Analysis Shows Strong Sequestration Potential of Corn, Energy Grasses, Tree Systems

Bioenergy interests will be gratified by a recent paper from a team of U.S. researchers that demonstrates the soil sequestration potential of corn, energy grasses and tree systems.

By undertaking the gargantuan task of reviewing approximately 100 studies from around the globe, the researchers have produced an analysis that can be used by advocates and policy makers as an extremely creditable source of support for a wide range of bioenergy feedstocks.

Scientists with the DOE’s Argonne National Laboratory, the University of Illinois and the International Food Policy Research Institute in Washington, D.C. have collected and analyzed data from worldwide field observations of major land use change from cropland, grassland and forest to land producing biofuel feedstocks, including corn, switchgrass, Miscanthus, poplar and willow.

Given that soil organic carbon can be impacted by a change in land use, the team used their findings to estimate soil organic carbon response ratios and carbon sequestration rates, as well as evaluate the effects of soil depth and time scale on changes in soil organic carbon. These are critical numbers as policy makers and regulators estimate the carbon emitted over the lifecycle of a bioenergy source before establishing the criteria upon which that source is deemed eligible under programs designed to cut back on emissions and address climate change.

Much of the debate over the federal designation of “advanced” biofuels (those that offer at least a 50-percent decrease in emissions compared to its petroleum equivalent) under the Renewable Fuel Standard or biofuels fully credited under state regulations like California’s Low Carbon Fuel Standard are based on how much in the way of emission-reducing benefits that federal and state regulators, respectively, determine those alternative energy sources carry.

The Argonne paper offers a first-time overview of a wide array of findings and shows virtually all of these feedstocks offer sequestration values – the amount of carbon that is left in the soil – greater than the level generally assumed by those charged with implementing emission-reducing policies. The paper also acknowledges that both the amount and rate of change in soil organic carbon are highly dependent on the specific land transition.

The team also noted that, in most cases, the response ratios in organic carbon were similar in both 0–30 centimeter and 0–100 centimeter soil depth. That, the team says, suggests that the use of top soil only for carbon accounting in biofuel lifecycle analysis might underestimate total soil organic carbon changes.

Furthermore, soil carbon sequestration rates varied greatly among studies and land transition types, the paper reports. Generally, the rates of SOC change tended to be the greatest during the 10 years following land conversion and had declined to approach zero within about 20 years for most land use changes. Observed trends in soil organic carbon change were generally consistent with previous reports, the researchers say, adding that soil depth and duration of study significantly influence carbon change rates and so should be considered in carbon emission accounting in biofuel lifecycle analyses.

The research team does not claim to have all of the answers, acknowledging that a high uncertainty remains for many perennial systems and forest transitions. The scientists call for additional field trials and modeling efforts that they say are needed to draw conclusions about the site- and system-specific rates and direction of change.

For example, high corn yields like those achieved here in the Midwest offer a higher carbon credit than that attained in other parts of the world where yields may be lower. A study of South Dakota agricultural practices over the past 25 years by a team of ten scientists at South Dakota State University (SDSU) shows that modern farming practices measurably increase the yield potential of the soil, and capture carbon in the environment. The study specifically determined the South Dakota surface carbon sequestration potential and associated partial carbon footprint for corn-based ethanol production. Findings support the theory that many of the surface soils in the region became carbon sinks when seeded with corn.

One of the paper’s authors, Steffen Mueller, with the University of Illinois-Chicago’s Energy Resources Center, notes that carbon sequestration varies widely across the United States. But he also cites recent peer-reviewed studies that show rapid corn-yield gains from intense selection, hybridization/heterosis, and biotech/GMO design is constantly adding more and more carbon to soil from roots and above ground residue. That, in turn, makes soil carbon sequestration associated with the growing U.S. trend of continuous no-till corn with cover crops even greater.

The paper represents a huge undertaking, accumulating a huge amount of data from an incredibly wide range of studies. The bioenergy community owes this research team a very big thank you.

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