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Root systems of annual wheat (on the left in each panel)
and intermediate wheat-grass, a perennial, at four times
of the year. Although roughly 25% to 40% of the wheat-grass
root system dies off and must grow back each year, its
longer growing season, and consequently greater access to
resources, results in greater above- and below ground
productivity than its annual counterpart. If carbon in the atmosphere is our problem then we should look to plants as one of the solutions. Not just to provide energy, but to reverse the greenhouse effect. After all, plants consume carbon dioxide (CO2). The carbon goes into above ground biomass, which includes the parts we and our livestock eat such as fruits, vegetables, grain, and leaves. If we are to harvest above ground biomass and use it as biofuel feedstock, the roots are obviously the business end of carbon sequesteration. Long-lived perennials have excellent capacity for this as seen in this image from The Land Institute showing relative below ground biomass of perennial and annual grasses.
Other viable sources of next generation energy may be carbon neutral, but they are not carbon negative like biofuels have the potential to be. They are also perhaps unique in that there is great potential to go the other way. Agro-energy moves towards carbon neutral and even positive when (1) land such as forests that already sequester carbon are converted to farmland - Deforestation has contributed some 20% of the increase in anthropogenic CO2 over the past decade - and (2) agricultural inputs that include diesel fuel and nitrogen fertilizer offset the gains.
Back to discussing the work of David Tilman's group at the University of Minnesota where they demonstrated a positive relationship between species diversity and biomass yield, i.e., mixtures of sixteen prairie species out yielded plots with lower diversity. A similar relationship was observed for root and soil carbon sequestration. How fantastic is this? The biofuel feedstock removed CO2 from the atmosphere and put it below ground. This is exactly what we are shooting for. Not only does this action ameliorate the greenhouse effect, it increases soil tilth. The long term experiment leading to these results was conducted on severely degraded farmland, which is the type of areas targeted for production of biofuel crops so as to not interfere with food production. In theory, within 50 to 100 years, those acres will have 'recovered' and be useful for food crops. The amount of energy produced by biofuels will not be sufficient to replace the use of fossil fuels for transportation, but it can replace some of it. The amount of CO2 placed below ground from biofuel crop production alone will not launch us to 450ppm, but it will be a step in the right direction. The aspects of the biology appear to be sound, but not resistant to idiot behavior, so let's continue the research and development of policy.
