Nitrogen Losses in Soil–Plant System - Nitrogen Management in Crop Production - page 72

fixing soil, and 3.9% of the added 15 NH 4 occurred in the low fixing soil. Extractable and fixed NH 4
fractions were interrelated pools. When fertilizer NH 4 was added to the soil, a proportional amount
was fixed by the clay minerals. When nitrification and immobilization depleted extractable NH 4 ,
ixed NH 4 was released. The fixed NH 4 pool appeared to be a slow-released reservoir, with fixed
NH 4 released being slower than the rate of fixation. Juma and Paul (1983) reported that NH 4 fixation
was enhanced when nitrification was inhibited with 4-amino-1-, 3-, 4-triacole (ATC), a nitrification
inhibitor. Neeteson et al. (1986) observed that over one-half of added NH 4 disappeared and reap-
peared after 5 weeks. Since the clay minerals in these soils were not found to fix added NH 4 , they
postulated that the immobilization occurred as an osmoregulation mechanism and then NH 4 was
remineralized when the microbes died and decomposed. One NH 4 is fixed by clay mica and it is
protected against nitrification until it is released from fixation sites (Scherer and Mengel, 1986). It
can be concluded that there are a number of ways by which NH 4 + fixation can be regulated. He et al.
(1990) reported that about 16% of the 15 NH 3 -N injected was fixed by the soil. Of the N fixed, 48%
was accounted for by chemical fixation into the soil organic fraction, most of which was removed
by sequential extraction and 52% was accounted for as clay-fixed NH 4 . These authors also reported
that NH 3 -N fixed by organic matter occurs in forms that are less stabilized and more biologically
available than the native soil N. Nommik and Vahtras (1982) presented an extensive review of the
mechanisms and influential factors affecting NH 4 fixation.
A large quantity of N accumulates in seeds of cereals and legumes, which are not recycled and
lost from the soil system. Fageria et al. (2011) reported that about 50% of the total N accumulated
was in the grains of upland rice cultivars IAC 47 and IR 43. The remaining 50% was in the roots and
shoots. In legume crops, grains remove still higher N. This means that in a N management strategy,
its removal by a crop should be taken into account.
Animal agriculture is an important component of N cycle in a soil-plant system that is generally
ignored when discussing this topic. Agricultural activity is estimated to be responsible for 90% of
anthropogenic NH 3 emissions (Boyer et al., 2002), with the majority coming from livestock pro-
duction and 12% coming from N fertilizer application (Ferm, 1998). As more animals are added
to a fixed land base, and more feed is imported onto a farm, the excretion of manure nutrients can
surpass the cycling capacity of local land, air, and water resources (Powell and Broderick, 2011).
In many industrialized countries and regions of the world, higher incomes have greatly increased
the demand for meat, milk, and other animal products. This trend of tremendous livestock expan-
sion raises concern over excessive nutrient use, nutrient loss, and environmental contamination (Ma
et al., 2010a,b). Dairy cows excrete urea N in urine, which is a source of NH 3 emission into the
atmosphere. Of the total feed N consumed by lactating cows on commercial dairy farms, a general
range of 20-35% is secreted in milk (Jonker et al., 2002). Ration formulation may influence not only
feed N transformation into milk but also the proportion of N excreted in feces and urine (Powell
and Broderick, 2011). As dietary crude protein increases and N intake exceeds the requirement, the
efficiency of the feed N use (i.e., the relative amount of consumed feed N that is secreted as milk)
declines and the excretion of urinary N increases (Broderick and Clayton, 1997; Nousiainen et al.,
2004). Urea N comprises from 55% to 80% of the total urinary N, depending on the concentration
of crude protein in the ration (Olmos Colmenero and Broderick, 2006). Urease enzymes that are
present in feces and soil rapidly hydrolyze urea to ammonium, which can be quickly converted into
NH 3 gas and lost to the atmosphere. Thus, the increase in urea N excretion due to excessive crude
protein in dairy rations increases NH 3 emissions during the collection, storage, and land application
of manure (Powell et al., 2008; Arriaga et al., 2010).
2.3 LOSSES THROUGH AMMONIA VOLATILIZATION
Ammonia volatilization is an important process of N loss from soil-plant systems when N fertil-
izers are surface applied, especially in alkaline soils (Harrison and Webb, 2001; Ma et al., 2010a,b;
Singh et al., 2012a,b). This can result in low N use efficiencies by crops (Bouwman et al., 2002;