A tractor harvesting a cropIn working towards a Midwestern clean fuels policy, there’s a great deal of interest in compensating farmers providing feedstock for fuels for the greenhouse gas (GHG) reduction benefits they’re creating on the farm. Research from South Dakota State University is playing an important role in informing those discussions.

Here are four takeaways:

  • Carbon credits can incentivize farming practices that reduce GHG emissions.
  • Agricultural conservation practices like reduced tillage and fertilizer management are demonstrated to offer GHG emissions reductions on the farm.
  • Research underway improves the measurement of the GHG impacts of biofuel feedstock production.
  • A clean fuels policy should include farm-level carbon reductions in carbon intensity determinations when reductions can be accurately measured or modelled.

For the past two years, the Great Plains Institute has facilitated discussions with a broad-based stakeholder group—the Midwestern Clean Fuels Policy Initiative—of biofuel producers, agriculture, environmentalists, electric vehicle proponents, government officials, and others. The Initiative’s recent white paper A Clean Fuels Policy for the Midwest outlines how a clean fuels policy in the region sets a significant and attainable goal for carbon intensity reduction. In a clean fuels policy, fuel producers with carbon intensities above a baseline can purchase credit offsets from fuel producers who earn credits for their lower carbon intensity. The policy is technology-neutral and would support all carbon-lowering fuels, including electricity (for electric vehicles), hydrogen, renewable natural gas, biofuels, and even better-than-average conventional fuels.

The Midwestern vision for the policy suggests that farmers supplying feedstocks to biofuel producers ought to be able to earn credits for better-than-average performance on the farm.  

Research looks at carbon reduction through farming practices

Fuel carbon intensities are calculated by the Greenhouse gases, Regulated Emissions, and Energy use in Transportation (GREET) model developed by the US Department of Energy’s Argonne National Laboratory. GREET uses national averages for farm-level GHG emissions occurring during the production of biofuel feedstocks. Farm emissions come from various sources, including fuel use on the farm and in corn or soybean transportation, upstream emissions associated with manufacturing of fertilizer and other inputs, nitrous oxide emission from denitrification of fertilizer in the field. Some of these factors are relatively easy to measure or model, some are more challenging. Emission reductions associated with soil organic carbon storage and nitrous oxide are considered more difficult to account at the farm-level. Recent research confirms, however, that certain farming practices reduce GHGs below the average, and can even be a carbon sink that absorbs additional carbon dioxide (CO2).

“Over the past 10 years, we’ve seen corn yields that have increased by approximately two bushels per acre per year,” explains David Clay, a distinguished professor of soil science in the Department of Agronomy, Horticulture & Plant Science at South Dakota State University. Increasing yields means increased amounts of crop residue, the stalk, and other organic matter left after the corn is harvested. At roughly 42 percent carbon, the residue returns more carbon to the field and presents an opportunity for the cropping system to have the net effect of removing CO2 from the atmosphere. An average corn yield increase of two bushels per acre per year results in 1,400 pounds of additional plant residue (488 pounds of carbon) returning to the soil over five years, of which a portion is sequestered in the soil. According to the US Environmental Protection Agency’s Greenhouse Gas Equivalencies Calculator, that’s equivalent to 549 miles driven by an average passenger vehicle. Quantifying the amount of carbon sequestered in the soil is the subject of ongoing research, including by Clay.

A major focus of Clay’s work at South Dakota State University is examining corn’s carbon footprint alongside monitoring the impact of conservation tillage on the soil and researching best farming practices.     

It is clear that best practices in fertilizer management and tillage, in general, contribute to GHG reductions that would have a large impact on corn ethanol’s life cycle analysis. However, there is variability from one farm to the next. The challenge is to refine the science to make accurate predictions that quantify the benefit of a certain practice in a specific location. The team of South Dakota State University researchers, led by Clay, deserves credit for the great work they’ve done in this area.

Corn crops increase soil organic carbon

The South Dakota research team took a close look at what was happening with soil organic carbon (SOC), the amount of carbon contained in soil, in South Dakota farm fields. The 2012 paper, Corn Yields and No-Tillage Affects Carbon Sequestration and Carbon Footprints,” published in Agronomy Journal, reported many of the region’s soils are carbon sinks when planted in corn.

The study, which looked at 81,391 composite soil samples, found that even in the dryer 2004-2007 time period with lower corn yields and reduced crop residue, carbon sequestration occurred in all regions across South Dakota. A big part of that was due to the increased adoption of no-till methods, which reached levels of between 68 and 97 percent in the central regions and between 11 and 29 percent in the east.

No-tillage declined between 2008-2010, particularly in the east, due to higher rainfall that delayed spring planting. More recently, a 2017 USDA Census of Agriculture report on corn production tillage methods across the Midwest indicated that overall, 37 percent use no-till, 35 percent use reduced till, and 28 percent use conventional, intensive tillage.

Based on the carbon stored in the soil, the data collected from the farmers’ fields indicate a carbon footprint between 2004 and 2007 of -10.4 grams of CO2 equivalent per megajoule (gCO2e/MJ). Slightly higher levels of -15.4 gCO2e/MJ were observed for the 2008 to 2010 period. The negative value indicates a carbon sink. GREET’s default carbon intensity for corn ethanol is 55.1 gCO2e/MJ, so incorporating soil organic carbon storage from feedstock production could lower the carbon score by 19-28%. 

