Environmental Engineering Reference
In-Depth Information
Less than 5% of all stover is regularly harvested as cattle feed, and the rest is
recycled. Stover availability l uctuates with yields by as much as 20%, and Pordesimo,
Edens, and Sokhansanj (2004) argued that its typical fresh-weight supply is 20%
less than the total resulting from a commonly assumed 1:1 stover:grain ratio.
Another common assumption is that 3-4 t of stover could be harvested per hectare
on a sustainable basis, but Blanco-Canqui et al. (2007) found that stover harvesting
at rates above 1.25 t/ha changes the hydraulic properties of soil and has a pro-
nounced effect on earthworm activity (complete removal cut the number of earth-
worm middens by 95%).
A fairly conservative approach is to assume that with conventional tilling, about
35% of stover could be removed without adverse effects, and that the rate rises to
about 70% for no-till farming. This would give a weighted national mean of about
40%, and it would mean that about 80 Mt (in dry weight) of stover could be removed
annually, a theoretical equivalent of roughly 20-25 GL of ethanol—worth no more
than 3% of the U.S. gasoline consumption in 2010. And using phytomass-based liquid
biofuel is even more questionable given the utter lack of system appropriateness: the
negative consequences would be easier to accept if the fuels were used in highly efi -
cient vehicles (in excess of 50 mpg), not by a l eet that now averages less than 30 mpg.
A combination of environmental, agronomic, social, and economic impacts thus
makes modern liquid biofuels a highly undesirable choice. Resource constraints (the
availability of farmland, competition with food crops) mean that even their large-
scale production would have a marginal effect on the overall supply. The production
of cellulosic ethanol would use relatively abundant crop residues, but they are often
difi cult to collect and expensive to transport and store, and, once gathered, their
structural cellulose and lignin are not easy to break down to produce fermentable
sugars. Moreover, crop residues are not useless wastes whose best use is an enzy-
matic conversion to ethanol; they are an extremely valuable resource that provides
a number of indispensable and irreplaceable agroecosystem services that range from
the recycling crop of the three macronutrients as well as many micronutrients and
the replenishment of soil organic matter to the retention of moisture (through their
spongelike action), and prevention of both wind and water erosion. Consequently,
any excessive harvesting of this only apparently waste phytomass is inadvisable.
Wood as a Raw Material
Crop residues incorporate more than half of the world's crop phytomass, and their
traditional off-i eld uses for feed, bedding, and household fuel remain widespread.
