Environmental Engineering Reference
In-Depth Information
spring-planted grains, and ending in October and November for many root and
vegetable crops.
A second reason is that the global multicropping ratio is now close to 1.5, ranging
from single crops harvested annually in high temperate latitudes (such as Canadian
or Russian spring wheat) to triple-cropping (and for vegetables even harvesting as
many as i ve crops) in some of the world's most intensively cultivated regions in
China, Indonesia, or California. As a result, the maximum global preharvest phyto-
mass (during the late summer in the Northern Hemisphere) is at least a third smaller,
no more than about 5 Gt (2.5 Gt C), a small fraction of the terrestrial phytomass.
Phytomass that is directly consumed by humans was about 1.2 Gt, while the phy-
tomass consumed by animals—about 1.2 Gt of feed crops and crop-processing resi-
dues, 1.2 t of forages, and 1.6 Gt of crop residues—added up to 4 Gt of dry matter.
Fiber crops and their residues incorporated only about 50 Mt of phytomass.
For comparison, Haberl et al. (2006) estimated the global crop harvest for the
years 1992-1994 at 6.28 Gt of dry matter, composed of 2.53 Gt of crops intended
for human consumption, 1.04 Gt of animal forage crops, and 2.71 Gt of crop
by-products, and Wirsenius (2000) presented the following global l ows (all totals
also in dry weight) for the years 1992-1994: food crops, 2.54 Gt (dominated by
1.65 Gt of cereals); their by-products, 3.46 Gt; animal forages, 1.15 Gt; and 5.82
Gt of feed from pastures. Imhoff and Bounoua (2006) calculated that in 1995, 4.09
Gt C (median estimate, with the low-high range of 2.83-5.85 Gt C) of phytomass
were required to produce the global food supply: 1.73 Gt C were in crops destined
for direct food consumption and 2.36 Gt C in phytomass for the production of
animal foods, with meat accounting for just over 80% of the phytomass used for
feeding.
All of these totals (with the obvious exception of root crops) refer to the above-
ground dry matter of harvested crops and their residues. To express them as total
crop phytomass, they should be enlarged by 20% for annual cultivars, and approxi-
mately doubled for all perennial forages. This aggregate would indicate the total
standing phytomass at (or shortly before) the time of harvest, but it should not be
used in any accounts of human appropriation, as the root biomass is usually com-
pletely recycled during regular plowing of annually planted i elds or is left untouched
for years to decades in no-till i elds and in multiyear or perennial cover crops.
My reconstructions show that the total phytomass produced by American crop-
ping tripled during the twentieth century, with more than 75% of the gain taking
place after 1950 and more than 40% of it realized after 1975. The country's
phytomass output per hectare of cultivated land increased about 2.4 times, from
