Study finds removing corn residue for biofuel production can decrease soil organic carbon and increase CO2 emissions; may miss mandated 60% GHG reduction
21 April 2014
|Contribution of modeled CO2 emissions from SOC to the life cycle of biofuel from corn residue. Error bars are ± one standard deviation. Liska et al. Click to enlarge.|
Using corn crop residue to make ethanol and other biofuels reduces soil carbon and under some conditions can generate more greenhouse gases than gasoline, according to a major, multi-year study by a University of Nebraska-Lincoln team of researchers published in the journal Nature Climate Change. The findings cast doubt on whether biofuels produced from corn residue can be used to meet federal mandates for cellulosic biofuels to reduce greenhouse gas emissions 60% compared to gasoline.
The study, led by assistant professor Adam Liska, was funded through a three-year, $500,000-grant from the US Department of Energy, and used carbon dioxide measurements taken from 2001 to 2010 to validate a soil carbon model that was built using data from 36 field studies across North America, Europe, Africa and Asia. Using USDA soil maps and crop yields, they extrapolated potential carbon dioxide emissions across 580 million 30-meter by 30-meter geospatial cells in Corn Belt states.
|Changes in SOC|
|Changes in SOC occur via two dominant processes: soil erosion by water and wind; and soil respiration where SOC is oxidized to CO2. Crop residue conventionally has been left on the field after harvest to reduce soil erosion and maintain the SOC stocks and soil fertility of the Corn Belt.|
|Liska et al. note that accurately measuring SOC change is limited due to high spatial variability in SOC stocks, inability to detect a small annual percentage change, short-term studies, and failure to express SOC results in an equivalent mass basis to account for changes in soil bulk density. Furthermore, when crop residue is removed, it is essential to determine whether SOC loss is due to erosion or respiration, to accurately estimate the resulting net CO2 emissions.|
A massive amount of data was used to produce the results. Liska and his colleagues analyzed the data using high-performance computer clusters in the Holland Computing Center (HCC) at University of Nebraska-Lincoln that employ parallel programs to speed up computation. The uncompressed input data totalled ∼3 terabytes (TB) and the uncompressed output data totalled > 30 TB.
The program split each state’s input file into ∼40 megabyte (MB) files, and then executed computations on the smaller files in parallel. The output files were then joined together in a single state file, for each of the 12 states. If input files had not been split, the computational speed would have been significantly reduced owing to opening and closing of files and because loading an entire large disk file into memory at once is infeasible.
Removal of corn residue for biofuels can decrease soil organic carbon (SOC) and increase CO2 emissions because residue C in biofuels is oxidized to CO2 at a faster rate than when added to soil. Net CO2 emissions from residue removal are not adequately characterized in biofuel life cycle assessment (LCA). Here we used a model to estimate CO2 emissions from corn residue removal across the US Corn Belt at 580 million geospatial cells. To test the SOC model, we compared estimated daily CO2 emissions from corn residue and soil with CO2 emissions measured using eddy covariance with 12% average error over nine years.
The model estimated residue removal of 6 Mg per ha−1 yr−1 over five to ten years could decrease regional net SOC by an average of 0.47–0.66 Mg C ha−1 yr−1. These emissions add an average of 50–70 g CO2 per megajoule of biofuel (range 30–90) and are insensitive to the fraction of residue removed. Unless lost C is replaced, life cycle emissions will probably exceed the US legislative mandate of 60% reduction in greenhouse gas (GHG) emissions compared with gasoline.—Liska et al.
The results showed that the states of Minnesota, Iowa and Wisconsin had the highest net loss of carbon from residue removal because they have cooler temperatures and more carbon in the soil.
Total annual production emissions, averaged over five years, would equal about 100 grams of carbon dioxide per megajoule—7% greater than gasoline emissions and 62 grams above the 60% reduction in greenhouse gas emissions as required by the 2007 Energy Independence and Security Act. They found the rate of carbon emissions is constant whether a small amount of stover is removed or nearly all of it is stripped.
… development of other bioenergy systems, such as perennial grasses or forestry resources, may provide feedstocks that could have less negative impacts on SOC, GHG emissions, soil erosion, food security and biodiversity than from removal of corn residue.
Soil CO2 emissions from residue removal, however, can be mitigated by a number of factors and management options. As residue is a source of N2O emissions, residue removal would lower these emissions by ∼4.6 g CO2e MJ−1, or ∼8% of SOC emissions. The lignin fraction of residue can also potentially be burned to produce electricity, off-setting coal- generated electricity and saving emissions of up to ∼55g CO2e MJ−1. Furthermore, use of improved soil and crop management practices, such as no-till cover crops, forage-based cropping systems, animal manure, compost, biochar and biofuel co-products, could replace the estimated SOC loss after residue removal. These management options require more research under different residue removal practices to ensure SOC stocks are maintained where crop residue is removed.—Liska et al.
Liska said his team tried, without success, to poke holes in the study.
If this research is accurate, and nearly all evidence suggests so, then it should be known sooner rather than later, as it will be shown by others to be true regardless. Many others have come close recently to accurately quantifying this emission.—Adam Liska
Until now, scientists have not been able to fully quantify how much soil carbon is lost to carbon dioxide emissions after removing crop residue. They’ve been hampered by limited carbon dioxide measurements in cornfields, by the fact that annual carbon losses are comparatively small and difficult to measure, and the lack of a proven model to estimate carbon dioxide emissions that could be coupled with a geospatial analysis.
The research has been in progress since 2007, involving the coordinated effort of faculty, staff and students from four academic departments at UNL. Liska is an assistant professor of biological systems engineering and agronomy and horticulture. He worked with Haishun Yang, an associate professor of agronomy and horticulture, to adapt Yang’s soil carbon model, and with Andrew Suyker, an associate professor in the School of Natural Resources, to validate the model findings with field research. Liska also drew upon research conducted by former graduate students Matthew Pelton and Xiao Xue Fang. Pelton’s master’s degree thesis reprogrammed the soil carbon model, while Fang developed a method to incorporate carbon dioxide emissions into life cycle assessments of cellulosic ethanol.
Liska also worked with Maribeth Milner, a GIS specialist with the Department of Agronomy and Horticulture, Steve Goddard, professor of computer science and engineering and interim dean of the College of Arts and Sciences, and graduate student Haitao Zhu to design the computational experiment at the core of the paper. Humberto Blanco-Canqui, assistant professor of agronomy and horticulture, also helped to address previous studies on the topic.
Adam J. Liska, Haishun Yang, Maribeth Milner, Steve Goddard, Humberto Blanco-Canqui, Matthew P. Pelton, Xiao X. Fang, Haitao Zhu & Andrew E. Suyker (2014) “Biofuels from crop residue can reduce soil carbon and increase CO2 emissions,” Nature Climate Change doi: 10.1038/nclimate2187
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