Midwest soil could be huge carbon reservoir


Cover Crops and Soil Organic Carbon Increase Potential for Farmland to Act as a Carbon Sink


By Dr. Zhichao WangShashi Menon, and Jim Ramm, P.E., EcoEngineers


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1.0 Background

Cover crops have long and widely been valued as an effective means for soil and water conservation. The benefits of reducing soil erosion and improving soil health have been long-observed.1 In recent years, the potential benefits of cover crops on water quality are gaining more attention as the impact of intensive agriculture on water quality has become a national concern. Cover crops can decrease nitrate leaching into surface water, not only alleviating the nutrient loss from the cropland, but also potentially reducing fertilizer usage in agriculture.2 In Iowa, cover crops are a key practice being promoted to meet goals associated with the Iowa Nutrient Reduction Strategy.3 Further and often-overlooked impacts of cover crops include supporting wildlife and beneficial insects — and potentially increasing cash crop productivity.

More recently, two other environmental benefits of cover crops have been gaining attention. Cover crops can provide significant amount of biomass for renewable energy production while increasing soil organic matter and sequestering carbon in the soil. Indeed, agricultural soils are the largest terrestrial carbon reservoir, and therefore represent a large potential sink to store carbon.4 Therefore, cover crops could serve as a greenhouse gas (GHG) reduction strategy while providing low-carbon fuels.

This study focuses on the GHG reduction potential of growing cover crops. The purpose is to provide a certain level of quantitative analysis of the scale of the GHG benefits from producing biofuels using cover crops by summarizing previous studies. We hope this paper will provide some basis for further studies and future agricultural policies.

2.0 Basics and Current Status of Cover Crops Adoption

The adoption of cover crops has been increasing at a rapid rate throughout the country, according to data from the 2017 Census of Agriculture.5 Cover crops were planted on 15.4 million acres in 2017, an increase of 50% over five years, with Iowa leading the way with a 156.3% increase during that period. In addition, a study by Feyereisen et al.6 identified corn and soybean areas by county using National Agricultural Statistics Service (NASS) data, excluding irrigated land and areas already supporting a winter small-grain crop. Within this area, they calculated biomass production using RyeGro, a cover-crop simulation model. The spatial analysis indicated that nationally 7.44 M ha (18.4 M acres) in continuous corn and 31.7 M ha (78.3 M acres) in a corn-soybean rotation are suitable for producing winter rye. These simulation results projected that 120–170 million U.S. dry tons of rye biomass can be harvested from this land.6

Different cover crops are used for different purposes.7 Common cover crops used in Iowa include winter hardy plants like rye and wheat. Other less common — but also effective — species are also used. Below are the major cover crops grown in Iowa:

  • Winter hardy grains: cereal rye, winter wheat, triticale (a cross of cereal rye and winter wheat)
  • Winter killed grains: oats, spring wheat
  • Winter hardy legumes: hairy vetch, red clover, sweet clover
  • Forage covers (winter killed): turnips, rapeseed, radishes
  • Forage covers: turnips, rapeseed, radishes

Rye plant cover crop (Shutterstock)

In this study, winter rye is selected as the one to perform further analysis with, as it is the most commonly adopted cover crop in Iowa.

Currently, the main purpose of growing cover crops is not to harvest biomass. Therefore, cover crops are usually terminated before spring cash crops (typically corn or soybean in the Midwest). Interviews with Iowa farmers suggest that the biomass yield from winter rye could reach as high as 4 MT/acre if the purpose of growing cover crop is to maximize harvested biomass.

The study by Feyereisen et al. calculated biomass production after corn harvest and before the subsequent corn or soybean crop for 30 locations using the RyeGro simulation model. The average RyeGro biomass yield for the 30 locations for six planting– harvest date scenarios is 4.2 MT/ha, or 1.7 MT/acre.6

In a study conducted by Argonne National Laboratory on the impact of land use management, change on soil organic carbon and the life-cycle greenhouse gas emissions of biofuel, a corn-soybean rotation is assumed, and winter rye was assumed to be planted after corn harvesting and terminated before soybean planting. The yield adopted in this study was 1.1 MT/acre. The rye was assumed to be killed 14 days before soybean planting and therefore only 50% of the rye yield can be achieved. This is to minimize nitrogen tie-up and conserve soil moisture.8

Based on above studies, it is reasonable to assume that the biomass that can be harvested from winter rye in the Midwest could range from 1-2 MT/acre, though higher biomass yield may also be possible.

