Last year at the Annual General Meeting of the Consultative Group on International Agricultural Research I don’t believe I heard the word “biomass” once. This year, along with its kissing cousin “climate change,” it was a cold sore on everybody’s lips. For whatever reason—and you can think of as many as I can—these are ideas whose time seems to have come. And by a strange coincidence, two fascinating papers in this week’s Science (as ever hard to access in full for non-subscribers) tackle biomass and bio-energy head on. (They’ve had quite some coverage, but I thought it worthwhile adding a slightly different angle.)
Bio-energy is a complicated business, that’s for sure. Quite apart from the science, of which more in a minute, there are rafts of policy implications and skeins of ramifications. To take just one. Governments may find a subsidy for biomass more palatable than a subsidy for food. It’s the same stuff, right—maize, sugarcane, wheat—but if you pay farmers to grow it for energy, you aren’t paying them to grow it for food and that might be globally acceptable even though it costs as much and keeps food prices as high.
All this means that well-rounded and non-dependent expertise on biomass, bio-energy and biofuels is pretty thin on the ground. But that is no reason not to explore, so lets start with some of that science.
David Tilman and his colleagues at the University of Minnesota headline their paper “Carbon-negative biofuels from low-input high-diversity grassland biomass”. The inportant bits are “carbon-negative” and “high-diversity”. First, high diversity.
Back in 1994 Tilman’s group planted a series of degraded prairie plots with 1, 2, 4, 8 or 16 species derived at random from the normal occupants of the prairie. Since then they’ve shown by careful measurement and in a series of papers that the more diverse plots—regardless of the actual species on the plot—are more productive and less variable from year to year than low diversity plots. The amount of dry plant matter above ground and in the perennial root systems is considerably higher. Measuring energy as simply the amount of heat released by burning the dried plants, the 16-species plots produced almost 2½ times more energy (238%) than monocultures.
Relative to food crops diverted into energy, such as ethanol from maize or biodiesel from soybeans, the impact is even greater, because annual crops occupy good land, they are often irrigated and require cultivation with machinery that uses fuel, and the fertilizers and pesticides they need also require energy to make and transport. Put the best possible conversion technology to work on maize, soybeans and prairie biomass, and the prairie produces more than 50% more energy than maize and almost 75% more than soybeans.
Add in the fact that low-input high-diversity prairie does all that on degraded land, of which there is plenty, and you have a rather nice advance on current bio-energy systems. Better yet, as the paper notes, bio-energy is carbon-negative. In addition to substituting for fossil fuels, and thus decreasing their contribution to greenhouse gases, diverse prairies go further and store additional carbon in their roots. Monocultures of prairie species add almost no carbon to the soil from year to year. The 16-species diverse plots sink 4.4 Megagrams (that's 4.4 tonnes in old money) per hectare per year into roots and soil, and can do so year on year for at least a century.
Most biofuels are not, in fact, carbon-negative. Their production and use actually adds some carbon dioxide to the atmosphere, the exact amount depending on the species and how its energy is extracted. Looking at the ratio of energy in to energy out, ethanol from maize gives just 1.25 times more energy than it uses. Turning prairie biomass into ethanol gives almost 5.5 times more energy than it uses. Making synthetic transport fuel from biomass offers 8 times more energy than it uses.
One can play with the figures in many different ways, but all point in the same direction. Highly-diverse prairie species are a far better option as a feedstock for bioenergy than anything else on the horizon. Taking an admittedly optimistic view, Tilman and his colleagues reckon that biomass grown on agriculturally degraded and abandoned land could supply “13% of global petroleum consumption for transportation and 19% of global electricity consumption”. Even without carbon sequestration in roots and soil, that represents 15% of current global carbon-dioxide emissions.
Like I said, though, bioenergy is a complicated business; next time, practicalities and policies.
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