Agriculture Reference
In-Depth Information
(Van Cleemput, 1998; Thomas et al., 2012). Water content and water-filled pore space is often a key
determinant of subsoil denitrification (Van Groenigen et al., 2005). The denitrification and N
2
O pro-
duction have been demonstrated on both saturated subsoils (Wells et al., 2001) and in subsoils that
are predominantly aerobic (Muller et al., 2004). In a few studies, the release and transport of N
2
O
from
NO
3
−
-contaminated shallow groundwater into the overlying vadose zone has been estimated
(Bottcher et al., 2011; Minamikawa et al., 2011). Typically, the low availability of C substrates is
considered a major limiting factor (Yeomans et al., 1992; Brye et al., 2001; Murray et al., 2004).
Nevertheless, it has been proposed that even low rates of denitrification may be important as a sink
for leached
NO
3
−
where vadose zones are deep and, in some cases, hundreds of meters thick (Jarvis
and Hatch, 1994).
2.5 LOSSES THROUGH LEACHING
N losses through leaching are one of the significant processes in soil-plant systems. Leaching is the
physical movement of
NO
3
−
through the soil (Stevenson, 1986). As water moves through the soil,
so does the
NO
3
−
in solution. In soils where water moves rapidly through the soil profile and where
water (either from rainfall or irrigation) exceeds evapotranspiration, nitrate is commonly found
below the rooting zone and is no longer available for plant uptake. Numerous studies have shown
that fertilizer applications of N that exceed crop requirements can result in
NO
3
−
leaching and can
contaminate groundwater (Baker and Johnson, 1981; Angle et al., 1993; McKenney et al., 1995).
Soil texture (percentage of sand, silt, and clay) controls the magnitude and rates of many physical,
chemical, and hydrological processes in soils. The process of NO
3
-N leaching from the soil profile
is more severe in light-textured soils as compared to heavy-textured soils. It is also related to the
quantity of precipitation and irrigation frequency and the quantity of water applied through irriga-
tion. Nitrate (
NO
3
−
) ion is negatively charged and easily leaches under heavy rainfall or irrigation.
Owing to high rainfall, the N leaching is more in humid and subhumid area soils as compared to
dry region soils. Singh et al. (2012a,b) reported that due to heavy texture and lower percolation rate
(<1.0 mL h
−1
) of the Greenville loam, it had significantly lower NO
3
-N leaching losses as compared
to the Lakeland sand.
The leaching of NO
3
-N not only occurs from N applied to crops. However, it may also be a prob-
lem in intensively managed pastures. Intensively grazed pastures generate nitrate N (NO
3
-N) and
the leaching of this nitrate N can contaminate groundwater creating grave environmental concerns
or problems globally (Jabro et al., 2012). The high levels of NO
3
-N (>45 mg L
−1
) are toxic to human
infants (Lehrsch et al., 2001). The NO
3
-N is also toxic to mammals, although in greater concentra-
tions. Hence, increasing NO
3
-N in the groundwater used for drinking is of concern around the globe
(Strebel et al., 1989; Spalding and Exner, 1993; Lehrsch et al., 2001).
Stout et al. (1998) studied the effect of NO
3
-N leached from urine-impacted areas. Their results
support that NO
3
-N levels beneath pastures exceed the 10 mg L
−1
drinking water maximum accept-
able contaminant level. About 25% of the N in urine and 2% of the n in feces leache beneath the
root zone in the northeast United States (Stout, 2003). Numerous other studies support that large
amounts of N in animal excreta may be leached from intensively managed grazed pasture (Stout
et al., 2000; Di and Cameron, 2002; Stout, 2003; Decau et al., 2004; Sorensen and Rubek, 2012;
Jabro et al., 2012).
The amount of N leached depends on the soil type, the source of N fertilizer, the crops, and the
methods of fertilizer application. Also, nitrate is a major factor associated with the leaching of bases
such as calcium, magnesium, and potassium from the soil. The nitrate and bases move out together.
As these bases are removed and replaced by hydrogen, the soil becomes more acidic. N fertilizers
containing strong acid-forming anions such as sulfate increase acidity more than other carriers
without acidifying anions (Fageria and Gheyi, 1999). In addition, applied N that is not utilized by
plants can leach as NO
3
-N from the fields to ground and surface waters, causing environmental and
health problems stretching beyond the original agricultural fields (NRCS, 1997; Gupta et al., 2000;