3.0 Farming Practices and Their Impact on CI

3.1 Cultivation Energy Use

Cover crops can be seeded in the fall using a variety of methods including drilling after crop harvest, broadcasting after crop harvest, or aerially broadcasting before harvest. In this analysis, the planting is assumed to happen using a grain-drill, and the energy consumption is set at 0.47 gal diesel /acre.8

3.2 Fertilizer Use

In the study conducted by Argonne National Laboratory, it is assumed cover crops do not consume any fertilizers and do not alter the NPK content of the soil.8 This practice is in line with the interview with Iowa farmers. It’s possible for cover crops to suppress the N2O emissions during spring time, but also to increase N2O because of the increase soil organic matter input. However, altered N2O emissions due to cover crop cultivation have not been considered here. Reliable data on those processes, which are most likely species-specific, still need to be gathered from the field or developed via more precise models.

3.4 Harvesting Energy Use

It is not clear how cover crops will be effectively harvested. The fuel assumption of 1.5 gal diesel/acre for corn stover harvesting from GREET model is used for this study.

The total impact of these farming practices on the CI of compressed natural gas (CNG) from cover crops is in the 1-3 g CO2e/MJ range for the fuel produced.

4.0 Soil Organic Carbon (SOC) Change Caused by Cover Crops

Using cover crops to produce energy will result in an increase in soil organic carbon (SOC) and a consequent decrease in the life-cycle GHG emissions for a biofuel produced from the cover crop if the sequestered carbon is credited to such biofuel. The potential carbon sequestration resulting from planting cover crops varies significantly across different studies, but there is general consensus that cover crops can increase SOC and therefore act as a carbon sink.

Olson et al. performed a 12-year study on the impacts of cover crop on SOC in different tillage scenarios- no till, chisel plow, and moldboard plow. The SOC increased with application of cover crops in all three scenarios. SOC gain caused by cover crops were 0.35, 0.2, and 0.04 MT /acre/year, respectively, for the different tillage scenarios.9

Another study conducted by Ruis et al. estimated that the rate at which cover crops can sequester carbon in agricultural soils is 0.2 MT/acre/year.10

Rapeseed cover crop flowers
(Adobe Stock Photo)

The aforementioned study conducted by Argonne National Laboratory shows 0.05 to 0.07 MT/acre/year SOC increase when conventional tillage practice was assumed. However, these values were based on the assumption that cover crops are only planted once every two years in the corn-soybean rotation. If the cover crops are planted every year for harvesting biomass, we should expect a corresponding increase in yield and sub-surface biomass, and increased soil organic carbon sequestration. In this case, it is reasonable to expect an SOC increase greater than 0.1 MT/acre/year.

In a study conducted by SARE, 60 million MT CO2e can be sequestered when 20 million acres of cover crops are grown. This coverts to 0.82 MT C/acre/year, which is significantly higher than all the literature data mentioned above.1

Finally, in a meta-analysis completed by Poeplau et al., the researchers analyzed data from 139 plots at 37 different sites in 30 studies worldwide.11 The results showed a mean annual SOC sequestration of 0.13 MT/acre/year. 102 out of 139 observations had an annual change rate between 0 and 1 MT/ha/year (0-0.4 MT/acre/year). Assuming that the observed linear SOC accumulation would not proceed indefinitely, they modeled the average SOC stock change using the carbon turnover model RothC. The predicted new steady state was reached after 155 years of cover crop cultivation with a total mean SOC stock accumulation of 16.7 Mg/ha. In this analysis, 50% of the effect on SOC stocks is likely to occur in the first two decades after adoption of cover crops. This is well within the typical time frame a life-cycle analysis considers which usually is longer than 30 years.

