Taking Measure of Biofuel LimitsSeptember 24, 2009
The current edition of American Scientist has a very good article on the fundamental biological limits of governing the production of biofuels. Taking the Measure of Biofuel limits, by Thomas Sinclair addresses the two obvious limiting factors, light and water, and the perhaps less obvious limiting factor of nitrogen availability in the soil.
Thomas R. Sinclair is a professor in crop science at North Carolina State University with a Ph.D. from Cornell. He specializes in the relationships between plant physiology, the environment, and crop yields. He has edited several books as a Ballard Fellow at Harvard University.
Sinclair sets the stage by pointing out..
The U.S. Energy Independence and Security Act calls for 144 billion liters of ethanol per year in the U.S. transportation fuel pool by 2022. That equals 25 percent of the U.S. gasoline consumption today. No more than about 4o percent is to be produced with maize, an important food and export crop. Non-grain feedstock is supposed to provide the rest.
Before nations pin big hopes on biofuels, they must face some stark realities, however. Crop physiology research has documented multiple limits to plant production on Earth. To ramp up biofuel crop production, growers must adapt to those limits or find ways around them. Such advances may not be as simple a some predict. Plants and their evolutionary ancestors had hundreds of millions of years to optimize their biological machinery. If further improvements were easy, they would probably already exist….
Plants cannot be grown without three crucial resource inputs: light, water, and nitrogen. Each of these inputs is needed in substantial quantities, yet their availability in the field is limited…[T]he close relationship between the available amounts of these resources and the amount of plant mass they can produce – not human demand – will determine how much biofuel the world can produce.
Sinclair considers the conversion efficiency of sunlight and CO2 to sugars, which ultimately fuel the building of starch, cellulose, protein and lipids, for C3 plants (95% of all of Earth’s plants, but not highly CO2 fixation efficient) and the more efficient sugar-making C4 plants (corn, sugar cane and sorghum, for example). He points out that “After hundreds of millions of years of evolution, these systems [for converting solar energy into the chemical energy of sugars] are highly efficient within the physical and thermodynamic constraints of photosynthesis and plant growth.” Not much room has been left for improvement.
Bio-engineering advances may increase yields a little, but they cannot overcome the limits of the sunlight to sugar conversion ratios. After the numbers are crunched he reveals that if the U.S. is to reach its biofuel goal of 58 billion liters of ethanol grown from corn (40 percent of 144 billion liters), it would require an additional 15 million hectares planted. Similarly, the remaining 86 billion liters made from non-corn C4 grasses, which are not nearly as efficient as corn for this purpose, would require at least an additional 48 million hectares.
It may be obvious that in areas of limited water supply, plant growth will be limited by the amount of water available. As plants transpire water out through their leaves, the rate of transpiration is:
T = G x VPD/k
where T is the transpiration (g/m2)
G is the plant growth (g/m2)
VPD is the Vapor Pressure Deficit, or the difference in the saturated water vapor pressure of air inside the plant leaves and the water vapor pressure of the outside atmosphere
k is a plant specific constant
The difference in the vapor pressure inside and outside a leaf (VPD) is what controls the rate of water loss through the stomata. The VPD is large in arid regions because the vapor pressure of the water in the atmosphere is low.
For a given environment the VPD cannot be controlled – it is what it is. So the only way for a plant to affect the transpiration, and thus prevent itself from drying out and dying in a arid environment, is to close down its stomata to reduce water loss. But this also reduces the flow of CO2 into the leaves and O2 out, and consequently reduces or stops the plant’s growth. There is no magic to get around this. Sinclair says…
“Despite claims that crop yields will be substantially increased by the application of biotechnology, the physical linkage between growth and transpiration imposes a barrier that is not amenable to genetic alteration.”
Under these circumstances the plant mass growth is nearly linear with water transpired. So as more arid regions are put into crop use either crop yields per hectare will be lower, or the amount of irrigation will be higher. This leads to the production of biofuels at the expense of aquifer depletion.
Sinclair repeatedly points out that to be economically viable, biofuel crops must yeild at least 9 tonnes of plant mass per hectare of crop. For C3 and C4 type plants this 9 tonne minimum requires the removal of 166 kg and 118 kg or nitrogen per hectare, respectively. But, “Expectations for cellulosic yields are sometimes double or triple the 9-tonne-per-hectare yield” required for economic viability. So, nitrogen removal from the soil will sometimes be double or triple also. Some of this nitrogen is replaced by plant debris that is left behind and some comes from thunderstorms and some from organisms that fix atmospheric nitrogen. But these sources are not enough to replace all the nitrogen that is removed with every harvest, and the available nitrogen will be less every year. Sinclair explains…
“Although this decrease rate is usually small when compared to all the original organic matter in the soil, a cropping practice dependent on a continuous withdrawal clearly is not sustainable… Nitrogen fertilizer of annual biofuel crops will inevitably be needed once soil organic matter decreases to levels limiting plant growth.”
Taking the limits of light, water and nitrogen in to account, for corn he concludes…
“The equivalent of 40 percent of today’s U.S. maize crop will be required to ethanol production while other domestic and export demands for maize also must be met.”
And for cellulosic derived ethanol he concludes that up to…
“50 million hectares of new land must be brought into high and sustainable agricultural production to achieve the required yields… it would be the most extensive and rapid land transformation in U.S. history… [L]and used for cellulosic feedstock must be in regions with sufficient rainfall to achieve needed yields. The amount of water transpired by those crops could be large enough to influence the hydrologic balance of farming regions.”
and for in general…
“I]ncreased nitrogen supplementation required for the new crops will result in more nitrogen leaching into natural waterways…”
My final words
Sinclair indicates that between corn and other plants for ethanol, the U.S. may have to put as much as an additional 65 million hectares into crop production (15 million hectares for corn and 50 million hectares for other biofuel crops) to generate 144 billion liters of ethanol. This would replace only 25% of our gasoline usage.
How big is 65 million hectares? It is the same as 650,000 square kilometers, and about the same as 160 million acres. To put this in perspective, this is more than 10 times the acreage of corn planted in Iowa in 2007. It is more than 150% of the corn acrage planted in the entire United States in 2007.
Look at the figures below. The first image is from the USDA Census of Agriculture for 2002, and it shows the acreage planted in corn for grain in the United States that year. Each dot on the map represents 10,000 acres. To achieve 144 billion liters of ethanol we could need an additional 160 million acres of of corn and other crops, or more than 10 times the amount of corn acreage planted in Iowa. The second figure shows the corn acreage of Iowa multiplied ten fold and added to the map of the United States. This should give you some idea of the unprecedented agricultural multiplication that would be needed to satisfy the U.S. Energy Independence and Security Act.
Let’s face it – this ambitious goal of 144 billion liters of ethanol per year from biofuels is a very bad idea. Our most precious resources are the land, water and resources for making fertilizer (which is primarily natural gas for nitrogen fertilizers). The dumbest thing we can do is deplete our soil and aquifers, pollute our water with extra nitrogen fertilizer, and waste our natural gas to make ethanol. If you think living with a shortage of gasoline is rough, try living with a shortage of food.