The value of that carbon intensity reduction is significant. Ethanol producers say that at credit values near $200 per ton in the California low carbon fuel market, each point of carbon intensity reduction is worth 1.5 cents per gallon of ethanol. However, neither the California market nor the Argonne National Lab’s GREET model currently incorporates values for corn’s carbon sequestration. Stakeholders in the Midwestern Clean Fuels Policy Initiative support inclusion of farm-level carbon reductions in carbon intensity determinations. 

Clay cites two studies that reinforce the 2012 findings. The first study coordinated by Ron Alverson, a South Dakota corn producer with a degree in soil science and a keen interest in corn ethanol’s lifecycle analysis, magnified the data points, Clay says.

“Ron collected soil sampling data from private laboratories between the years of 1997 and 2013; the resulting data points numbered in the millions. Second, we conducted an analysis of published work in collaboration with Argonne National [Laboratory].”

The analysis reviewed 3,380 studies conducted globally. The peer-reviewed report, “A global analysis of soil organic carbon response to corn stover removal,” was published in 2019 in the journal GCB Bioenergy. The study looked at soil organic carbon sequestration rates in corn production, under different levels of residue management, to confirm that globally, stover (corn stalks and leaves) retention increased soil organic carbon over time. The global studies confirmed even with moderate stover removal SOC can be maintained or increased over time. “The three sets of data have nearly identical results,” Clay says.   

Accurately measuring GHG impacts

Another line of research at South Dakota State University looks at a major source of corn ethanol’s carbon footprint—the GHG impact of fertilizer practices. With funding from the South Dakota Corn Utilization Council, Clay’s research team recently acquired a spectrometer that can continuously measure greenhouse gases—primarily ammonia, nitrous oxide, and CO2 —at several locations, facilitating comparisons of different treatments. “We’re able to measure what actually is being emitted from different tillage and cover crop practices,” Clay says.

One early observation is that the timing of fertilizer applications has a big impact on emissions, Clay reports. Applying urea fertilizer when the plant is younger results in lower emissions than applying it at the six-leaf stage. When looking at cover crops, the researchers found cover crops to have 70 or 80 percent lower nitrous oxide emissions when growing. The next phase of that research will analyze what happens over the entire season.

The emissions research also finds that the current thinking about agricultural nitrous oxide emissions may be greatly overstated. Estimates of nitrous oxide emissions have a huge impact on corn ethanol’s carbon footprint because the GHG impact of nitrous oxide is nearly 300 times greater than CO2. “The IPCC [Intergovernmental Panel on Climate Change] says there’s a one percent loss, but our numbers are coming back much, much lower that,” Clay says.

“We’re working with Michael Wang and his team at Argonne [National Laboratory] to complete additional meta-analyses to look at some of these other questions on nitrous oxide emissions, manure, and cover crops,” Clay says, with a goal of making the estimates of GHG impacts from biofuel feedstock production more accurate.

Economic & environmental benefits of no-till farming

“We need to support farmers in the transition to no-till,” Clay says. The possibility of earning carbon credits for GHG-reducing practices would be an important incentive for farmers considering adopting no-till. It can take up to five years before farmers begin seeing positive returns from switching to no-till. Besides possibly requiring new equipment, the switch affects multiple management considerations such as the placement of seed and fertilizer and the timing of field operations, as well as requiring new skills in residue and weed management. To maintain yields, no-till also requires higher nitrogen rates for the first few years until the soils reach a new equilibrium and start releasing nitrogen to the growing crop.

Greater adoption of no-till would move the region even further along in rebuilding the carbon-rich soils that were heavily mined in the decades after settlement. South Dakota State University studies have found a 24 percent increase in soil organic carbon since 1985 as conservation tillage practices have been more widely adopted. A 2014 study, “Does the conversion of grasslands to row crop production in semi-arid areas threaten global food security?,”  put a dollar value on that by comparing two severe drought years. State-wide corn yields averaged 33 bushels per acre in 1974 compared to 101 bushels per acre in 2012, a three-fold increase with the same rainfall.

“We separated out genetic improvement and looked at just the improvement we could quantify from the soil improvements, which was mostly from water holding capacity. By improving our soils, it meant $1 billion more for South Dakota,” Clay says. Similar extreme climatic events are occurring across our region, he adds, and improved soil organic carbon can benefit producers and the environment through increased resiliency.

Farmers have benefited from the improvements in soil health, Clay says, and society benefits as a whole when the environmental impacts of farming practices are reduced. One of his goals is to expand the dialogue on how to manage land for its best use, be it no-till cropping or perennial grasses.

“We facilitated a workshop with farmers and environmental groups where we looked at where we thought agriculture should be in 20 years. We all had the same vision for sustained agricultural production and increased ecosystem services. The question is, how do we get there?”

“I value the Great Plains Institute’s ability to build a group of people with the same vision—how do we maintain our rural economies, minimize the effect on the environment, and maintain wildlife?” Clay says. “That’s where the [Midwestern Clean Fuels Policy Initiative] fits in. Let’s identify where we want to be and come up with a consensus on where to start.”

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