Based on the literature reviewed, we adopt 0.04, 0.13 and 0.35 MT/acre/year as the low, medium and high SOC sequestration rate, acknowledging that higher SOC sequestration rates are possible.

5.0 CI of transportation fuel derived from cover crops

CI of transportation fuel (for example, CNG derived from anaerobic digestion and biogas upgrading) produced from cover crops include several key components. The GREET model used by California’s Air Resource Board currently allows user-level inputs for biomass transport, biogas production, upgrading to RNG, compression of biogas/RNG, and transportation of the RNG to California. For the purpose of this study, we are assuming this to be set at 35 g CO2e/MJ, which is a reasonable value in the typical range for the industry. Of the 35 g CO2e/MJ, transportation of the RNG from the Midwest to California could contribute about 10 g CO2e/MJ. If the biogas is used locally at an Iowa ethanol or biodiesel plant as process heat, the fixed component of this CI would be closer to 15-20 g CO2e/MJ, since there will not be a gas transport penalty, and there will be no impact from intensive gas upgrading or compression. These values are assumed to be fixed in this study as they are not the focus of the study.

If the biogas is used locally at an Iowa ethanol or biodiesel plant, the CI reduction resulting from carbon sequestration would be passed through to the ethanol, making the ethanol more competitive in California and other states with similar low-carbon regulations.

The focus of this paper is to evaluate the impact on CI from SOC increase. Several factors, such as carbon sequestration rate, biomass yield, and biomethane yield, can influence the final results significantly. Based on information collected in the above sections, the below scenarios are developed based on low, medium, and high values for these parameters.

Table 1: Key factors and the low, medium, and high values used for CI analysis

Carbon sequestration rates should be closely correlated to biomass yield, though not necessarily in a linear manner. Therefore, when developing scenarios for low carbon sequestration rate, only low biomass yield was assumed, and for high carbon sequestration rate, only high biomass yield was assumed because it is unlikely a low biomass yield will result in a high carbon sequestration, and vice versa. On the other hand, if we assume the carbon sequestered is a fixed value while the biomass yield changes, then as biomass yield increases, the CI often increases. As the biomass yield increases, the resulting output of biofuel also increases, and the SOC increase is allocated across the entire production. Therefore, the CI reduction per MJ of biofuel produced falls. Table 2 shows the CI analysis results for different scenarios based on parameters in Table 1.

Table 2: CI analysis results for different scenarios

Depending on the harvestable biomass of cover crops and different yield assumptions of biomethane production, when the SOC change is considered, the CI of CNG produced using biogas from cover crops may be reduced by  -92 g CO2e/MJ to  -14 g CO2e/MJ. The CI of ethanol using such biogas may be reduced by  -35 to  -10 g CO2e/MJ. The scenarios with medium carbon sequestration rate, which resulted in a CI reduction between -54 to  -23 g CO2e/MJ for CNG, can provide a reasonable reference for policy decisions and further study.

Maximum CI reduction (-92 to  -61) from SOC increase from cover crops on the produced CNG happens when there is maximum carbon sequestration rates and maximum biomass yield. Mid-level carbon sequestration rates result in more moderate CI reductions (-54 to -27) with lower biomethane yields resulting in greater CI reduction. Even at the lowest carbon sequestration levels and low biomass yield levels, there is a CI reduction on the CNG in the  -14 to  -23 range.

If the biogas is used as process heat at an Iowa biofuel production facility like an ethanol or biodiesel plant, it will lower the CI of the fuel produced at the facility. The potential impact of the biogas on the CI of an Iowa ethanol plant producing dry distillers grains and solubles as co-product can vary from -10 to  -35 if it is used as process heat. The impact will depend on site-specific information. In the above scenario we are assuming 100% of process heat is substituted and the heat demand is 25,000 Btu NG/gal ethanol for a facility producing DDGS as co-product. Under these assumptions, it is reasonable to believe that at carbon sequestration levels in the mid-range, an ethanol plant can reduce its CI by 15-23 points.

6.0 Next Steps

The purpose of this study is to lay a ground work for more detailed and thorough future work that quantifies the GHG impact of producing biofuels from cover crops.

