ing and denitrification are economically and environmentally undesirable and add a large degree of un-
certainty to N fertilizer management.
The problems of leaching and denitrification have stimulated the identification and development of
nitrification inhibitors that block microbial conversion of NH 4 to NO 3 [277,278]. Research has shown that
the use of these inhibitors can reduce losses of fertilizer N, especially under soil and weather conditions
that favor N loss [276,279,280]. For several reasons, however, a consistent yield increase from the use of
nitrification inhibitors is not always observed [276,280]. These reasons include:
A lack of opportunity for the nitrification inhibitor to express its potential for reducing N loss
Inadequate duration of the inhibitory effect
Inadequate experimental sensitivity to permit statistical detection of small benefits that may occur
Adverse effects on other soil microorganisms
Genetic differences among cultivars to N level or to NH 4 nutritionAlthough nitrification inhibitors were originally developed to minimize N losses, they have also been
proposed as a means of altering the predominant form of N in the soil [245]. The use of ammoniacal fer-
tilizers along with nitrification inhibitors may alter plant nutrition by supplying a greater proportion of the
N to the plant as NH 4. Enhancing the supply and utilization of NH 4 -N may also be beneficial to plant
growth, as several crop species have been shown to absorb more N and to grow more rapidly when sup-
plied with mixtures of NO 3 and NH 4 (see Sec. IV.C).
Urease inhibitors represent another approach to preventing fertilizer N loss [281,282]. When applied
to the soil surface, urea [(NH 2 ) 2 CO] is rapidly cleaved to NH 4 and CO 2 by the action of urease enzymes
present in the soil and plant residue. This conversion gives rise both to high NH 4 levels and to elevated
soil pH, two properties that are conducive to volatilization of N as NH 3. Urease inhibitors temporarily de-
crease the activity of urease enzymes, maintaining urea-applied N as urea for several days. Because the
uncharged urea molecule is quite mobile in soil, rainfall can move surface-applied urea into the soil pro-
file, where it can hydrolyze with less opportunity for N losses via volatilization. As with nitrification in-
hibitors, use of a urease inhibitor will generally be effective only when the crop can respond to the N con-
served by the inhibitor and when environmental conditions are conducive to large losses of
surface-applied urea (such as warm soils with abundant plant residue). Conversely, urease inhibitors are
of limited value when urea-based fertilizers can be easily and inexpensively incorporated into the soil dur-
ing or immediately following their application [281,282]. They also require sufficient rainfall within a
few days to facilitate urea movement into the soil.
In summary, although the use of fertilizer N has the potential for negative environmental conse-
quences, several cultural practices can be used to minimize this possibility. These practices include:
Use of N rates appropriate for the historical productivity of the land and the yield of the crop being
grown
Timing of N applications to better fit plant N needs
Specific placement of N-containing fertilizers
Use of appropriate N sources
Use of nitrification inhibitors to slow the breakdown of NH 4 to NO 3
Use of urease inhibitors to minimize volatilization of surface-applied urea
Taking into account the soil’s capacity to supply the crop with N
Adequate fertilization with other mineral nutrients to maximize the plant’s use of N.VI. CONCLUSIONS
Because of the high requirement of crop plants for elemental N and its numerous important roles in
growth and development, N is the mineral nutrient element that most often limits crop productivity. Be-
cause N mineralization from the soil is normally too low to support desired production levels, soil N lev-
els are typically increased through fertilization. However, the complex cycle of N in the environment
causes uncertainty in N fertilizer management, increasing the chances for economic loss and environ-
mental damage. Nitrogen use and productivity of crop plants is also complex, resulting from an interac-
tion of biochemical, physiological, and morphological processes in the plant.
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