Keeping carbon in the ground
Featured image: No-till cropping uses specialized equipment to sow seeds directly into un-tilled soil to minimize soil disturbance and encourage carbon gain. (Photo credit: Wikimedia Commons).
Featured article: Walia, M.K., S.G. Baer, R. Krausz and R.L. Cook. 2017. Deep soil carbon after 44 years of tillage and fertilizer management in southern Illinois compared to forest and restored prairie soils. Journal of Soil and Water Conservation 72(4): 405–415. doi: 10.2489/jswc.72.4.405
The geometric rows of corn and soybeans you might see driving through farm country don’t look anything like the industrial smokestacks we often associate with the buzz-wordy phrase “climate change”. But under the yearly beat of the plow, much of the world’s farmland has been acting a lot like those offending smokestacks, invisibly releasing the same greenhouse gases through loss of carbon stored in the soil. Some recent research with the University of Southern Illinois by Dr. Maninder Walia and her colleagues shows that changing tillage practices can help stop these emissions and reverse soil carbon losses, turning working farmland back into a sponge for carbon and restoring it to conditions more like untouched wildland.
A carbon sponge, squeezed
Until the arrival of mechanized agriculture, much of the American Midwest was a sea of grass dotted with islands of forest. Every year the growing plants patiently drew carbon dioxide out of the air via photosynthesis to build their tissues, a fraction of which was left behind in the soil as decaying organic matter. This organic matter acted not only as a sponge for carbon, but also helped store nutrients and gave the region its characteristic black topsoil. Annual plowing served to remove weeds and boost short-term fertility, but in the decades since its arrival has also caused a great deal of farmland topsoil to be lost to erosion (e.g. The Dust Bowl). Less visibly, by breaking up soil and exposing it to air and warmth all that tilling has encouraged the breakdown by soil microbes of its stored organic matter. Once consumed, the carbon stored in soil organic matter is released back to the air as CO2 – the main climate-changing greenhouse gas.
As a result of this slow carbon loss, conversion of land to agricultural uses worldwide is a significant ongoing source of humanity’s greenhouse gas emissions. More recently developed conservation techniques like no-till can minimize the soil disturbance that leads to carbon loss by planting crops directly into un-tilled soil and leaving dead crop litter in place. However, the rate at which no-till soils accumulate carbon is slow, reversible, and difficult to measure. As a result, scientists are still not clear about how quickly soil might be made to recover its lost carbon, or how much it might be able to ultimately store after switching to no-till farming.
“Surprisingly, these fully fertilized no-till plots were also able to reach levels of carbon storage similar to nearby forests across the entire 1 m-deep soil profile.”
Since 1970, the University of Southern Illinois has operated an agricultural research station outside of St. Louis, Missouri. At the research station are a set of corn- and soybeans plots that every year for the past 44 years have repeated the same set of crop cultivation practices in the same place – including some plots grown under permanent no-till. This patient field trial has yielded a wealth of unique data that Walia and her colleagues have used to answer questions about the long-term potential for no-till and other management practices to reverse soil carbon losses.
In their recent paper, Walia and her co-authors compared carbon storage in soil under permanent no-till to soil beneath nearby forested area and a patch of restored prairie. By taking soil samples from as deep as 1 m below the surface and analyzing for carbon content, the researchers found that over the 44-year trial period soil kept under no-till stored up to 18% (6.4 metric tonnes per hectare) more carbon on average than soil under more intensive tillage types. Maintaining adequate fertilizer addition was also critical, and the highest carbon storage was found in no-till soil receiving a complete fertilizer regimen. Surprisingly, these fully fertilized no-till plots were also able to reach levels of carbon storage similar to nearby forests across the entire 1 m-deep soil profile. Carbon storage in these plots even exceeded that in restored prairie plots (though the authors note that the restorations, installed in 2006, might not have had time to accumulate their maximum storage capacity). Overall, carbon gain in the fertilized no-till plots was higher than any other treatment, absorbing an average annual net of 0.36 metric tonnes of carbon per hectare from the atmosphere since 1970. The results of this study imply that if no-till was universally applied to the roughly 72 million combined hectares of soybeans and corn planted in the U.S. in 2018, the carbon uptake each year could be equivalent to some 95 million tonnes of CO2 taken from the air each year – a small but measurable 1.8% bite out of the country’s total emissions.
The findings of this research offer hope that by changing crop production practices farmers may be able to both restore soil health and draw CO2 out of the atmosphere in the process. Policies, practical advice, and incentives enabling farmers to switch to no- or reduced-tillage practices could help turn agricultural soils into an effective carbon sink, at least over the medium-term. The work by Walia and colleagues adds a rare and important long-term experimental data point to our understanding of how we might grow food on our finite planet, while working to safely lock away climate-changing greenhouse gases.