Agriculture Reference
In-Depth Information
(Sinclair et al.
2000
). This is in line with the suggestion that photosynthetic
downward acclimation, which is related to leaf N and (Rubisco) protein, is less
pronounced or absent in well-fertilised plants. However, recent synthesis papers
suggest that even if high N supply counteracts decreases in N to some extent, N
m
remains significantly lower in leaves (Wang et al.
2013
) and grains (Taub
et al.
2008
).
Whilst decreases in tissue concentrations of N are reasonably consistent and well
established, results on changes of overall N demand and uptake of the crop per m
2
ground area are variable. N uptake into (above ground) biomass depends on the
relative magnitude of biomass increase and decrease in N tissue concentrations.
Most analyses suggest that biomass and yield stimulation are relatively greater than
the decrease in tissue N concentration, which would lead to greater N demands of
bigger crops under CO
2
enrichment (Lam et al.
2012a
,
b
,
c
) despite decreased N
concentrations in tissues. However, others suggest
that N uptake remains
unchanged (Wang et al.
2013
).
It is noteworthy that the plant growth response to elevated [CO
2
] in natural
ecosystems, which do not receive extra N input, may decrease over time, and this
was ascribed to a depletion of N (and other nutrient) reserves (
progressive
ΒΌ '
nitrogen limitation
(PNL) hypothesis, e.g. Hungate et al.
2003
).
'
Adjustment of N
mcrit
Under CO
2
Enrichment
It has been suggested early on that critical tissue concentrations of N (and other
mineral nutrients) must be adjusted under high [CO
2
] (Conroy and Hocking
1993
;
Fig.
9.3
). Tissue concentrations of nutrients such as N are often used as a diagnostic
tool to assess a plant
s nutritional status. Critical tissue (such as leaf) concentration
of N (N
mcrit
) would indicate sufficient N supply. N
mcrit
is the value of N
m
that is
needed for optimum growth, assuming otherwise non-limiting conditions. At leaf N
greater than N
mcrit
growth will not increase further (Fig.
9.3
). Conroy and Hocking
(
1993
) suggested that critical concentrations need adjustment towards lower values
under high [CO
2
] (Fig.
9.3
). As such results were derived from pot experiments,
field evaluation in cropping systems will be important.
'
Does CO
2
Enrichment Change N Response of Plants?
Similarly to other essential nutrients, insufficient N supply to crops can limit the
[CO
2
]-driven growth and yield enhancement; N-limited crops sometimes do not
show a significant effect of CO
2
enrichment on biomass, or the relative yield and
biomass increase is less than in well fertilised crops (Stitt and Krapp
1999
). As a
consequence, the N response curve may change under elevated [CO
2
]. N response
curves (Fig.
9.4
) are derived from fertiliser experiments (fertiliser added at different
