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Nitrogen Metabolism and Crop Productivity
Fred E. Below
University of Illinois, Urbana, Illinois
385I. INTRODUCTION
Among the mineral nutrient elements, nitrogen (N) most often limits the growth and yield of nonlegumi-
nous crop plants, which require relatively large quantities of N (from 1.5 to 5% of the plant dry weight)
for incorporation into numerous organic compounds. These compounds include proteins, nucleic acids,
chlorophyll, and growth regulators, all of which have crucial roles in plant growth and development. The
N composition of plant tissues also has important nutritional consequences, since plants are a major
source of proteins in the diet of humans and animals. Because N deficiency can seriously decrease yield
and crop quality, elaborate steps are often taken to assure that adequate N levels are available to plants.
Although plants can absorb small amounts of N from the atmosphere through their foliage, by far the
greater part of it is acquired from specific forms in the soil such as nitrate (NO 3 ) or ammonium (NH 4 ).
Most soils, however, do not have sufficient N in available form to support desired production levels.
Therefore, addition of N from fertilizer is typically needed to maximize crop yields; this requirement has
resulted in the development of a large N fertilizer industry. Some estimates suggest that N fertilizer ac-
counts for 80% of all fertilization costs and 30% of all energy costs associated with crop
production [1].
Although it is well accepted that sufficient N is needed to obtain high yields, growers each year must
determine how much fertilizer N to apply. This problem results from the complex cycle of N in the envi-
ronment, which can allow loss from the rooting zone. It is further complicated by mechanistic inconve-
niences associated with fertilizer N application and by uncertainty related to weather conditions, espe-
cially water availability. Unused fertilizer N is economically wasteful and can become an environmental
hazard if it is lost from the soil. Excessive use of fertilizer N has been implicated in the contamination of
ground water by NO 3 [2–5], which represents a potential health hazard to humans and animals [6,7]. As
public awareness focuses on environmental quality, there are increasing pressures on growers to improve
N management.
Additional knowledge regarding N use by crop plants is clearly one way to help improve N fertilizer
management. Although complex, factors such as N use that limit or enhance crop productivity do so by
affecting specific physiological processes within the plant. A better understanding of how N governs crop
growth and yield will add to information required to improve N management and will help to minimize
the adverse environmental impact of N fertilizer use.