Alternatively, partitioning effects may play a role in altered growth: NH 4 must be assimilated imme-
diately by the root, resulting in greater amounts of nitrogenous compounds in the roots and an altered par-
titioning of carbon between the root and the shoot [260]. Enhanced movement of sugars out of the leaves
has been shown to relieve feedback inhibition of photosynthesis due to carbohydrate accumulation [261],
which has been theorized to result in higher photosynthesis under mixed-N conditions [182]. However,
whereas some studies have reported higher photosynthetic rates for NH 4 -grown plants [262,263], others
have shown greater photosynthesis for NO 3 -grown plants [264,265]. In addition, field studies have shown
equivalent (or greater) rates, and a similar duration, of canopy photosynthesis, but lower grain yields for
maize plants supplied with predominantly NO 3 than with mixed N [247]. These findings, and the obser-
vation that mixed-N nutrition alters dry matter partitioning between shoots and roots [228,246] and be-
tween vegetative and reproductive fractions [235,239,246,247], suggest that altered partitioning may be
more important than photosynthesis in the enhanced productivity observed with mixed-N nutrition.
Other studies have shown that additional physiological processes are beneficially altered by mixed-
N nutrition. For example, increasing the proportion of N used by the plant as NH 4 usually results in an in-
crease in anion uptake, especially for P [258,266,267]. Because of its acidifying effect on the rhizosphere
[268], enhanced uptake of NH 4 may also make trace elements like iron and zinc more available [170,269].
In addition, mixed-N nutrition has been shown to increase root branching [84,270] and the supply of cy-
tokinins to the shoot [246] compared with NO 3 -grown plants. It is also possible that by utilizing both N
forms, plant cells are able to control their intracellular pH more tightly [271,272].
The experiments discussed in this section show that mixed-N nutrition can increase crop productiv-
ity as the result of alterations in several important physiological processes such as reproductive develop-
ment, N acquisition, dry matter production, and assimilate partitioning. Indirect effects on other mineral
nutrients and on endogenous phytohormone balance may also be important. For maximum yield en-
hancement, mixed N needs to be available during the period when reproductive potential is determined
and set. Thus, although additional work is needed to further elucidate the physiological basis for mixed
N–induced increases in crop growth and yield, the prospect of using mixed N to improve fertilizer use ef-
ficiency is encouraging.
D. Nitrogen Use Efficiency
The efficient use of N is an important goal in maximizing yield in ways that have a minimal impact on
the environment. Various methods have been used to define and characterize nitrogen use efficiency
(NUE), so care must be taken to specify the method or definition that is used [13]. These methods can re-
flect agronomic, economic, or environmental perspectives, and they can be characterized on an incre-
mental basis, on a cumulative basis, or as a yield efficiency index [13].
From an agronomic perspective, NUE refers to three main functions detailing the relationships
between:
N availability and yield
N availability and N recovered
Yield and N recoveredTo calculate these values requires measurements of grain yield, the total nitrogen in the plant, and the to-
tal available soil N. However, because the soil N availability and the total N recovered by the plant are
difficult to determine in field experiments, the N content in the aboveground plant parts and the N rate
supplied as fertilizer are typically used. In all cases, the most accurate estimates subtract the yield or plant
N accumulated in unfertilized plots from the values obtained in fertilized plots.
The relationship between yield and N rate is most often referred to as “yield efficiency” or “agro-
nomic efficiency” and is defined as the yield increase per unit of applied N for a specific portion of the
yield response curve. The yield efficiency is a function of the efficiencies of N recovery and N utilization,
which are known as “recovery efficiency” and “physiological efficiency,” respectively. The recovery ef-
ficiency represents the N accumulated by the plant per unit of applied N, while the physiological effi-
ciency is the grain produced per unit of N accumulated by the plant. Physiological efficiency integrates
the effect of plant factors on N use and yield, while recovery efficiency is a measure of how much fertil-
izer N is absorbed by the plant. The yield efficiency for N use can be improved by increasing the recov-
ery efficiency, or the physiological efficiency, or both.
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