Agriculture Reference
In-Depth Information
Fig. 9.5 Schema of plant based mechanisms that potentially contribute to decreases in mineral
nutrient (symbolized by
black circles
) concentrations in crops grown under elevated
[CO
2
].
1
Dilution in increased biomass by increased carbohydrate supply.
2
Decreased mass
flow due to decreased stomatal conductance.
3
Changes in root architecture and function. More
root mass in top soil may improve access to nutrients, but this may come at a cost of access to
deeper layers. Uptake physiology may change.
4
Decreased rate of nitrate (and possibly sulfate)
reduction.
5
Adverse changes in remobilisation from leaves and translocation to grains
Biomass dilution, however, does not entirely account for the total decrease in
nutrient concentrations. For example, when Poorter et al. (
1997
) cross-checked
decreasing nutrient concentrations for their direct dependence on increased TNC,
the minerals and proteins expressed on a TNC-free biomass basis remained signif-
icantly reduced, even if less so than on a total dry weight basis. This suggests that
processes other than dilution by TNC contribute to decreased nutrient concentra-
tions under CO
2
enrichment. Simple dilution by greater (structural) biomass pro-
duction remains a possibility, but if biomass dilution is exclusively responsible for
decreasing nutrient concentrations under CO
2
enrichment then all nutrients would
decrease equally in concentration. However, as reported earlier, decreases in
macro- and micronutrients can vary greatly among each other and across studies
(between 0.7 and 19.5 % or 3.7 and 18.3 %) (Fangmeier et al.
1999
; Fernando
et al.
2012
;H¨gy and Fangmeier
2008
;H¨gy et al.
2009
; Manderscheid et al.
1995
),
and there seems to be no general relationship with growth stimulation by elevated
[CO
2
]. It has been repeatedly suggested that factors relating to nutrient uptake
efficiency and metabolism are involved in decreased nutrient concentrations under
high [CO
2
]. It has to be acknowledged, however, that an exact quantification of
possible biomass dilution effects within experiments, particularly under free air
growth conditions, is still missing.