Iowa may be at the cusp of formulating policy that drives greater cover crop adoption as part of its nutrient reduction strategy. The benefits to soil health and water quality resulting from cover crops are typically the focus of cover crop planning and implementation. We hope to introduce the added benefit of soil organic carbon increase and the resulting CI reduction for biofuels produced from anaerobic digestion of the cover crop biomass. The biogas resulting from this process can be used at Iowa ethanol and biodiesel plants as process heat or it can be upgraded and injected into a pipeline to be used as CNG at other locations.

Red clover cover crop

Carbon prices in California’s LCFS markets have been around $200 / MT through most of 2018-19. At this price, a one-point reduction in the CI of an Iowa ethanol plant is worth $0.016 per gallon in LCFS credits. Assuming a 17-point reduction from carbon sequestration, a 100 MGY ethanol plant selling to California can potentially earn $27 million in added revenues per year if they were to switch 100% of their natural gas use to biogas from cover crops and get credit for carbon sequestration. Some of this money will be spent on expanding cover crop acreage and harvesting, storage, and pretreatment systems. Preliminary calculations suggest that the about 200,000 to 250,000 acres of farmland would be required to support the production of sufficient biomass to meet the heat demand of a 100 MGY ethanol plant.

The effect of cover crop on crop yields, soil health, and nitrate-nitrogen (N) leaching is complex and variable 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. And usually field study for SOC change takes years or even decades before the data can be deemed reliable. Furthermore, both modeling and field studies are needed in order to understand the interactions between these factors.

However, given the data we have so far, it is a reasonable conclusion that growing cover crops will have multifold 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 and improve the evaluation based on future data collection and better modeling. California Air Resources Board took a similar approach in their treatment of the indirect land use change (iLUC) issues for certain biofuels. The iLUC value of corn ethanol was set at 30 g CO2e/MJ when the Low Carbon Fuel Standard (LCFS) was initially adopted. It dropped to 19.8 g CO2e/MJ in current regulations as the data collection and modeling technique approved over the years, while allowing billions of gallons of corn ethanol contribute to the carbon reduction goal in California. This successful precedence of the approach provides an example, among many, for successful policy making.

Therefore, it is reasonable to set the carbon sequestration levels around 0.13 MT per acre per year and incentivize the practice through effective biofuel policies. An existing policy that could reward this practice is the LCFS in California, where carbon prices have gone beyond $200/MT of CO2e this year.


Zhichao Wang, Ph.D.

Dr. Zhichao Wang is a Carbon Analyst at EcoEngineers. He is a national expert in conducting life-cycle analysis on many renewable energy systems. Dr. Wang has published over 20 works in numerous industry-related papers and journals, six Department of Energy technical reports, and two book chapters. He holds a Ph.D. in agricultural and biological engineering from the University of Illinois at Urbana-Champaign and Master and Bachelor of Science in environmental engineering from Tsinghua University in Beijing, China. You can reach him at zwang@ecoengineers.us or 515.985.1278.


Shashi Menon

Shashi Menon is the CEO and cofounder of EcoEngineers. He has holds over 20 years of business strategy and business development experience in finance, commercial real estate investments, and renewable energy consulting. He has worked closely with federal and state regulators and the biofuel industry over the past 10 years to frame policy to enable successful projects. His formula for a successful renewable energy project are regulatory compliance, a value-add technology, and monetization of carbon premiums to finance startup and operations. You can reach him at smenon@ecoengineers.us or 515.985.1274.


Jim Ramm, P.E.

Jim Ramm, P.E., is the Director of Engineering and cofounder of EcoEngineers. He holds over 30 years of experience in civil and environmental engineering including work in the areas of management, project management, design, environmental investigation, audit, remediation, water supply, construction, solid waste and renewable fuel production processes. Mr. Ramm holds a Bachelor of Science in civil engineering from Kansas State University and is a licensed Professional Engineer in Arizona, Illinois, Iowa, Kansas, Nebraska and Tennessee. You can reach him at jramm@ecoengineers.us or 515.985.1266.



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