Renewable fuels are an integral part of the liquid fuels portfolio of the United States and in fulfilling the stipulations of the federal threshold of 80 billion liters of ethanol produced from cellulosic sources by 2022. This will necessitate planting approximately 21 million hectares with cellulosic crops such as switchgrass, a perennial grass native to the United States. Land currently enrolled in the Conservation Reserve Program (CRP) – about 12 million hectares – might be suitable for energy crops. Some of this area is seasonally wet, environmentally sensitive, and with limitation for annual cropping. Converting these CRP lands to energy crops may increase the emissions of nitrous oxide, a potent greenhouse gas, particularly when converting lands that are seasonally wet due to soil or topographic attributes. Low carbon footprint is critical to accrue the benefit of energy crops. For such crops to qualify as renewable, their greenhouse gas emissions must be at most 50% of those from fossil fuels. Therefore, nitrous oxide emissions must be kept low in the feedstock production phase. Controlling nitrous oxide emissions from these energy crops requires an in-depth understanding of the interactive effects of landscape properties, crops growth rates, nutrient, and hydrology.
The research conducted by Debasish Saha and colleagues at The Pennsylvania State University identifies the potential of growing sustainable energy crops on these CRP landscapes without increasing greenhouse gas emissions. The researchers measured nitrous oxide emissions from plots converted from CRP to switchgrass and Miscanthus in central Pennsylvania. The physiography of the experimental site, representative of the Appalachian Ridge and Valley region with cropped uplands and wet bottomland that are occasionally under CRP. The emissions from the plots of energy crops were compared to the emissions from adjacent, unconverted CRP land under different landscape positions with varying soil and hydrologic properties. The researchers also established an autonomous network of 144 soil moisture sensors installed at three soil depths in 48 monitoring points in the landscape to continuously measure soil moisture, an important factor for nitrous oxide emissions. The monitoring period extended from May to September of 2013 growing season, which includes a summer storm that saturated the soil – the perfect conditions to expect nitrous oxide emissions. Their research work is published in GCB Bioenergy.
Nitrous oxide is produced by soil microbes when soil mineral nitrogen from organic/inorganic fertilizer and other sources exceeds plant demand and coincides with wet soil conditions usually after a storm or snowmelt event. “The transition phase from CRP to energy crops is critical for nitrous oxide emissions as soil disturbance may increase nitrogen availability in excess of demand when plants are small and the root system is not extensive,” said Saha.
The authors realized that nitrous oxide emissions from energy crops increased above the CRP control baseline only in the wetter footslope positions. “While near-stream footslope soils with prolonged subsoil wetness had higher nitrous oxide emissions from energy crops than CRP, a large portion of the landscape had comparable emissions to those of CRP. The footslope positions of the landscape occupy at most a third of the lower part of the watershed. For this reason, about two-thirds of the set-aside conservation area (CRP) could be used for energy crops production.” Saha further added, “It is expected that large emissions from the footslope can eventually be curtailed as the grasses get established.”
Saha said “Apart from carbon benefits of energy crops, growing energy crops in these seasonally wet CRP lands usually on steep areas of the landscape can offer additional ecosystem services. Energy crops in these landscapes can function as riparian buffers to provide water-quality benefits by curtailing nutrient as well as sediment loads to surface and groundwater. Owing to vigorous biomass production by these grasses and little disturbance of the perennial rooting systems, these crops can store carbon in the soil because of their extensive below-ground carbon allocation.”
The outcomes of their study revealed that managing the conversion from CRP to energy crops while maintaining low nitrous oxide emissions could be optimized by designing a sufficient transition process that curtails co-occurrence of high mineral nitrogen and wet soils.
This research was funded by U.S. Department of Transportation Sungrant, the USDA, and the Richard King Mellon Foundation. Other research team members include Armen Kemanian, associate professor of production systems and modeling and Felipe Montes, research associate in Plant Science, Penn State; Jason Kaye, professor of soil biogeochemistry in Ecosystem Science and Management, Penn State; Paul Adler, research agronomist with the Pasture Systems and Watershed Management Research Unit, USDA-Agricultural Research Service; and Benjamin Rau, former USDA-Agricultural Research Service soil scientist, now a research ecologist with the USDA, Forest Service.
Debasish Saha, Benjamin M. Rau, Jason P. Kaye, Felipe Montes, Paul R. Adler, and Armen R. Kemanian. Landscape control of nitrous oxide emissions during the transition from conservation reserve program to perennial grasses for bioenergy. GCB Bioenergy (2017) 9, 783–795.Go To GCB Bioenergy