Nitrogen is unique among the mineral nutrients in that it can be absorbed by plants in two distinct
forms, as either the anion NO 3 or the cation NH 4 . The form of N absorbed has a pronounced effect on
the mechanisms for uptake, transport, assimilation, and storage and, in some cases, on the physiology and
morphology of crop productivity. The use of a specific N form also can be affected differentially by en-
vironmental or culture factors, such as plant N status, temperature, and pH. While N is usually applied as
NH 4 -fertilizer, the nitrification process renders NO 3 -N the soil form most available to the crop. In addi-
tion, NO 3 -N is the N form most susceptible to losses from the crop’s rooting zone. Several relatively new
techniques have been developed in an attempt to better assess the soil N supply; however, their useful-
ness is still being evaluated. Plant-based estimates of soil N supply are also receiving attention.
Although the maximum rate of N accumulation usually occurs during vegetative growth, the timing
of N acquisition can be altered by cultural and environmental factors. Extensive redistribution of N among
plant parts further confuses our understanding of when N has the greatest impact on crop productivity.
Nevertheless, the major roles for N in crop productivity can be divided into four general areas:
Establishment of photosynthetic capacity
Maintenance of photosynthetic capacity
Establishment of sink capacity
Maintenance of sink capacityAlthough the relative abundance of C and N in the plant dictates a predominant role for photosynthesis in
the productivity of cereal crops, some evidence suggests that the availability of N to and within the plant
is more variable than the availability of photosynthate and at least as limiting to grain development.
Improved crop productivity from N fertilization can result from increases in dry matter yield and/or
improvements in quality factors. In either case, increases with N supply follow the law of diminishing re-
turns; thus N is used most efficiently when available at low levels. Cultivars grown at the same location
may exhibit different responses to N supply that result from differences in how much N they need for
maximum yield or when in the life cycle they mainly acquire their N. Supplying N as mixtures of NO 3
and NH 4 can also increase productivity as the result of alterations in important plant processes (e.g., re-
productive development, N acquisition, dry matter production, assimilate partitioning). The efficient use
of N is an important goal in strategies to maximize yield potential while minimizing negative effects of
fertilizer N on the environment. Several methods have been used to assess N use efficiency, and its com-
ponents, in crop plants.
The use of nitrogen by crop plants is dictated by a complex interaction of plant metabolism with cul-
tural and environmental factors that alter the availability of N. Each of the plant processes involved in the
acquisition and utilization of N is under genetic control, and each may contribute to varying degrees de-
pending on the environmental conditions. A better understanding of these processes will undoubtedly
help in developing strategies to improve the management of fertilizer nitrogen.
ACKNOWLEDGMENT
The author expresses his sincere gratitude to P. S. Brandau for providing critical comments on the text
and for help with the graphics.
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