Agriculture Reference
In-Depth Information
Low NUE in crop production is explained by N losses from the system, including plant senes-
cence, denitrification, surface runoff, volatilization, leaching, and immobilization (Sato et al.,
2012). Nitrogen in the N-NO
3
form is readily available to plants and also easily lost by leaching
since its electrostatic adsorption is lower than of other anions present in the solid-liquid interface
of the topsoil. Therefore,
NNO
−
is not absorbed to the soil, and remains in the soil solution
from where it is easily lost by leaching to deeper soil layers out of reach of plant roots (Sartor
et al., 2011).
There is a variation in NUE at lower and higher N rates. At high nitrogen input, variation in NUE
was explained by variation in nitrogen uptake capabilities, whereas at low nitrogen input, variation
in NUE was mainly due to differences in NUE defined as the ratio grain yield/nitrogen uptake
(Hirel et al., 2001). These differences in the expression of genetic variability were further confirmed
following the detection of specific quantitative trait
loci
(QTL) for a given level of fertilization
(Bertin and Gallais, 2001). This suggests that several sets of genes are differentially expressed
according to the amount of nitrogen provided to the plant (Bertin and Gallais, 2001).
In parallel with these agronomic studies, several investigators found that it is possible to
detect genetic variation and select new genotypes that show increased or decreased activities of
several enzymes involved in the nitrogen assimilatory pathway (Groat et al., 1984; Degenhart
et al., 1992; Harrison et al., 2000; Hirel et al., 2001). In particular, in corn hybrids, several
studies have been done to correlate the efficiency of primary nitrogen assimilation and nitrogen
remobilization with yield and its components (Reed et al., 1980; Purcino et al., 1998). As a result
of these studies, it was concluded that increases in grain yield observed during the last two
decades were not due to additional enhancement in inorganic nitrogen assimilation but rather
due to a better NUE as a result of a more efficient nitrogen remobilization (Hirel et al., 2001). In
particular, leaf longevity was shown to be one of the main factors responsible for yield increase
in modern corn hybrids (Tollenaar, 1991; Ma and Dwyer, 1998). Extension of leaf metabolism
activity improved the ratio between the assimilate supply from source leaves and demand in sink
leaves during grain filling and was independent of the level of fertilization in the soil (Racjan
and Tollenaar, 1999a,b). During this metabolic process, the putative role of enzymes involved
in inorganic nitrogen assimilation and recycling such as nitrate reductase (NR), cytosolic Gln
synthetase (GS1), and Glu dehydrogenase (GDH) was suggested (Lea and Ireland, 1999; Hirel
et al., 2001).
Moll et al. (1982) reported that efficiency in uptake and utilization of N in the production of grain
requires that those processes associated with absorption, translocation, assimilation, and redistribu-
tion of N operate effectively. Taking into consideration these aspects of N use, definitions of NUEs
have been grouped or classified as agronomic efficiency (AE), physiological efficiency (PE), agro-
physiological efficiency (APE), apparent recovery efficiency (ARE), and UE (Fageria and Baligar,
2001, 2003, 2005; Fageria et al., 2003, 2011; Fageria, 2013, 2014). The determination of NUE in
crop plants is an important approach to evaluating the fate of applied chemical fertilizers and their
role in improving crop yields.
Raun and Johnson (1999) defined NUE as the percentage of fertilizer N removed in grain, and
averages about 33% worldwide for cereal grain production. A low NUE is a result of fertilizer N
losses through various physical, chemical, and biological pathways of the soil-plant-atmosphere
systems (Zhang et al., 2012). These losses are proportional to the amount of excess fertilizer N
applied (Johnson and Raun, 2003), especially for adverse climatic conditions. Hence, the most
effective way to improve NUE is to reduce excess N throughout the entire growing season by
synchronizing N supply with plant N demand. Given the dynamic nature of weather and other fac-
tors affecting plant N demand, N synchronization is best done using process-based, dynamic crop
simulation models (Zhang et al., 2012).
Crop N requirement is a physiological component, which is directly related to the genetic poten-
tial of the crop and to plant growth conditions. This component is determined by the overall crop N
uptake under optimum growing conditions (Zotarelli et al., 2009). The fertilizer uptake efficiency
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