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January 19, 2018
New research from Argonne National Laboratory, the University of Chicago at Illinois and the International Food Policy Research Institute shows that incorporating conservation practices can both increase soil organic content and improve the greenhouse gas profile of biofuels produced from crop residues. A January 11 study in Global Change Biology: Bioenergy, looks at how adapting conservation practices in a typical corn-soy rotation can add organic content to soil and improve the GHG profile of cellulosic biofuels. Researchers examined the offset to soil organic carbon loss when stover is removed by various conservation practices. Importantly, the research shows that without additional conservation methods to offset the loss of organic content in the residues, organic stocks of soil may decrease over time, thus decreasing the greenhouse gas (GHG) benefit of cellulosic biofuels production.
With certain land management practices, an 80 percent reduction in greenhouse gases compared to gasoline could be achieved. Conversely, without adding some organic matter back to the soil through various conservation mechanisms, the researchers found that cellulosic ethanol would not quite meet the 60 percent GHG reduction threshold for cellulosic fuels, as required by the Renewable Fuel Standard (RFS).
Cellulosic feedstocks – sourced from agricultural wastes like corn stover (corn husks, leaves, stalks) oat and rice hulls, as well as purpose-grown crops -- are set to make up the majority portion of renewable fuels under the RFS going forward. Using lifecycle analysis (LCA), researchers estimated the impact of various conservation practices, coupled with stover removal rates of 30, 60 and 100 percent. Three conservation practices were examined: manure application, reduced tillage (conservation tillage, low-till and no-till), and cover crops. They found a combination of 30 percent stover removal rates, combined with cover crops or manure application, to provide the largest GHG benefit – over an 80 percent reduction as compared to conventional gasoline.
LCA is a process where environmental impacts for every stage of fuel production, from growth to harvesting, transport and refining, are considered to assess the total lifecycle greenhouse gas emissions (GHG) of a particular fuel pathway. According to the RFS, cellulosic fuels must be 60 percent less greenhouse gas (GHG) intensive than traditional gasoline, compared to 20 percent GHG reduction for ‘1st gen’ corn ethanol. Recent research from USDA has confirmed that improvements at both the field and refinery level mean that corn ethanol is, on average, 40 percent less GHG intensive than traditional gasoline.
Typically, stover is left on the field to provide valuable nutrients and carbon back to the soil. But two recent developments have turned this ‘residue’ into a potential resource. First, rising corn yields means there is more residue than ever before on the field, leading producers to talk about ‘residue management’. Secondly, the nascent cellulosic ethanol industry could create widespread markets for crop residues. In response, agronomists have developed a range of stover removal recommendations, depending on soil type and other field-level factors. Generally, a rate of no more than 30 percent stover rate is the accepted rule of thumb.
The researchers found that depending on the region, various conservation measures provide a bigger benefit to soil organic carbon than others. For example, in the southern United States, cover crops grow faster and thus can return organic matter back to the soil more quickly than in colder regions such as the Dakotas and Minnesota. These potential changes to soil health and the GHG footprint of biofuels highlights the importance of conservation measures in increasing soil carbon. According to USDA, conservation tillage rates are increasing over the past two decades. The researchers suggest that with increased stover removal, tillage rates could further be reduced.
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