Environmental Engineering Reference
In-Depth Information
3.2 t/ha in 1900 to almost 7.8 t/ha in the year 2000. The productivity of cereal
phytomass more than tripled, from about 3.4 t/ha to 10.6 t/ha, and hay productivity
doubled, from 2.7 to 5.5 t/ha. Lobell et al. (2002) used a biogeochemical model
driven by satellite observations to estimate the productivity of U.S. agriculture
between 1982 and 1998, and their results (expressed in carbon units) are in close
agreement with my calculations. The total NPP for 48 states was 620 Mt C/year,
or about 20% of total NPP, and there was an average annual increase of nearly
4 Mt C during the studied period. With annually harvested area averaging 126 Mha,
this prorates to nearly 5 t C/ha, with rates ranging from 2.5 t C/ha for Kansas wheat
to 6.5 t C/ha for Iowa corn.
Similarly, China's total crop phytomass expanded 3.2-fold during the twentieth
century. Cereal phytomass accounted for nearly 85% of the total in 1900 and, after
a 2.7-fold increase in the production, its share was down to just over 70% in 2000.
The share of oil crop phytomass doubled and that of sugar phytomass rose more
than sixfold. Vegetables and fruits recorded remarkable gains after 1980 under
privatized farming, while the cultivation of green manures declined sharply as syn-
thetic fertilizers replaced the traditional practice of rotating leguminous cover crops
with food or industrial crops. The total phytomass productivity was almost 11 t/ha
in 2000, when China's multicropping index stood at 1.2. By the year 2000, French
agriculture produced 2.5 times as much cereal phytomass as it had in 1900, and
the total phytomass production had expanded 2.2-fold. Notable trends included
halving of the tuber phytomass, a large expansion of the legume (nearly 10-fold)
and oil crop (about 50-fold) phytomass output, and a declining hay harvest. The
aggregate French crop phytomass productivity rose from about 3.8 t/ha of cultivated
land in 1900 to 10 t/ha in 2000, a 2.6-fold increase.
But it would be wrong to put all crop phytomass in the category of human
appropriation because not all crop residues are removed from agroecosystems: a
considerable share remains available in i elds for microbes and other heterotrophs.
Quantifying the postharvest l ows of the residual phytomass is very difi cult, but
rough approximations indicate the extent to which the harvested phytomass is not
removed from (or, to a lesser extent, is returned to) i elds. The estimate will obvi-
ously be most inl uenced by apportioning the fate of cereal crop residues, whose
eventual fate ranges from direct recycling and in situ burning to removal for feed
and bedding, household fuel, and construction material.
The Intergovernmental Panel on Climate Change estimated that in low-income
countries, about 25% of all residues were burned, and that the corresponding share
in afl uent nations was just 10% (IPCC 1995). Actual rates are higher, especially
