Agriculture Reference
In-Depth Information
or 10 mg L
−1
N in the nitrate form (Follet and Walker, 1989; Brady and Weil, 2002). The European
Economic Community (EEC) recommends 50 mg
NO
3
−
per liter (11.5 mg N per liter) as an upper
limit in potable water (Jurgens-Gschwind, 1989). In addition to
NO
3
−
N, ammonia, which is in equi-
librium with
NO
4
+
, depending on the pH, is also toxic to humans and animals (Hooda et al., 2000).
Any technology or nutrient management system that provides a strong control over nitrate forma-
tion or its direct release into the soil may reduce its adverse effects (Shaviv, 2001). It is the general
feeling today that the nitrate directive in the EEC (Goodchild, 1998) and the efforts made in the
United States (Livingston and Cory, 1998) are not effective enough in reducing the problems associ-
ated with nitrate. Additional efforts have been made to further improve or even increase the control
over the release of nitrate into the environment (Goodchild, 1998; Joosten et al., 1998; Livingston
and Cory, 1998; Wendland et al., 1998).
The loss of N from the soil-plant system to river and lake water may also create environmental
problems. The N and other nutrients stimulate the growth of algae and other organisms, which sink
to the bottom when they die. When these materials decompose by bacteria, other organisms use the
oxygen from the water that could not sustain aquatic life. The fish and other aquatic species either
die or move to other areas that are more adequately supplied with oxygen. This state of low oxygen
in the water (<2-3 mg O
2
per liter) is known as
hypoxia
, and the process that brings it is called
eutrophication
(Brady and Weil, 2002).
Glass (2003) reported that, unfortunately, analyses of the costs of excessive fertilizer application
of this sort generally fail to factor in environmental costs. In terms of dollars, these are exceed-
ingly hard to evaluate and this difficulty is reflected in the paucity of papers dealing with this topic
(Schlegel et al., 1996). In contrast, there is a large literature reporting the damaging effects on the
environment associated with nutrient overloading. Many sources suggest that some forested lands
in Europe and North America have been strongly impacted by N pollution and may be exhibiting
symptoms of N excess (Fenn et al., 1998; Campbell et al., 2000). Perakis and Hedin (2002) reported
that over 70% of the N in woodland rivers in Europe and North America is in the form of
NO
3
−
. In
contrast, in these more remote South America streams in temperate forests in Chile and Argentina,
NO
3
−
made up only 5% of the N, while 75% was in organic form. N budget calculations performed
in the major subbasin of the Upper Mississippi River watershed show that fertilizer is the most
important input of N to the basins (Glass, 2003).
N fertilizer consumption has increased more rapidly than that of P and K to support world food
supplies (FAO, 2006; Jantalia et al., 2012). In addition, N fertilizer prices have increased signifi-
cantly in the last few decades. In the United States alone, N fertilizer prices have more than doubled
since the 1990s (Economic Research Service, 2010), reaching historic heights in mid-2008 due
to high fertilizer demand and the inability of manufacturers to increase production levels (Huang
et al., 2008). The findings of many studies (Glover et al., 2010; Dalal et al., 2011) support the con-
tention that bringing the land under cultivation or crop production reduces soil total N content and
increases N losses. In the future, the N demand for crop production will likely increase and the cost
of production will also increase. Under these scenarios, improperly managed N fertilizers can form
gases or soluble compounds with the potential to pollute air (NH
3
, NO
X
, N
2
O, and N
2
) or contami-
nate surface and groundwater with nitrate N (Aneja et al., 2003). Furthermore, it is essential to know
how N losses occur in the soil-plant system to take necessary measures to reduce these losses and
improve the efficiency of use of N in crop plants. Appropriate N management not only improves N
use efficiency by crops but also reduces environmental pollution. The objective of this chapter is to
discuss N losses mechanism or processes in the soil-plant system, factors that are responsible for
these losses, and suggest management practices to reduce these losses.
2.2 CYCLE IN SOIL-PLANT SYSTEM
The knowledge of nutrient cycle in a soil-plant system is an important aspect to understanding its
availability to plants and adopting management practices for maximizing its uptake and efficiency
