Tuesday, December 12, 2006

Carbon-negative biofuels: too good to be true?

A paper in this week’s edition of Science reports that low input high diversity (LIHD) biofuels may offer a carbon negative energy solution. Let’s define some terms to make sense of that statement. By low input, the authors mean that fields require minimal or no irrigation, pesticides, or fertilizers. In contrast, current biofuels (corn and soy) require all three inputs, which take energy and create pollution. High diversity means that plots contain a mixture of species, probably between 8 to 16 for maximum efficiency, rather than a monoculture, the conventional mode of agricultural production. LIHD biofuels are carbon negative because over time they sequester carbon dioxide in the soil, whereas monocultures are carbon neutral or even carbon positive. It gets better. Unlike corn and soy which must be grown on fertile soil, directly competing with food production, LIHD production can take place on degraded agricultural land. This means that not only do LIHD biofuels not compete with food production, but they don’t necessite additional habitat destruction. I wonder if LIHD biofuels might cure cancer too?

Besides the obvious practical implications, there is some really interesting ecology behind this study. The research was led by ecologist David Tilman and is an extension of research in a sub-discipline called Biodiversity and Ecosystem Function (BEF). Researchers in this field have shown that, more or less, biodiversity improves the health of an ecosystem, often measured as the amount of biomass production. I should note that, due to their complexity (e.g. What does ecosystem function really mean? and, How do you measure it in a naturally meaningful way? are not simple questions), results from the field are often contentious. Nonetheless, the emerging synthesis is that biodiversity really does improve ecosystems in nature. Most BEF experiments, the present study included, involve sowing many fields with varying diversities. In this study, they used fields with either 1, 2, 4, 8, or 16 plant species. Why does biodiversity improve biomass production? The BEF literature suggests several possible, non-mutually exclusive possibilities that are worth mentioning here:

  1. Resource partitioning - Imagine a forest. If every tree were exactly the same height, they would all compete for sunlight at the canopy. Inevitably, some sunlight would get through the canopy, but there would be no trees there to capture it. If instead, trees came in diverse heights, they could partition the sunlight resource, some utilizing light that passes through the canopy. Similarly, fields with higher species diversity can parition light, as well as many other resources, compared to a monocuture.
  2. Avoiding pests – Many herbivores and pathogens specialize on a single species, population, or even genotype. In a field with 16 species, a pest may be able to adversely affect one species, but it is very unlikely it would be able to harm all 16 simultaneously.
  3. Temporal variance – Weather and other conditions vary day to day and year to year. A single species may do well at one time, but quite poorly at another. With 16 species, each reacting somewhat differently to changes in the environment, it is likely that some species will be doing well at all times.
  4. Functional diversity – Not all plant species are created equal. Legumes, for example, are a functional group of plants that can transfer atmospheric nitrogen into soil nitrogen which can be used a fertilizer. A diverse field is more likely to have more functional groups represented.
The authors do not actually investigate specific mechanisms in this case (or at least they were not reported), but their results are highly suggestive. For example, high production occurred without the use of pesticides, likely because no single pest could seriously affect all 16 species at once. Their inclusion of legumes in the study likewise obviated the need for nitrogen fertlizers.

What does this study mean for energy production on a massive scale? The authors estimate that using only degraded agricutural land, LIHD biofuels could reduce carbon emissions by 15%. Not bad. Are there reasons to be skeptical? Certainly – as the old adage goes “If it sounds too good to be true, it probably is.” There are many steps that go into industrial energy production. A major obstacle at any one of them could doom a potential new energy source. On the environmental side of things, unlike commercial corn and soy bean, LIHD fields would presumably contain semi-natural communities that could interact and interbreed with surrounding communities, potentially producing unforseen consequences. In any case, these are simply reasons to be cautious, not to abandon any prospect of carbon negative biofuels. At the very least, the present study presents a strong case for spending some fraction of the billions allocated annually to corn and soy subsidies on research and development of LIHD biofuels.

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