Saturday, April 04, 2009

Carbon negative Biofuels: 2 - Polyculture of wild grasses

Illustration - The wild prairie grasses have abundant and deep roots reaching 3 to 4 meters under the surface. (source: United States Department of Agriculture, the depth in meters was added by the author of this blog)

With petroleum fuels, one takes carbon that was underground, burns it and constantly increases the CO2 content of the atmosphere. The idea behind biofuels is not to emit CO2 from carbon sequestered in geological formations (géocarbone) and use in place of biocarbon found in plants. Biocarbon enters in what is called the carbon cycle, where the carbon we send into the atmosphere by burning biofuels is reabsorbed by plants that grow to produce biofuels. It does not constantly adds to the atmosphere if we do not use fossil fuels to produce biofuels. In this ideal case we say that biofuels are carbon neutral.

This ideal case is not achieved in practice and net CO2 emissions are also associated with biofuels, but at lower levels than CO2 emissions from petroleum fuels. This reduction is only 20% for ethanol produced from corn, and some even say it is zero if one takes into account the greenhouse gas involved in the fabrication of the machinery. Add to this the problems of land degradation and water pollution caused by fertilizers and pesticides, and we understand why many environmentalists do not like biofuels.

However, second-generation biofuels have the potential to reduce greenhouse gas emissions by 80% to 90%, using the whole plants instead of only grains and fruit, as it is currently the case. But the problems of degradation and soil erosion by intensive monocultures must also be taken into account.

To solve these issues, in the last few decades researchers have studied the advantage of wild tall prairie grasses, as switchgrass. First, these plants are perennials, they do not need to be sowed each year, such as corn or soybeans. In addition, these grasses have well-developed and deep root systems (illustration at top of post). With these two features, the wild prairie grasses protect soil from erosion, rather than increasing it as do the intensive in row monoculture of annuals.

Furthermore, wild grasses need not be watered because their roots are very efficient to retrieve soil moisture, 3 to 4 meters deep. By comparison, corn requires often watering with several hundreds of liters of water per liter of ethanol produced.

But the cultivation of wild grasses becomes particularly interesting when it is grown in a mixture, including plants that fix nitrogen. This was studied by researchers at the University of Minnesota for 10 years on degraded land. They cultivated 152 different parcels of land containing different mixtures of up to 16 different plants in the same plot. The astonishing results of their study were published in 2006 (Tilman, Hill and Lehman, Science, Vol. 314, December 8, 2006, pages 1598 to 1600).

First, the quantities of fertilizers and pesticides required are significantly reduced compared to corn and soybeans, as shown in the chart below, taken from their publication. The word "Biomass" in this graph represents the high diversity mixture of 16 different plants.

Now the surprise is that second generation biofuels from such high diversity prairie grasses mixture are strongly carbon negative! This means that in addition to avoid net emissions of CO2 in the atmosphere (carbon neutral), these cultivations are literally removing CO2 from the atmosphere to reduce its concentration. The reason is simple, the carbon is stored underground in large quantities in the roots.
It's a bit like charcoal in the Terra preta, buried by the Amazon natives (see previous post).

However, to get stong carbon negative biofuels (-150% to -250%), we must cultivate several plants together. For example, plots with a mixture of 16 plants store 31 times more carbon in the soil than in monoculture plots!

No, definitely, biofuels of tomorrow will have nothing to do with those of today, hence the importance of not throwing the baby out with the bathwater. Sustainable development of biofuels is entirely feasible, if done intelligently and if we produce only small quantities.

In my last book ”Driving without oil”, I demonstrate that cultivating energy crops to produce biofuels equivalent to 5% of current petroleum fuels would be sufficient to stop oil consumption in road transport. The electricity networks would, of course, provide the main ”fuel”. One would also use municipal waste, forest residues and recycled oils and fats from the food industry to produce biofuels, which can easily supply the equivalent of 2.5% of current petroleum fuels, for a total of 7.5% in biofuels (including dedicated energy crops).

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