Agriculture Reference
In-Depth Information
The interactions between elevated [CO
2
] and mineral nutrition may be markedly
different for different nutrients: in contrast to N, where critical nutrient concentra-
tions are reduced (Fig.
9.2
), critical concentrations of P generally remain the same
or are increased when C3 plants are grown under elevated [CO
2
] (Ghannoum
et al.
2007
). Increases in critical P concentrations of C3 plants grown under elevated
[CO
2
] have been attributed to changes in competition for P. As photosynthetic
carbon fixation is preferred over the photorespiratory cycle, more P is needed for
the energy carrier ATP (Ghannoum et al.
2007
). Similar to N, several studies have
noted that growth (and yield) response to high [CO
2
] depends on P nutrition. The
relative importance of this interaction between P and high [CO
2
] however varies
with species. Lam et al. (
2012c
) found that high [CO
2
] increases biomass of two
pulse species, chick pea (
Cicer arietinum
) by 18-64 % and field pea (
Pisum
sativum
) by 24-57 %, as well as the pasture legume barrel medic (
Medicago
trunculata
), and that this effect was greater when P supply was non-limiting.
Proposed Mechanisms for Decreased Nutrient
Concentrations in Elevated [CO
2
]
The mechanisms responsible for and controlling decreased nutrient concentrations
under elevated [CO
2
] are not completely understood, and there are a number of
hypotheses under consideration. Most of these hypotheses are connected to studies
conducted on N but, depending on the nutrient, similar principles may apply for
other nutrients. An overview is given in Fig.
9.5
.
Dilution by Increased Biomass and Carbohydrate Production
CO
2
enrichment increases C fixation and dry matter accumulation in plants.
Increased dry matter is mostly derived from greater carbohydrate (C, H and O)
accumulation and concentrations of other macro and micro nutrients will decrease
in biomass if their uptake is not increased (Taub and Wang
2008
). For example,
Poorter et al. (
1997
) investigated the chemical composition and construction costs
of leaves of 27 wild and agricultural species at ambient and elevated [CO
2
]. They
found that the strongest response of plants to CO
2
enrichment in respect to their
chemical composition is the increase in concentrations of total non-structural
carbohydrates (TNC) resulting from stimulated photosynthesis under high
[CO
2
]. The next strongest responses to CO
2
enrichment were decreases in protein
and mineral concentrations leading the authors to conclude that a dilution effect
caused by accumulating TNC significantly contributes to decreasing nutrient con-
centrations under CO
2
enrichment. Similar conclusions were made by Taub and
Wang (
2008
), who used graphical vector analyses to study biomass dilution effects
under CO
2
enrichment.
