Black gold: Iowa’s soils can be a Fort Knox for carbon storage


Analysis of multiple studies shows cover crops’ potential in the Midwest


By Dr. Zhichao Wang, P.E., and Jim Ramm, P.E., EcoEngineers


Iowa’s fertile farmland could be a soil-based Fort Knox that — instead of gold — stores a wealth of carbon within its underground vaults.

How farmers can tap into the potential riches of carbon capture and sequestration and how biofuels producers can benefit from forming a partnership with those farmers is the subject of a white paper written by the experts at EcoEngineers, a Des Moines, Iowa-based renewable energy consulting firm whose vision is to build a sustainable economy by monetizing clean carbon.

Using information gleaned from a wide range of academic and governmental sources, the white paper evaluates the impact of cover crops’ ability to increase soil organic carbon (SOC) and reduce the carbon intensity (CI) of biofuels produced from those cover crops.


Iowa Farmland. (Shutterstock)

Although several factors can influence the CI score that is used to measure the carbon footprint of biofuels, the analysis shows clearly that biofuels producers who substitute renewable natural gas (RNG) from the anaerobic digestion of cover crops for petroleum-based natural gas can market their biofuels at higher prices in the low-carbon fuel markets, such as California.

By releasing the paper, EcoEngineers aims to spark more understanding of the benefits of using farmland as a carbon sink and to influence future agricultural policies, so farmers will be rewarded financially for storing carbon in their soils. Until now, Iowa’s land-use policies have viewed cover crops primarily as a component in a nutrient-reduction strategy. Their benefits include promoting soil health and water quality by curbing the runoff of agricultural chemicals and fertilizers. What EcoEngineers hopes to introduce is the idea that there are additional benefits from cover crops that should be recognized and rewarded, such as increasing SOC content.

Anaerobic digestion of cover crop biomass produces biogas, which can be used at Iowa’s ethanol and biodiesel plants (or other manufacturing facilities) as process heat. It can also be upgraded and used as transportation fuel in the form of renewable compressed natural gas (CNG). Upgraded biogas is chemically indistinguishable from fossil natural gas and can be injected into the natural gas pipeline distribution network and exported out of the state.

The white paper concludes that using cover crops to produce renewable energy will result in an increase in SOC and a consequent decrease in the CI or life-cycle greenhouse gas (GHG) emissions for biofuels produced from the cover crop. The resulting low-carbon fuel could be worth tens of millions of dollars a year for an ethanol plant that sells its production into a state, like California, that provides economic incentives for biofuels produced with a lower CI. Farmers who grow cover crops for this purpose should be rewarded for the benefits that they contribute.

Carbon prices in California’s Low Carbon Fuel Standard (LCFS) markets have been around $200 per metric ton throughout most of 2018 and 2019. At this price, a one-point reduction in the CI of an Iowa ethanol plant is worth $.016 per gallon in LCFS credits. Assuming a 17-point reduction from carbon sequestration, a 100-million-gallon-a-year (MGY) ethanol plant selling its ethanol into the California fuel market can potentially earn additional revenues of $27 million a year if it switched all of its petroleum-based natural gas use to biogas from cover crops. Preliminary calculations suggest that 200,000-250,000 acres of farmland would be required to support the production of enough biomass to meet the process heat demands of a 100 MGY ethanol plant. That is an area that covers about half of a typical Iowa county.

Soil Organic Carbon (SOC) Saved by Cover Crops

The potential carbon sequestration that results from planting cover crops varies significantly according to different studies, but there is a general consensus that cover crops can increase carbon stored in the soil and, therefore, act as a carbon sink.

Based on the academic literature reviewed by EcoEngineers, a high, low, and medium rate of carbon sequestration was used to determine how much a cover crop could be worth if it is used to produce clean energy. The lowest SOC sequestration rate was 0.04 metric tons a year, the medium rate of SOC sequestration was 0.13 metric tons a year, and the highest rate was 0.35 metric tons an acre.

field tillage


According to a 12-year study, different tillage methods – no-till, chisel plow, and moldboard plow – have different impacts on cover crop SOC. The SOC increases with the application of cover crops in all three scenarios. SOC gain from cover crops totaled 0.35, 0.2, and 0.04 metric tons per acre per year, respectively, for the three tillage scenarios. Another study estimated that the rate at which cover crops can sequester carbon in agricultural soils is 0.2 metric tons per acre per year.

The white paper cites a study conducted by the Argonne National Laboratory that shows an increase in SOC of 0.05 to 0.07 metric tons per acre per year when conventional tillage practices were performed. However, these values were based on the assumption that cover crops are only planted once every two years in a corn-soybean rotation. If cover crops are planted every year for harvesting as biomass, it can be expected that a corresponding increase in yield and sub-surface biomass (such as roots) would occur, which would increase SOC sequestration. In this case, it is reasonable to expect an SOC increase of more than 0.1 metric tons per acre per year.

Another analysis showed that 50% of the SOC stocks built up in the soil is likely to occur in the first two decades after cover crops are planted. This is well within the typical timeframe that is considered in a life-cycle analysis (LCA), which usually is longer than 30 years.

The Bottom Line

Decarbonization is the most significant disruptive trend of the 21st century, and it has the potential to revolutionize agriculture. Today’s market leaders are being asked to measure and account for the environmental impacts of production and implement sustainable solutions for them. The “first mover” who charts a pathway for low-carbon agriculture and animal farming will control the most important discussion of the century.

Agricultural soils are the largest terrestrial carbon reservoir, and therefore, represent a large potential sink to store carbon. EcoEngineers highlights the benefit of SOC increase and the resulting carbon intensity (CI) reduction for biofuels produced from the process.

The effect of cover crops on row crop yields, soil health, and nitrate-nitrogen leaching is complex and varies from year to year. The amount of carbon sequestered by cover crops can vary with soil type, cover-crop type, management, elevation, climate, and many other factors. In addition, a study of SOC sequestration usually takes years or even decades. Furthermore, both modeling and field studies are needed in order to understand the interactions between these factors.

However, according to EcoEngineers, these should not stop policy support that will ultimately help farmers. It is a reasonable conclusion that growing cover crops will have multiple positive impacts on agriculture and energy sectors. One of the easiest and fastest ways to promote such positive impacts is to set a reasonably conservative estimate for SOC and improve the evaluation based on future data collection and better modeling.

By monetizing the value that cover crops provide when used to produce RNG through anaerobic digestion, farmers will have a strong economic incentive to increase their use of them. This will lead to a wide variety benefits for many groups and sectors.


Zhichao Wang, Ph.D., P.E.

Jim Ramm, P.E.

Dr. Zhichao Wang, P.E., is a Senior Engineer and Carbon Analyst at EcoEngineers. He is a national expert in conducting life-cycle analysis on many renewable energy systems. You can reach him at or 515.985.1278.

Jim Ramm, P.E., is the Director of Engineering and cofounder of EcoEngineers. He has over 30 years of experience in civil/environmental engineering including work in the areas of ethanol, cellulosic ethanol, and solid waste and renewable fuel production processes. He is EcoEngineers’ ethanol line of business manager. You can reach him at or 515.985.1266.