"Just to put that in context, global cropland is around 3.8 billion acres. So this is not a small number; it's something like a quarter of the total amount of cropland globally," said Field, who is also director of the Carnegie Institution's Department of Global Ecology at Stanford.
To determine how much fallow cropland there is around the world, they used a combination of satellite imagery and historical maps of agriculture.
Then, they estimated the amount of land that had either been lost to urbanization or had gone back to forest, and reduced the total of available land accordingly.
Land has fallen out of agricultural production for a variety of reasons. In some instances, new technologies or infrastructure made land with better soil available, as when farmers in the eastern United States left their farms for the richer prairie soils of the Midwest.
In other areas, soil erosion may have triggered the abandonment, or soil nutrients may have been depleted through poor farming practices but might still be sufficient to support grasses.
Field said there are three broad categories of crops that have potential for bioenergy: food crops, local native plants and special bioenergy crops such as switchgrass or elephant grass, both of which are hardy enough to grow well on marginal agricultural land.
"These abandoned agricultural lands are distributed throughout the world, in places with a variety of different climates," Campbell said. "So the type of plant species that might give you the most biomass per year would probably depend on the local climate."
Corn is not necessarily the crop of choice for the abandoned lands because some of these areas are prone to erosion, and corn needs intense cultivation and tilling, Campbell said.
"Perennials, crops that don't need as much tilling or cultivation would probably be more advantageous, so you would
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