Research roundup: Is cropland ag carbon sequestration as effective as we want it to be?

by Trevor Russell
Cover crops and no-till farming may not be as effective at sequestering carbon as we hoped. (Photo by Colette Kessler, USDA-NRCS, CC BY-SA 2.0 )
 

Storing greenhouse gases in cropland soils is all the rage these days, as fortune 500 companies, conservation initiatives and even the Biden administration are pursuing new opportunities to reward farm cropland operations for sequestering carbon beneath our feet.

At FMR, we’ve long been excited about these opportunities, as both a climate mitigation solution, climate adaptation opportunity, and as an innovative way to improve agricultural conservation and reduce farm runoff pollution. 

But only if the science backs it up.

After all, getting this ‘right’ is vital for the health of the Mississippi River, in part because of the link between climate and water quality. Unmitigated climate change is expected to supercharge runoff pollution in Minnesota by an additional 24% this century — an increase that would make achieving our state’s clean water goals virtually impossible.

As Minnesota’s recent climate change report shows, we’re not meeting our climate goals and not making progress on agriculture climate mitigation. But are the offered solutions actually effective?

A key question: What really works?

Our enthusiasm for cropland climate sequestration shouldn't cause us to look past new information that doesn’t align with our hopes for the future of farming. As new climate science emerges, we're committed to reviewing research findings and asking ourselves (and our peers) an important question: What agricultural climate mitigation solutions are real, and what solutions do we “want” to be real (even if the science doesn’t yet fully support them)?

A raft of recent articles and research suggests the enthusiasm for some agricultural carbon sequestration strategies might be getting ahead of the science. While these finds are disappointing they're certainly worth closer examination.   

Time to ‘tap the brakes’?

A handful of recent publications strongly urge a more cautionary approach to agricultural carbon sequestration proposals, highlighting the scientific and logistical uncertainties we face:

These articles are a small sample of an emerging conversation within the conservation and climate community.

While we know some strategies do provide some carbon sequestration benefits (land retirement, silviculture*, wetland restoration, permanent perennial vegetation, managed rotational grazing), other proposed strategies are far less certain. This includes two flagship soil health practices often held as climate solutions: no-till and cover crops.

Limits to no-till and cover crop carbon sequestration

When deployed on Midwestern row-crop fields, these are great tools to improve water quality and overall soil health. However, there are valid concerns that these tools may fail to result in meaningful long-term carbon sequestration. As we noted in a recent blog post, there are three lines of uncertainty with regard to the potential of row-crop systems (like corn and soybeans) to sequester carbon in the soil for any length of time.

Two of these uncertainties are deeply scientific, the other logistical.

(1) Carbon stratification: Most studies look at the so-called “plow layer” — the top-most layer of cropland soils which tend to show increased carbon in response to standard conservation strategies like cover cropping and no-till farming. But what if we look deeper? 

Renewed attention is being paid to the possibility that these practices may simply redistribute carbon in the soil, rather than truly store it from the atmosphere. Some examples include:

  • University of Minnesota research that looked deeper into the soil profile found that roughly an equal amount of carbon disappeared in the layers below 30 centimeters.
  • UC Davis research likewise found that cover cropping conventional soils can increase carbon in the “plow layer” while losing significant amounts of carbon below that depth.

(2) Soil microbiology: Soil is alive with microbial life that is constantly at work. While soil carbon storage can vary greatly between soil types, recent research indicates that some standard soil health and nutrient management practices may have an unintended effect. Healthy soils enhance soil microbial activity, which in turn could release much of the crops’ deposited carbon back into the atmosphere. It's too soon to be sure, but some examples include: 

(3) Tillage impermanence: Even if the first two issues above could be resolved, we often fail to address the elephant in the room: Tillage practices are not permanent.

Even if no-till practices do sequester additional carbon over time, virtually any subsequent tillage will quickly give it all right back to the atmosphere. Absent a permanent tillage easement (which doesn’t exist, and which would be virtually impossible to enforce), we can’t honestly consider no-till practices to be permanent.

This is especially relevant as land ownership and management practices change over years and generations, so we can't be certain that future farm operators would continue no-till practices. In fact, since the average Minnesota farm operator is almost 59 years old, it’s certain that farming practices — and farmland ownership itself — will change over time. Will tillage practices change with it?

As a result, even modest cropland carbon sequestration may not amount to much more than a short-term carbon rental. 

Soil nitrogen and fugitive methane emissions.

Alongside the uncertainty described above, there is also a pair of potentially significant knowledge gaps regarding nitrogen fertilizer and soil nitrogen cycles.

These interrelated issues are both important (given the relative potency of both methane and nitrous oxide as greenhouse gases) and subject to an at-times bewildering level of complexity and uncertainty. 

What this means for cropland climate solutions

This sort of choose-your-own-adventure suite of divergent scientific conclusions makes it difficult to advance meaningful climate policy with any degree of certainty. So caution is warranted. 

As new research emerges, it's reasonable to conclude that cover crops and no-till are certainly not yet a home run from a carbon sequestration perspective. Despite the many benefits that these practices offer for water quality, flood resilience and wildlife habitat, we must acknowledge that greenhouse gas storage in annual row-crop soils may be more limited and temporary than initially hoped.

Much more research is required to better understand the complexities of soil dynamics, nitrogen cycling and permanence before taxpayer funds and public policy are directed toward agricultural interventions that may not yield long-term climate mitigation benefits.

A few next steps

As state and federal leaders look toward investing in agricultural climate solutions, FMR is taking action to better explore these uncertainties and urge a science-first approach to agricultural carbon sequestration.

This includes:

While the journey toward agricultural carbon sequestration is sure to be long and challenging, we are confident that if we work together and stick to the science, we can eventually find the right solutions that protect our environment while enhancing the prosperity of Minnesota’s farm operations.

You can help

One great way to help us advance this work is to sign up as a River Guardian. We'll email you when there's a chance to act quickly online for the river on issues like this. Plus, you'll be invited to special events like educational happy hours (online for now and in person…eventually).

Read more from the water blog.
 

(*): For the purposes of this article, we're not focusing on lag-time concerns related to some practices such as tree planting that take time to mature before achieving modeled sequestration potential. However, whenever trading/offset proposals are considered, it is vital to consider lag-time (along with permanence and additionality-over-baseline-conditions) in program design.