The potential of US croplands to provide an effective carbon sink may be much less than was previously thought, analysis by University of Montana scientists suggests.
The work found that by defining what they describe as a “more appropriate land base from cover cropping for carbon sequestration purposes,” the amount of carbon that could potentially be sequestered would be about a fifth of what had been estimated before, representing 32% of US cropland.
Managing climate resources
Realistic assessments of soil carbon potential are essential to truly understand the contribution of the land-based climate change mitigation strategy, so as to effectively allocate resources, Nazli Uludere Aragon, who led the work, wrote in the journal Earth’s Future.
As such, the team’s calculations are based on only considering stable croplands (croplands remaining croplands) in annual production, such as corn and soybean, that can employ cover cropping without using irrigation, and so avoid excessive water use. They found that 62% of the potential under the terms of the study would be found in the US’s highly productive Heartland region, and 91% where the top crop is either corn or soy.
The researchers also applied more conservative estimates of soil carbon sequestration rates from cover cropping to account for how soil carbon measures work over large areas.
More reliable effects — but pricey
This “constrained land base” would make carbon sequestration more durable, restricting subsequent losses through land-use changes needed to qualify for specific carbon incentives. However, the new calculations also suggested that this robust carbon storage is likely to cost more than was previously thought, with $100 per metric tonne of carbon dioxide equivalent needed to achieve half of the revised potential. As such, financial incentives would be needed to bring about such large-scale cover cropping in the US.
“Our economic analyses suggests that without financial incentives, implementing cover cropping could be expensive for most U.S. farmers,” warned Aragon. “This high cost hurdle needs to be evaluated against the cost‐effectiveness of alternatives that compete for the same limited resources for climate mitigation.”
Contextualising carbon farming’s benefits
Many claims around carbon farming are based on performance under ideal conditions, and while the findings may serve to take the edge off some of the more optimistic claims around carbon farming in the world’s largest economies, they make a valuable contribution to current thinking, noted Aragon.
“Our work points to the importance of assessments that consider benefits and costs for a broader range of conditions to demonstrate how a program or practice would perform at scale and approach its full technical potential. This is complementary to demonstrations of practice efficacy under ideal settings,” she wrote.
The calculations used in the work could be replicated elsewhere, with Aragon describing such an approach as “imperative” for future work on natural climate solutions.
“While our findings call for tempering expectations…the many soil health and ecosystem benefits of cover cropping mean it may be socially desirable to promote and financially support this practice more broadly with possible enhancement of soil organic carbon as a co‐benefit,” she added.
