Agriculture Reference
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rates), and are an important tool to assess N limitations and requirements of crops
(Hawkesford 2011 ). Figure 9.4 illustrates the important questions: (1) Is the amount
of N required for maximum growth different between high and ambient [CO 2 ]?
(2) Is the N response (either in % yield increase per kg N or in kg yield increment
per kg N) greater under high [CO 2 ] than under ambient [CO 2 ]? Work on potted
wheat under relatively high CO 2 enrichment concluded that the total amount of N
required for maximum growth remains unchanged, and N-responsiveness both in
absolute (g biomass per g added N) and relative terms (in % biomass per g added N)
was greater under high [CO 2 ] (Conroy and Hocking 1993 ). Whilst this is in line
with the notion of generally greater resource use efficiency of plants under CO 2
enrichment, there are few, if any, reports of field data (such as full N response
curves under free air conditions).
Nitrogen Use Efficiencies Under CO 2 Enrichment
Changes in tissue concentrations in conjunction with biomass and yield changes
lead to changes in Nitrogen Use Efficiency (NUE) in their various definitions and its
components (Good et al. 2004 ; Hawkesford 2011 ; Table 9.1 ). A frequently used
definition for NUE is the ratio of biomass accumulation or yield over the amount of
available N, and according to this definition NUE is closely related to yield
(Hawkesford 2011 ). As yield increases under CO 2 enrichment, so will by definition
NUE of the crop, provided available N remains unchanged. NUE as defined above
can be divided into Nitrogen Uptake Efficiency (NUpE, N taken up into the biomass
as a proportion of total available N) and Nitrogen Utilisation Efficiency (NUtE, the
ratio of yield or biomass over the N taken up (Hawkesford 2011 ). Depending on the
relative magnitude of changes in N uptake, growth and yield, and N concentrations
in biomass, the different components of NUE may change (Table 9.1 for an
overview).
On a physiological level, changes in Photosynthetic Nitrogen Use Efficiency
(PNUE) are the result of lower concentrations of leaf N in conjunction with greater
assimilation rates under CO 2 enrichment leading per definition to greater PNUE,
where PNUE is defined as carbon assimilation rate (expressed in
mol C fixed per
m 2 leaf area and s) per unit leaf N (expressed as g N per m 2 leaf area; N area ). A
review of FACE experiments confirmed an increase in PNUE, and this increase was
mainly due to the large increase in assimilation rate, and to a minor extent to a
decrease in N area (Leakey et al. 2009 ).
An important aspect of NUE in agricultural systems is the proportion of N
applied to crops (as fertiliser) that is utilised by the target crop. A significant
proportion of the applied N can either remain in the soil (although potentially
available to subsequent crops) or can be lost from the soil-plant systems by several
processes, including denitrification, volatilisation and leaching. For example, in
Australian wheat production systems actual fertiliser N recovered in the wheat plant
itself
μ
ranged from approximately 20-60 % (Chen et al. 2008 ). Recent
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