Environmental Engineering Reference
In-Depth Information
biosphere's most abundant aromatic polymer has a carbon content ranging between
about 61% and 66% (Wald, Ritchie, and Purves 1947). Lamlom and Savidge (2003)
and the USDA (2010) offer the following information for the heartwood of 41
North American trees. The carbon content of absolutely dry wood is between 46%
and 50% (average about 48%) for hardwoods (the lowest value is found in birches)
and between 47% and 55% (average about 51%) for conifers (with the highest
value for redwoods). The differences are due to a higher lignin content of softwoods.
In addition, the carbon content of early wood is consistently a few percentage points
higher than that of late growth, which contains more cellulose.
Carbon is signii cantly lower in marine phytomass because of the latter's often
high mineral content. Diatoms, silicol agellates, and radiolarians use Si(OH) 4 (silicic
acid) to build their intricate opal (hydrated, amorphous biogenic silica, SiO 2
0.4H 2 O)
structures, and their dry-matter carbon content can be as low as 20%. Similarly,
nannoplanktonic coccolithophores use calcite (CaCO 3 ) to build their exoskeletons.
The generic mean of 50% C in phytomass may be acceptable for very large-scale
approximations of terrestrial phytomass but will have errors of 4%-5% when
applied to specii c tree stands, and it will exaggerate the carbon content of crops
and their residues (by about 10%) and, even more so, of marine phytoplankton (by
as much as 50% or more).
Although this topic is concerned with harvesting plants and animals, I should
point out (given that bacteria make up the bulk of the biosphere's mass) that particu-
larly large errors can be introduced by converting microbial biomass to carbon
equivalents. Traditionally, it was assumed that the dry-matter content of bacterial
cells is about 20%, but Bratbak and Dundas (1984) examined three common strains
( B. subtilis , E. coli, and P. putida ) and found that the real value may be more than
twice as high, ranging mostly between 40% and 50%, and hence they recommend
converting bacterial volume to carbon content by assuming an average of 0.22 g/cm 3 .
Autotrophic and heterotrophic species have been selected for harvest by humans
because of exceptionally high proportions of individual biopolymers or because of
their specii c combinations. Prominent food examples include cereals, selected for
their combination of high carbohydrate (typically more than 70%) and a relatively
high (7%-15%) protein content; wheat, selected for its uniquely high content of
gliadin and glutenin, proteins whose pliability makes leavened breads possible;
soybeans, for their unmatched percentage of protein (typically around 35%); sun-
l ower seeds or olives, for their high lipid shares (respectively almost 50% and about
20%); and meat, for its combination of high-quality protein (around 20% of fresh
weight) and saturated fat (on the order of 40% in many cuts of beef), whose inges-
tion provides a feeling of satiety.
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