Benefits of Switchgrass and Cottonwood for Carbon and Nitrogen Capture

Linda Benedict, Blazier, Michael  |  9/6/2014 2:03:58 AM

Michael A. Blazier, Hal O. Liechty, Matthew H. Pelkki, Jim J. Wang and Montgomery W. Alison

The Lower Mississippi Alluvial Valley is the 25 million-acre floodplain of the Mississippi River that extends from the confluence of the Mississippi and Ohio Rivers southward to the Gulf of Mexico. The region has abundant agricultural production, with approximately 17 million acres of farmland. This agricultural infrastructure, along with its central location within the United States, river and port access, and climatic and soil conditions that foster growth of a wide variety of crops, make the Lower Mississippi Alluvial Valley an attractive potential region for the development of bioenergy production facilities that produce energy and fuels from crops.

Private and public interests and investments in bioenergy are affected by concerns not only about its financial feasibility but also about the sustainability of bioenergy crop production. Since 2009, a collaborative research and extension effort between the LSU AgCenter and the University of Arkansas at Monticello has focused on the production and ecological sustainability of eastern cottonwood (Figure 1) and switchgrass (Figure 2) as bioenergy crops in the Lower Mississippi Alluvial Valley. This study has focused on switchgrass and cottonwood because both were identified as model energy crops by the U.S. Department of Energy after a comprehensive evaluation of potential bioenergy crops. These two crops are being grown on retired agricultural fields with soil conditions that led to chronic low yields when in agricultural production.

These sites were selected because converting highly fertile agricultural lands to bioenergy crop production is unlikely because of the economic and social benefits associated with producing conventional agricultural crops on such sites. At these sub-optimal sites, cottonwood has produced up to 3 dry tons of above-ground biomass per acre per year, and switchgrass has produced up to 6 dry tons. In addition to providing this biomass as biofuel feedstock, research has shown potential ecological benefits associated with cottonwood and switchgrass.

The bioenergy market is heavily influenced by domestic and international public policies that promote fuels that are relatively “carbon neutral,” by which carbon emitted during the cultivation of crops and burning of fuels is offset by capturing carbon in crop biomass (above- and below-ground) and in soil. Cottonwood and switchgrass have relatively high capacity to capture carbon above and below ground because they are perennial plants and have high growth rates.

At three locations in Arkansas and Louisiana over a threeyear period, switchgrass had three times more carbon and cottonwood had 2 times more carbon in total above- and below-ground biomass than a soybean-grain-sorghum crop rotation that was conventional for similar soils. Switchgrass and cottonwood at these sites were similar in their above- and below-ground carbon allocation patterns, with each having approximately 70 percent of carbon above ground and 30 percent below ground.

Because of the differences and frequency in harvesting techniques, cottonwood and switchgrass differ in the accrual of biomass residue at the soil surface. Annual cottonwood leaf deposition forms a litter layer at the soil surface, but such a residue layer is much lower in switchgrass because most above-ground biomass is annually removed in harvesting. Greenhouse gas emissions from soil can also differ between cottonwood and switchgrass; in this study cottonwood had lower nitrous oxide (which is considered a greenhouse gas) emissions than switchgrass because it was only fertilized with nitrogen in its first year, whereas switchgrass was fertilized with nitrogen more frequently.

Previous land use can affect soil carbon trends when land is converted to cottonwood and switchgrass. The study sites in Arkansas, which were in agricultural production until immediately prior to the establishment of the study, had 9 to 23 percent increases in mineral soil carbon three years after conversion to cottonwood and switchgrass. The study site in Louisiana had fields that were kept fallow and in pasture for several years prior to the study’s establishment. At this site three years after conversion, mineral soil carbon decreased by 2 percent in switchgrass and 7 percent in cottonwood.

Nitrogen in soil water
Cottonwood and switchgrass are sometimes planted in buffers along waterways on farms in the area due in part to their propensity to enhance water quality by preventing erosion and filtering nutrients. At the Arkansas and Louisiana study sites, eastern cottonwood and switchgrass have shown lower nitrogen concentrations in leachate water than soybean and grain sorghum rotations grown on the same sites. Soybean and sorghum rotations had 5 to 10 times higher concentrations of nitrate as nitrogen, organic nitrogen and total nitrogen in soil water to a 1-foot depth than cottonwood and switchgrass. Nitrate nitrogen leaching losses at some locations were 20 to 40 times greater in the soybean-sorghum rotation than in cottonwood and switchgrass. The soil water nitrogen concentrations from soybean and sorghum likely were higher than cottonwood and switchgrass because of soybean’s nitrogen fixation and the presence of living roots yearround in cottonwood and switchgrass. In contrast, soybean and sorghum roots were an annual source of nitrogen as they decomposed after the fall harvest.

Study implications
Results from this study indicate that if cottonwood and/ or switchgrass were to increase in the landscape of the Lower Mississippi Alluvial Valley with the development of bioenergy production facilities that require such feedstocks, these crops could enhance the region’s capture of carbon and nitrogen in biomass and soil. This increase in carbon and nitrogen sequestration can improve air and water quality. A more immediate implication of these results is that the hardwood- switchgrass buffer systems being established within the landscape to protect water quality appear effective at reducing nitrogen losses from farms.

Acknowledgements: This article contains information obtained from a project funded by Sun Grant and USDA Sustainable Agriculture and Research Education programs until 2013. The project is currently supported by the Agriculture and Food Research Initiative of the National Institute of Food and Agriculture, Grant #2011-67010-20078.

Michael Blazier is an associate professor at Hill Farm Research Station, Homer, La.; Hal O. Liechty is George R. Brown Endowed Professor, and Matthew H. Pelkki is a professor and Clippert Chair in the School of Forest Resources, University of Arkansas at Monticello. Jim J. Wang is a professor in the School of Plant, Environmental and Soil Sciences, and Montgomery W. Alison is an extension specialist at the Scott Research/Extension Center, Winnsboro, La.

This article was published in the summer 2014 issue of Louisiana Agriculture Magazine.


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