comparisons and manage our global soil resource coherently.
Table 4 from the Smith et al. paper summarizes the models commonly used to estimate greenhouse gas emissions and potential for carbon (“C”) sequestration in agricultural soils, but both the models and where they are applied are limited in geographic and scientific scope.
The challenges ahead
Methods
Though the world is finally turning overdue attention to carbon removal and the untapped potential for carbon sequestration in agricultural soils, the methods that underpin our ability to do so reliably and cost-effectively are lagging behind. We need tools that allow us to measure SOC non-destructively (as in, without having to dig a hole) and model how different agricultural practices affect soil carbon. The dream would be to have the ability to scan a sample in Montana, then instantly add it to a global library of soil samples, where it can be referenced across a database to help improve future predictions of soil SOC in similar contexts.
Geographic bias
In order to harness the power of soils to draw down atmospheric carbon, we need to expand soil carbon monitoring across the globe. Today, many regions lack both long-term monitoring and infrastructure in order to adequately quantify and track soil carbon stocks changes across time. And long-term experiments are also limited in geographic scope, restricting the broad applicability of the results to mostly temperate ecosystems. Ramping up capacity and expanding soil monitoring across the globe is critical.
Variation
As soil scientist Tony Hartshorn (Montana State University) likes to say, soils are sneaky — you can walk a few meters and find yourself on a different soil type, with inherently different properties and soil carbon sequestration capacities. That means we need to collect soil samples from a lot of spots to get a better sense of how soil carbon inherently varies and how different management approaches affect it.
And we need to do these assessments frequently enough that we capture the “good” and “bad” years, so farmers and ranchers are not penalized for reduced carbon sequestration rates during a drought year, for example.
Impatience
Soil C gains can be slow and small enough from one year to the next to be undetectable. That means we need to think about MRV protocols that take the speed of carbon accrual into consideration, and payment schemes that support farmers through the 3–5 years it takes for soil carbon to accumulate at measurable rates.
Diversity of stakeholders
Many groups rely on soil MRV today, and many more groups will need to in the future as natural climate solutions become more mainstream. The groups that need to accurately measure and track soil carbon include:
- Farmers and ranchers who can make management decisions to optimize soil carbon sequestration
- Businesses that rely on agricultural products and have made climate commitments, including building soil health and maximizing soil carbon sequestration
- Government entities (at the local, regional, national, and international scales) that have expressed greenhouse emission reductions goals that include carbon drawdown by plants and soil or that regulate emissions
- Scientists who are improving modeling approaches and developing new tools for soil carbon measurement and verification
- Policymakers developing incentive structures that rely on trusted and accurate measurement of soil carbon
- Philanthropies that are building strategies around carbon removal and need to understand both the challenges and opportunities of soil carbon sequestration.
So, what comes next?
We have a long way to go to address the challenges in front of us and make the most of soil carbon sequestration’s potential. Luckily, we’re not starting from scratch. We know how to measure soil organic carbon, we have modeling platforms that are getting better and better every day, and we have a growing community of farmers and ranches who are excited to test out new agricultural practices and track soil changes.
In their paper, Pete Smith and his colleagues are calling for a global soil MRV platform — one that includes benchmark demonstration sites on representative soils across the globe, in conjunction with models that can take the data from benchmark sites and project changes in soil carbon into the future. A global soil monitoring system of this scale will require international cooperation, capacity-building, and technology transfer, often from countries with ample resources to under-resourced countries. It will require collaboration and ambition that matches the scope of the climate change challenge.
Smith et al. also advocate for long-term experiments that can serve as platforms to track soil carbon change, verify models, and provide testbeds for new ways to manage for soil carbon sequestration. Official MRV processes can be time-consuming and costly, so Smith et al. suggest self-reported soil carbon sequestration activities be spot-checked and added into the data pool, both to bolster models and help assess uncertainties in soil carbon measurements or predictions. By lowering the barrier to entry, more folks can participate, add their data, and help scale soil carbon sequestration in agriculture.
As we plan ahead, we should prioritize working towards congruence across metrics of soil health and carbon sequestration that can be applied across a range of geographies, agricultural practices, and jurisdictions. We also need to develop and benchmark tools and metrics that are easy to use and inexpensive.
And we should remember — despite clear opportunities to improve how we measure soils, carbon sequestration in agricultural systems is ready for prime time today. The farming practices are not new, not reliant on inventing a specific technology, and can be implemented on millions of acres across the world. We just need the political will and scientific support to make it real.
