Nitrogen Losses in Soil–Plant System - Nitrogen Management in Crop Production - page 82

and emissions of N 2 O, a highly potent GHG with shared responsibility for global warming. When
evaluating among management systems, it is important to consider options that have the potential
to improve agronomic productivity, economic return, and/or environmental sustainability (Johnson
et al., 2012). An increase in soil N storage reduces N losses through leaching, volatilization, denitri-
fication, surface runoff, erosion, and N 2 O emissions (Sainju et al., 2012b). Hence, adopting appropri-
ate management practices to reduce N losses not only improves N use efficiency and reduces the
cost of crop production but also reduces environmental pollution. These practices include use of an
adequate rate of N, the use of an effective source of N, use of appropriate timing of N application, use
of conservation tillage, adopting appropriate crop rotation, and planting N-efficient crop species and
genotypes within species. A detailed discussion on these practices is given in Chapter 8. However,
some part of the management practices associated with the reduction of emission of gases from the
applied fertilizer is more pertinent to this chapter and is discussed here.

2.9.1 I rrIGatInG after t opdressInG n ItroGen f ertIlIzers
When N is top dressed during crop growth, the application of irrigation (wherever possible) water
in adequate amounts (>15 mm) may significantly reduce N loss by volatilization. In dryland agri-
culture when the crop is totally dependent on rainfall, the application of N in top dressed can be
done a little earlier than the rain is expected. Meteorological forecasting can be used to define the
N topdressing timing under dryland farming system. In addition, applied N incorporated into the
soil can also reduce N losses significantly. Research on effective irrigation depth has varied from 5
to 75 mm to minimize volatilization losses by NH 3 (Harper et al., 1983; Bouwmeester et al., 1985;
Black et al., 1987; Mugasha and Pluth, 1995). Holcomb et al. (2011) reported that the application
of NH 3 losses can be significantly reduced by ammonia losses by irrigation immediately after urea
application to wheat crop. These authors developed a regression equation by plotting irrigation rate
versus cumulative total N loss as NH 3 yielded the following equation (R 2 = 0.91):

NH -N loss ( % of N applied) = 62 655exp . .( − 0 . 1559 × irrigationr ate in mm)

3

Using the model to solve for an 80% reduction in NH 3 loss resulted in a need for 10.3 mm of
irrigation, a 90% reduction needs 14.8 mm of irrigation, and a 95% reduction needs 19.2 mm of
irrigation. Holcomb et al. (2011) further reported that as the irrigation rate increases, NH 3 volatiliza-
tion loss decreases but with a diminishing return, particularly at irrigation rates of >14.8 mm. Black
et al. (1987) reported that a simulated rate of 16 mm of precipitation was enough to reduce NH 3 loss
to 2%. Kissel et al. (2004) applied 24 mm of simulated rain immediately after a urea application and
they found <1% loss, compared with 5% when simulated rain was applied at day 7.
Black et al. (1987) reported that precipitation was not effective after 48 h because hydrolysis
had already occurred. Engel et al. (2011) reported that after application of urea on the soil surface,
if precipitation events that follow are light (<8 mm), NH 3 loss may be reduced to 10-20% and
if the events are heavy (>18 mm), the loss may be <10% of the applied N. Jantalia et al. (2012)
reported that irrigation with 16-19 mm of water 1 day after N fertilization reduced NH 3 -N loss
from surface-applied N fertilizers to a range of 0.1-4% of total N applied depending on N sources.
Halvorson and Del Grosso (2012) reported that irrigated soils in semiarid climates have relatively
low N 2 O-N losses, provided irrigation is well managed to avoid water-logged conditions and poten-
tial for denitrification.

2.9.2 u se of o rGanICallY e nhanCed f ertIlIzers
There are several sources of N as inorganic fertilizers. However, there are three most common
sources that are largely used as a source of N in crop production worldwide. These sources are