From a soil standpoint, the overall NUE depends on the interaction of factors responsible for N loss
(leaching, denitrification, volatilization, and immobilization) with such N management variables as N
rate, N source, N placement, and timing of N application. In conjunction with soil factors, NUE from a
plant standpoint depends on the processes associated with the absorption, translocation, assimilation, and
redistribution of N. The NUE is greatest at low levels of N and is highly influenced by soil type, which
determines the mineralization and N loss characteristics [13,173,214]. The NUE can also be influenced
by plant characters such as tissue N concentration and the size and number of reproductive sinks
[173,253].
Another measure of NUE uses data on yield and plant N content, without correcting for dry matter
or N accumulation by unfertilized plants [273]. This procedure was developed to assess genotypic varia-
tion in response to N supply, where evaluation of a large number of genotypes by traditional methods is
constrained by the size of the necessary field experiments. The procedure denotes dry weight and N val-
ues as a series of ratios, all expressed in the same unit—often grams per plant [273]. As with the tradi-
tional measures of NUE, this method defines NUE as grain production per unit of fertilizer N; there are
two main components: the efficiency of N absorption (uptake efficiency), and the efficiency with which
the N absorbed is utilized to produce grain (utilization efficiency). The uptake efficiency is denoted as the
N in the plant divided by the fertilizer N applied, while the utilization efficiency is the grain produced di-
vided by the N in the plant. Thus, the overall NUE can be expressed as a product function of uptake and
utilization efficiencies.
Further subdivision of uptake and utilization efficiencies can be made to reflect more specific as-
pects of plant N use (e.g., translocation, remobilization, distribution, timing of N acquisition) [273]. In
addition, converting the appropriate dry matter and N ratios to logarithms provides a means of parti-
tioning variation in NUE into the proportion attributable to each of its components. Such data have
shown that genotypic differences in NUE of eight maize hybrids were primarily the result of differences
in N utilization efficiency when the crops were grown with a low supply of N and differences in up-
take efficiency at a high N supply [273]. The data also showed that either high or low values of NUE
could be attained by different combinations of uptake and utilization efficiency. Similar cases of such
variation have been noted for wheat [219]. The NUE and its components have also been shown to vary
as a function of N fertilizer rate and the timing of N availability [126,173,274]. Collectively, these data
emphasize that each of the plant traits involved in the acquisition and utilization of N is subject to ge-
netic diversity, which may contribute to N use and crop productivity in different degrees under differ-
ent environmental conditions.
V. ENVIRONMENTAL ASPECTS OF NITROGEN FERTILIZER USE
In addition to being removed with the crop, N can be lost from agricultural ecosystems in large amounts
as the result of several processes. These include leaching, denitrification, volatilization, surface runoff,
and soil erosion. Nitrogen can also be temporarily removed from the available soil pool because of ad-
sorption, fixation, and microbial immobilization. The economic implications of these losses are self-evi-
dent, especially when they are large enough to limit crop productivity. These losses can also have envi-
ronmental consequences with regard to water and air quality.
Losses of N are highly affected by which ionic N form (NO 3 or NH 4 ) predominates in the soil
[275,276]. Both forms are lost by soil erosion, but only NH 4 is lost directly to the atmosphere from
volatilization. Ammonium can also be temporarily removed from the plant-available N pool by cation ex-
change with soil particles, fixation by clay lattices of the soil, fixation by organic matter, immobilization
into microbial biomass, and conversion to NO 3. Conversely, NO 3 -N is not readily used by soil microbes,
nor does it bind to soil particles or organic matter. It is, however, subject to losses from leaching and
denitrification.
The least controllable of these N losses, which are determined by soil type and rainfall, are the leach-
ing and denitrification of NO 3. Leaching is a physical process that occurs because NO 3 is repelled by neg-
atively charged soil colloids and readily moves with soil water. However, if too much downward move-
ment of water occurs, NO 3 can be leached below the plant’s rooting zone, ultimately to accumulate in
ground water [2]. In denitrification, a separate microbial process that occurs under waterlogged or anaer-
obic conditions, NO 3 is converted to gaseous compounds, which are lost to the atmosphere. Both leach-
NITROGEN METABOLISM AND CROP PRODUCTIVITY 399