Environmental Engineering Reference
In-Depth Information
because of its high density, is easier to store and transport than stover; (2) ethanol from grain
starch is produced close to the theoretically predicted efficiency (Patzek 2006); (3) grain ethanol is
already a mature industry whereas cellulosic ethanol is still several to many years away from being
commercially profitable; and (4) conversion of grain to stover will also mean more total lignin,
assuming its concentration in the stover does not change, which will more than offset the expected
biomass gain from converting protein and oil into carbohydrates (see below).
Comparative bioenergetic costs associated with constructing tissues of different compositions can be
used in estimating the amount of biomass that can be produced from a given amount of photosynthate.
It was estimated from the underlying biochemical pathways that 1 g of photosynthate could be used to
make 826 mg of storage or structural carbohydrates, 465 mg of lignin, 404 mg of protein, or 330 mg
of lipid (Penning de Vries et al. 1974; Sinclair and de Wit 1975; Bhatia and Rabson 1976).
Maize varieties adapted to tropical regions when grown in temperate zones may not flower and
keep producing vegetative biomass, resulting in increased biomass production. However, energy-rich
lignin in stover would negate most of the expected gain in biomass from the elimination of oil and
protein in the form of grain. For example, bioproductivity of a typical maize grain is approximately
720 mg/g photosynthate (Figure 16.11) (Dhugga and Waines 1989). Similarly, for maize stover of a
typical composition, bioproductivity would be 704 mg/g of photosynthate (Figure 16.11). Additional
biomass gain can be realized from a slightly prolonged growing season because of an expected
delay in leaf senescence and hence continued photosynthesis from the lack of a strong N sink in
the form of grain. Reducing lignin, which has been difficult to accomplish because of undesirable
pleiotropic effects, can further aid in increasing biomass in stover-only maize. For example, a
reduction in lignin content by 50% (from 18% of dry mass to 9%) may produce an additional 6-7%
biomass (Figure 16.11).
108
Lignin
106
Protein
104
Oil
102
100
98
96
94
92
-50
-30
-10
10
30
50
% Change
FIGure 16.11
Predicted effects of altering oil or protein (grain) or lignin (stover) on bioproductivity in
maize. Starting composition for grain was taken as 90 g/kg protein, 40 g/kg oil, 20 g/kg ash, and (by subtrac-
tion) 850 g/kg carbohydrates (starch and fiber). The initial composition for stover was 730 g/kg carbohydrates,
180 g/kg lignin, 40 g/kg protein, and 50 g/kg ash. Replacement of lignin, oil, or protein only by carbohydrates
is assumed. Although oil requires the most energy for its synthesis, relatively steeper slopes for lignin and
protein are due to their higher initial amounts in the respective tissues. (Adapted from Dhugga, K.S.,
Crop
Sci
., 47, 2211-2227, 2007.)
