Agriculture Reference
In-Depth Information
for a specific production system depends on factors, including environmental conditions, manage-
ment, rate, timing, and source of nitrogen (Zotarelli et al., 2009).
5.2.1 a
GronomIC
e
ffICIenCY
AE is defined as the economic production obtained per unit of N applied. It can be calculated with
the help of the following equation:
GY in kg GY in kg
Nratein kg
−
f
uf
AE kg kg
(
−
1
)
=
where GY
f
is the grain yield of fertilized plot and GYf
uf
is the grain yield of unfertilized plot. If the
experiment is conducted under greenhouse or controlled conditions, the unit of AE is in mg mg
−1
and grain yield and N rate should also be expressed in mg.
AE for lowland rice was calculated and the values are presented in Table 5.2. AE was higher at
lower N rate and decreased with increasing N rates. This indicated that rice plants were unable to
absorb N when applied in excess because their absorption mechanisms might have been saturated.
Under these conditions, the possibility exists for more N being subject to loss by NH
3
volatilization,
leaching, and denitrification. Decreases in N uptake efficiency at higher N rates have been reported
by Kurtz et al. (1984). Similarly, Eagle et al. (2001) reported that NUE in rice, which has both
physiological and soil N supply components, decreased with increases in soil N supply, indicating
that some of the decrease in NUE may have been due to the increased soil N supply. Sowers et al.
(1994) also reported that a reduction in N availability efficiency with an increasing N rate indicates
a greater proportion of the N supplied was not recovered in the plant tissue or retained in the soil
profile to the depth of rooting. This suggests N loss was a controlling factor in reducing the N uptake
and use.
There is a general trend of decreasing NUE with increasing N fertilizer rates (Barbieri et al.,
2008; Abbasi et al., 2012). Jokela and Randall (1997) reported that the N uptake in corn grain was
increased by higher N rates in all cases, while the total N recovery ranged from 31% to 60% at the
low N rates and decreased to 24-45% at the high rates. Fertilizer N recovery by the crop may some-
times be greater when N application is delayed as compared to application at planting (Russelle et al.,
TABLE 5.2
Agronomic Efficiency of Lowland Rice under Different N Levels
Nitrogen Rate (kg ha
−1
)
Agronomic Efficiency (kg kg
−
1
)
30
35
60
32
90
22
120
22
150
18
180
16
210
13
Average
23
0.93
**
R
2
Source:
Adapted from Fageria, N. K., and V. C. Baligar. 2001.
Commun. Soil Sci.
Plant Anal
. 32:1405-1429.
**Significant at the 1% probability level.
