Agriculture Reference
In-Depth Information
2
Nitrogen Losses in Soil-Plant
System
2.1 INTRODUCTION
Nitrogen (N) is the mineral nutrient that is most limiting for crop production around the world and
is often applied as a fertilizer to maintain adequate soil levels for crop production. Geisseler et al.
(2012) reported that adequate N supply is crucial to obtaining high yields in intensive crop pro-
duction. While insufficient N application can have serious economic consequences for the farmer,
excessive fertilization increases the risk of environmental pollution, especially groundwater pollu-
tion with
NO
3
−
, NH
3
volatilization, and emissions of N
2
O. Even in a well-managed cereal production
system, a substantial fraction (typically 40-60%) of N fertilizer inputs can be lost (Galloway et al.,
2002). Dinnes et al. (2002) reported that N is essential for the growth and reproduction of all life-
forms, and except for legume crops and virgin soils with relatively high soil organic matter, soil N
must usually be supplemented to sustain food and fiber production.
Globally, about 100 Tg of N (100 million metric tons) is applied to farmland every year as a fertil-
izer (Gruber and Galloway, 2008; Kim et al., 2011). Among various N sources, urea has been the most
preferred, mainly due to its effectiveness and cost, accounting for 50% of the total world consumption
of N fertilizer (Vaio et al., 2008). A major part of the applied N is lost in the soil-plant system and
its use efficiency in crop plants is low. N losses in a soil-plant system are among the most important
mechanisms or processes responsible for the low recovery efficiency of applied chemical fertilizers.
In important food crops, N recovery of applied chemical fertilizers never exceeds more than 50%.
Many researchers have reported that 30-40% of the applied N is utilized by plants (Dobermann and
Cassman, 2002; Cisse and Vlek, 2003; Shah et al., 2004; Fageria et al., 2011; Fageria, 2013, 2014).
In fact, up to 50% of the surface-applied urea could be volatilized as NH
3
(Gioacchini et al., 2002)
because urea broadcast onto the soil surface is quickly hydrolyzed into NH
3
and easily oxidized into
NO
x
. In spite of the obvious risk of high emissions, nearly half the amount of urea is required to be
broadcast over the soil surface for a quick supply of N (Wang et al., 2008).
The most important N losses in the soil-plant system occur through the combination of ammo-
nium volatilization, leaching of
NO
3
−
N from soil profile to lower depths where the roots cannot
absorb it, losses through denitrification, losses through surface runoff and soil erosion, and also
losses as NH
3
gas through the foliage of plants (Fageria and Baligar, 2005; Hull and Liu, 2005;
Bauer et al., 2012; Sainju, 2013). These losses of applied N in cropping systems not only increase
the cost of production but also increase the risk of environmental pollution by surface and ground-
water contamination and the alteration of atmospheric composition. A marked loss of N applied
to agricultural soils has raised concerns about the environmental impacts of N that escape from
the rooting zone (Blackmer, 2000). Furthermore, a high concentration of
NO
3
−
N in water is asso-
ciated with public health problems (Owens, 1994). High
NO
3
−
N concentration in water is asso-
ciated with human disorders and diseases such as methemoglobinemia and cancer (Smith et al.,
1990).
Methemoglobinemia,
also called blue-baby syndrome, occurs when nitrates are converted
into nitrites in the guts of human infants and ruminant animals. The nitrites decrease the bloods
ability to carry oxygen to the body cells. Since inadequately oxygenated blood lacks the red color,
infants with the condition take on a bluish skin color (Brady and Weil, 2002). Brady and Weil (2002)
reported that death from
methemoglobinemia
is quite rare; however, in many countries, there is a
limit to the amount of nitrates in drinking water. In the United States, this limit is 45 mg L
−1
nitrate
67
