A study published this week contributes more details than previous reports on biomass crops most suited to bioenergy feedstock use and what long-term factors may affect their growth.
A new long-term analysis conducted by 26 researchers across the country and published in “GCB Bioenergy,” examined the potential of growing switchgrass, Miscanthus, sorghum, energy cane and prairie grass mixtures.
The study attempts to provide farmers with general guidelines on how energy crops best fit in given regions. The project research spanned from 2008-2013 and was sponsored by the U.S. Department of Energy and the Sun Grant Initiative.
In addition to the specific biomass crops, the researchers examined how those crops might fit on Conservation Reserve Program, or CRP, lands.
BILLION TON STUDY
The promise of converting biomass to bioenergy on a grand scale was the topic of the so-called “Billion Ton Study” by the U.S. Department of Energy in 2005 — at around the time biofuels production was exploding on the scene as a result of the first Renewable Fuel Standard.
The report essentially laid out a best-case scenario as to the availability of all biomass materials in North America.
For farmers, it set the stage for what could be a future of growing biomass for biofuels production.
What the study lacked, however, was the finer details of the types of biomass crops best suited for a given region. The study lacked details on how farmers could best use land to take advantage of the potential bioenergy boom ahead. The new study published in GCB Bioenergy addresses that issue.
“Our long-term field study during 2008-2013 indicates that the annual biomass yield was 2.82 tons per hectare (2.47 acres) for warm-season mixture CRP land and 5.1 tons per hectare for cool-season mixture CRP land under best management practices (Anderson et al., 2016),” the researchers write.
The scientists said nitrogen fertilizer was the key management factor determining biomass yield on CRP land.
“By far, the greatest impacts on seasonal biomass production and changes in vegetation composition were due to location-specific precipitation,” the scientists found.
In most CRP sites evaluated in the study, they said precipitation was the main factor in turning out higher yields. The researchers also examined how changes in the composition of plant species on CRP land could affect the long-term sustainability of those lands.
“The results demonstrate that CRP land will shift vegetative composition over time based on harvest and fertilization management for biomass feedstocks,” the study said.
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“Any shift by mismanagement over time to less-desirable or less-productive species will hinder the ability of CRP land to adequately provide a sustainable or reliable resource for bioenergy feedstock production.”
While there continues to be tens of millions of acres in CRP contracts, the study said higher price signals in grain markets have led to diminishing CRP acres. So finding the best way for producers to make profits on CRP could have multiple benefits, the study found.
Profitable CRP land can “feed an emerging cellulosic biofuel industry, but also protect environmentally sensitive land and improve soil and water quality,” the study said.
“Accordingly, in order for CRP lands to be a reliable source of sustainable biofuel feedstock, management considerations must be taken into account that can produce sustainable stands of desirable species and provide ongoing conservation services,” the authors concluded.
Non-irrigated trials showed CRP in the eastern half of the United States and isolated locations to the west of that area are “capable of producing significant biomass for a national bioeconomy.”
In 2011, an update to the Billion Ton study was completed. That update found up to 24.7 million acres of CRP land could be used to produce 50 million tons of dry bioenergy feedstock annually.
More than a decade ago, switchgrass was considered to be the most promising feedstock for producing large volumes of biomass for cellulosic ethanol production, because it is grown in all regions of the country.
Researchers found large yield variations depending on where switchgrass is grown. In addition, they found switchgrass response to nitrogen was “highly variable, but greatest in South Dakota and Virginia.
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In states like Alabama where some of the highest switchgrass yields were recorded, nitrogen response was just 13%. In trials in Oklahoma and New York, the study said, “there was no benefit of added N across years, and in some production seasons, the effects of N on switchgrass in New York were significantly negative.
Researchers also found some difficulty in establishing switchgrass.
“The subpar establishment rates that arose at several sites in this study would negatively influence economic outcomes in a real-world setting and point to challenges for deploying biomass systems on marginal sites with difficult edaphic conditions, seed banks laden with weed seeds, or both,” the study said.
“Although manageable, these issues present additional costs in terms of lower yield with the slow establishment or the cost of weed control. Of course, the value of a ton of switchgrass will remain the key driver for feasibility for marginal land use and fertilization inputs.”
The study also examined the use of sorghum as a biomass crop. Researchers found annual rainfall to be the single most important variable for sorghum’s success.
“In general, the southeastern United States had the highest and most stable yields, indicating that this is the most stable region for sorghum biomass production,” the study said.
When it came to sorghum grown on CRP land, the study said yield was “significantly impacted by N rate, harvest timing, and year. Biomass yield increased as N fertilization rate increased.”
In addition, harvest timing that resulted in the highest biomass yield depended on the mixture of plant species, the number of harvests, and the amount of precipitation received during the growing season.
“The effect of year on biomass yield was mainly attributed to the amount of precipitation received during the most critical period of the growing season, with most locations experiencing moderate to severe drought conditions for at least one season,” researchers said.
“This effect of year, and precipitation in particular, highlighted the importance of conducting long-term field studies to more accurately predict expected biomass yields from CRP lands.”
Read the study here.
Todd Neeley can be reached at email@example.com
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