Agriculture Reference
In-Depth Information
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n D e r s t a n D i n g
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Many experiments were conducted during the 1960s, 1970s, and early 1980s to learn
more about the effects of elevated CO
2
on plant growth and development. Kimball
(1983) reviewed more than 70 greenhouse and growth chamber studies of the effects
of CO
2
enrichment on the yield of 24 agricultural crops and 14 other species. In spite
of data limitations, the meta-analysis concluded that crop yields could increase 33
± 9% if the atmospheric CO
2
concentration were to double from 330 to 660 ppmv
(parts per million by volume).
In 1990, a new technology was implemented in Arizona to study the response of
crops to elevated CO
2
under field conditions and, eventually, the terrestrial feedback
of CO
2
exchange with the atmosphere (Hendrey and Kimball, 1994). The technol-
ogy, called FACE (free-air carbon dioxide enrichment), allowed plants to grow in the
field in microclimates with a CO
2
level of 550 ppmv, which was expected to prevail
around the middle of this century. Yields of irrigated cotton grown under elevated
CO
2
were 43% greater than those grown under current ambient conditions (Mauney
et al., 1994). The FACE treatment also increased WUE. Quickly, the FACE technol-
ogy was applied to other crops and vegetation types, including grasslands, forests,
and arid shrublands. Furthermore, the studies not only covered aspects of yield and
biomass accumulation but also a range of ecosystem properties (e.g., water balance,
carbon cycling).
Conditions of elevated CO
2
are expected to interact with many other factors, such
as temperature, availability of water and essential nutrients. Thus, Amthor (2001)
reviewed 50 studies on the effects of elevated CO
2
on wheat. The studies reviewed
revealed that wheat yields increased by 31% when CO
2
concentrations doubled from
350 to 700 ppmv, with increased yields due mainly to an increase in ear density.
Relative to ambient conditions, the
harvest index
(HI; defined as the fraction of plant
biomass harvested due to its alimentary or economic value) of wheat under elevated
CO
2
tended to increase under water stress conditions and decrease with nutrient limita-
tions. Increased yields due to elevated CO
2
, however, may reduce grain quality due to
lower protein content and inferior bread-making qualities. These are very relevant find-
ings since, as Amthor (2001) pointed out, wheat contributes on average about 20% of
the calories and about 22% of the proteins consumed daily by the world population.
In addition to the negative impacts caused by warmer temperatures and water
stress, crops exposed to elevated levels of tropospheric ozone (O
3
) may not real-
ize the beneficial effects of CO
2
enrichment due to the counteracting effect of this
phytotoxic molecule formed at ground levels by industrial activities. Wheat and
maize plants responded differently when grown in open-top chambers under ambi-
ent and elevated CO
2
(500 ppmv) with and without 40 ppbv (parts per billion by
volume) of O
3
(Rudorff et al., 1996). Under ambient CO
2
conditions, wheat grown
under elevated O
3
yielded 23% less than wheat grown under ambient levels of O
3
.
Correspondingly, under elevated CO
2
, wheat yielded only 9% less when exposed to
elevated concentrations of O
3
. Similar results were obtained with maize, suggesting
that the stoma-closing effect of elevated CO
2
could ameliorate the inhibitory effects
of elevated O
3
on photosynthesis.
