Texas A&M study: Bioenergy sorghum could help with GHG emissions

By Texas A&M AgriLife | June 03, 2014

Bioenergy sorghum may offer more than another energy supply; it may offer a “sink” for greenhouse gases, according to a Texas A&M AgriLife Research study.

Researchers in the Texas A&M University soil and crop sciences department have been measuring greenhouse gases from biofuel production scenarios to help quantify the carbon footprint of a bioenergy cropping system and evaluate compliance with federally mandated reduction goals.

The study, “Impacts of Biomass Sorghum Feedstock Production on Carbon Sequestration and Greenhouse Gas Emissions,” was funded by the AgriLife Research Cropping Systems bioenergy program and a USDA National Institute of Food and Agriculture grant.

Frank Hons, professor of soil science and AgriLife Research Faculty Fellow; Joe Storlien, postdoctoral research associate; Jason Wight, assistant research scientist; and James Heilman, professor of environmental physics, jointly worked on the research.

Hons explained that bioenergy sorghum may play a significant role in future biofuel production as a high quality biomass feedstock. To date, no studies have quantified life cycle greenhouse gases from this source.

“Bioenergy crop production represents an opportunity for greenhouse gas mitigation in the U.S.,” said Hons. “Crop production systems can be a net sink or net source of atmospheric carbon dioxide, depending on a number of factors, including land management practices.”

Life cycle analyses are used to evaluate biofuel efficiency by balancing the direct and indirect greenhouse gases associated with production with the total energy output and soil carbon storage, said Storlien.

One objective of their study was to determine the effects of crop rotation, nitrogen fertilization and residue management on net greenhouse gas emissions from bioenergy sorghum production. The study analyzed direct and indirect greenhouse emissions, soil carbon sequestration to a 3-foot depth, and theoretical biofuel yield from eight different bioenergy sorghum production scenarios.

The study centered on a field near College Station under bioenergy sorghum production since 2008. The researchers collected soil samples prior to the start of the study and each following spring in order to study changes in soil carbon storage and nutrient availability over time.

Annual accrual rates of soil organic carbon were much higher than anticipated, ranging from 1.2 to 3.3 tons of carbon per acre across all treatment combinations, Wight said. The corn-sorghum sequence had a significantly lower annual accrual rate than monoculture sorghum.

The high carbon sequestration rates from sorghum may be attributed to the high yield potential, more than 20 dry tons per acre, and the fact that the study was conducted in carbon-depleted soil, Storlien said. Moderate and severe drought conditions in 2010 and 2011 may also have contributed to soil organic carbon accrual if greater carbon production was allocated to roots scavenging for water.

Unfertilized, monoculture sorghum with half the yield returned to the field to provide nutrients and organic matter had the greatest overall biofuel production efficiency based on net greenhouse gas emissions savings, he said. However, crop rotation and fertilization would be recommended to minimize pest pressure and sustain long-term crop yield.

“These results have significant implications for net greenhouse gas emissions, soil organic carbon sequestration and life-cycle analyses,” Hons said. “Few studies have quantified greenhouse gas emissions and below-ground carbon inputs from bioenergy sorghum, and further investigation is warranted.”