This figure shows how much water is used to produced one unit of ethanol (defined as water use intensity) for each energy crop. Credit: ACS, Atul Jain. Click to enlarge.

The integrated model-data framework simulated the dynamic growth patterns of bioenergy grasses (e.g. carbon allocation, vegetation structure, and phenology). The dynamic approach inherently accounts for the impact of temporal and spatial climate variability, water quantity and NL, the researchers said.

Specifically, this study evaluated three factors important to producing bioenergy feedstocks in each yield zone:

1. the potential demand of land and water use intensity on a per unit of ethanol production basis;

2. the progressive interaction between water quantity and biomass feedstock production over time; and

3. the spatial changes in soil water quantity and NL as a result of producing bioenergy grasses relative to the existing land cover (e.g., row crops, herbaceous plants, and forests).

Broadly, they found that:

• Water use intensity tends to be lower where grass yields are modeled to be high—e.g., in the Midwest for Miscanthus and Cave-in-Rock and the upper southeastern US for Alamo. However, most of these regions are already occupied by crops and forests and substitution of these biome types for ethanol production implies tradeoffs.

• Growing Miscanthus generally consumes more water, Alamo consumes less water, and Cave-in-Rock consumes approximately the same amount of water as existing vegetation.

• Bioenergy grasses can maintain high productivity over time, even in water limited regions, because their roots can grow deeper and extract the water from the deep, moist soil layers. However, this may not hold where there are frequent and intense drought events, particularly in regions with shallow soil depths.

• One advantage of bioenergy grasses is that they mitigate nitrogen leaching relative to row crops and herbaceous plants when grown without applying N fertilizer; and bioenergy grasses, especially Miscanthus, generally require less N fertilizer application than row crops and herbaceous plants.

Growing bioenergy grasses, in general, can mitigate nitrogen leaching across the United States. However, the greatest reduction in nitrogen leaching occurs when bioenergy crops displace other cropland or grassland, because energy crops consume more water and less nitrogen fertilizer than the crops and grasses that they replace, resulting in less water runoff and nitrogen loss.

—Yang Song, lead author

By using a combination of crop growth, hydrological, carbon and nitrogen cycle models, researchers found that the estimated land suitable for bioenergy grasses—particularly Miscanthus, the most productive bioenergy crop—is limited, despite its relatively high biomass productivity and low water consumption per unit of ethanol.

Specifically, the most suitable regions to grow bioenergy grasses in terms of impact on water (and ultimately ethanol production) are eastern Ohio, eastern Kentucky, eastern Tennessee, and the Northern Atlantic regions. Miscanthus and Cave-in-Rock are less suitable in areas such as Missouri, southern Illinois, and Mississippi River watershed regions of eastern Arkansas.

Finally, the researchers found that bioenergy crops do best in regions with higher precipitation rates. They are more likely to fail in dryer regions with less frequent and predictable precipitation, such as the Great Plains, where environmental conditions limit production of bioenergy grasses. In the Midwest, on the other hand, the grasses are generally able to withstand periodic dry conditions because their roots can grow toward deeper and moister soil.

The research was supported by the National Science Foundation and the US Department of Energy Office of Science.

Resources

• Yang Song, Matthew C Cervarich, Atul Jain, Haroon Kheshgi, William Landuyt, and Ximing Cai (2016) “The Interplay Between Bioenergy Grass Production and Water Resources in the United States of America” Environmental Science & Technology doi: 10.1021/acs.est.5b05239