Agriculture Reference
In-Depth Information
5
Nitrogen Use Efficiency
in Crop Plants
5.1 INTRODUCTION
In the last six decades, nitrogen fertilization has been a powerful tool in increasing the grain yield
of food crops, especially wheat, rice, and corn. However, in the current agricultural and economic
environment farmers must optimize the application of nitrogen fertilizers to avoid pollution by
nitrate and to preserve their economic margin (Hirel et al., 2001). In this context, nutrient use
efficiency, including nitrogen in crop plants, has special importance due to economic and environ-
mental implications. The importance of improving nutrient use efficiency in modern agriculture or
crop production is higher because of the use of large amounts of chemical fertilizers. It has been
estimated that almost 10
11
kg of N per annum is applied as fertilizer worldwide, a 20-fold increase
over the past 50 years, at a cost of 50 billion US dollars (Glass, 2003). The majority of crops except
nitrogen-fixing legumes receive an application of N; the major requirements are for the production
of seeds (Mengel et al., 2006) and forage (Kingston-Smith et al., 2006). However, crop plants are
only able to convert 30-40% of this applied N into useful food products such as grain (Raun and
Johnson, 1999). There is therefore extensive concern in relation to the N that is not used by the
plant, which is lost by leaching of nitrate, denitrification from the soil, and the loss of ammonia to
the atmosphere, all of which can have deleterious environmental effect (Vitousek et al., 1997; Glass,
2003). The possibility of more precise use of N in crop production is now being taken seriously
( Day, 20 05).
The widespread availability of N fertilizers from the 1950s onwards has enabled many farmers
around the world to abandon exploitative, low-yielding agricultural practices that had minded the
soil for macronutrients (Duan et al., 2011). Most of the food crops are highly responsive to N fertil-
ization. According to FAO (2004), corn grain yields are highly responsive to N fertilization, leading
to annual applications of an estimated 10 million Mg N fertilizer worldwide. Corn is not the only
crop that requires N fertilization for higher yields but other crops such as rice, wheat, barley, and
oats also need an adequate amount of N for maximizing yields. The use of chemical fertilizers in
modern cropping systems is a prerequisite to maximize crop yields (Fageria, 1992). Nearly half or
less of the N applied to soil, however, is unutilized by plants and is subject to leaching to ground-
water or the emission of gaseous compounds into the atmosphere (Bundy and Andraski, 2005).
This not only increases crop input costs but also threatens human health and the stability of natural
ecosystems (Kaiser, 2001; Duan et al., 2011). Hence, this topic deserves ample discussion in order
to provide information available in the literature on NUE in crop plants, which may be helpful in
reducing the cost of crop production while at the same time reducing the risk of environmental pol-
lution (air, water, and soil).
Global food security requires yield improvements or an expansion of land area used for agricul-
ture. In addition, optimum resource use efficiency is a prerequisite for sustainability (Hawkesford,
2012). A major driver for yield, especially in intensive agriculture systems, is N fertilizer. Canopy
growth requires N, and it is canopy photosynthesis that ultimately drives yield. The canopy also
acts as a reservoir of N and other minerals, which are recycled into grain with potentially higher
efficiency (Hawkesford, 2012). Inappropriate use of N can lead to a lower uptake of N, higher cost,
and also environmental pollution. Well-adopted agronomic practices have a crucial role in optimizing
fertilizer use to exploit the full potential of crop cultivars (Hawkesford, 2012).
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