In a recent Viewpoint on biofuels (BioScience 59: 926–927), Tom Simpson addressed important shortcomings of corn ethanol production: low net energy gain, competition with food production, and the loss of nitrogen from fertilizers and its environmental consequences. Simpson proposed an agricultural landscape that could produce food, feed, and fuel while improving water, air, and habitat quality by relying more on perennial biomass. However, other environmental problems are related not only to ethanol production but also apply to production of perennial biomass.
To call anything a sustainable biofuel strategy requires specifying a time frame. If the chosen time frame is short enough, anything becomes sustainable and degenerates to an oxymoron (Flueck and Smith-Flueck 2006). Generally accepted sources put US reserves of easily mined phosphorus deposits as sufficient to last for another 25 years at current application levels. Corn grain harvest, though, removes about 7.2 kilograms (kg) of phosphorus (P, expressed as P2O5) per metric ton of corn grain, and 1.64 kg per metric ton of corn silage. Any crop will, however, remove P and thus necessitate replacement.
Twenty-five years is a mere blink of time. China, having a major deposit of easily accessible P deposits, has very recently taken measures to prevent P leaving China, for obvious reasons. Since the year 2000, P prices have increased several-fold, and “peak phosphorus” is commonly mentioned.
Another consequence of biomass export is soil acidification, which requires corresponding countermeasures such as liming. As inputs necessary for biofuel production are becoming scarcer, their further production in itself will require more external energy (as with P), or compete with easy energy sources (natural gas for nitrogen fertilizer). If corn ethanol production does not pass the sustainability test regarding the environment according to Simpson, I posit that neither can perennial biomass pass the test.
Switchgrass removes about 4.55 kg of P per metric ton, and because it leads to higher biomass export than corn grain, it also results in greater P loss. Moreover, others have pointed out that it requires much more pesticide than corn, and considering the initial two years of no harvest and declining yields after a few years, corn yields outperform switchgrass by a factor of 2 or more.
Overall, it is not appropriate to label biofuels as sustainable alternatives, as if we can get something for nothing (i.e., as if it involves only solar energy conversion). Given the mentioned constraints, in addition to several others (Flueck and Smith-Flueck 2006, Flueck 2009), it would be more precise to label biofuels as temporary solutions to give us time to rethink the current level of dependence on energy.