Agriculture Reference
In-Depth Information
urea, ammonium sulfate, and anhydrous ammonia. Generally, N sources that contain nutrients in
complex organic compounds are more efficient environmentally and agronomically due to their
slower N release (Singh et al., 2012a,b). Singh et al. (2012a,b) reported that organically enhanced
fertilizers (fertilizer manufactured by using sterilized and chemically converted organic additives
extracted from municipal wastewater biosolids) could be an attractive N source. These authors
further reported that the environmental and food security benefits of organically enhanced fertil-
izer result from both recycling of sterilized and converted organic wastes (C, amino acids, and
micronutrients) and minimizing N losses from land to atmosphere and from land to water, which are
characterized by a disrupted N cycle and meet the standards on nutrient management set by Natural
Resources Conservations Service of the USA. However, the use of such products at a commercial
scale in crop production is still debatable.
2.9.3 u
se
of
s
low
n
ItroGen
r
elease
f
ertIlIzers
Many types of controlled-release or slow-release fertilizers (S-coated, thermosetting resin-coated,
and thermoplastic resin-coated fertilizers) have been developed and evaluated for increasing crop
yield, fertilizer N use efficiency, and decreasing N losses (Shaviv, 2001; Shoji, 2005; Yang et al.,
2012). Enhanced efficiency of N fertilizers that control N release has been available in the U.S.
fertilizer market for several years, but their use has been limited due to their higher cost (Snyder,
2008; Stewart, 2008). However, increasing N fertilizer prices, heightened environmental awareness,
increasing area under conservation tillage, and improved manufacturing technology have led to an
increased interest in the enhanced efficiency of N fertilizers (Rochette et al., 2009a,b; Halvorson
et al., 2010b).
Yang et al. (2012) reported that controlled release of urea can improve activities of N metabolism
enzymes in leaves, and the efficiency of use of N of rice. Similarly, Cao et al. (2008) reported that
enhanced N recovery by rice is accomplished in part through increased activity of the N assimila-
tory enzymes. Sun et al. (2009) reported that GS (glutamine synthetase) content of leaves could be
adopted as an index to assess N accumulation. Significant positive correlation exists between the
activity of GS, a key N assimilatory enzyme, and the N status of some higher plants. In addition,
growth, yield, and/or protein content are sometimes correlated with GS levels in seeds and/or leaves
(Hageman, 1979; Srivastava, 1980; Yang et al., 2012).
The use of slow-release fertilizers can regulate N release from soil according to plant demand
and can reduce N losses by leaching and/or denitrification (Mosier et al., 1998; Halvorson et al.,
2010a,b; Venterea et al., 2011; Drury et al., 2012). This is especially important in regions where the
soil moisture is high in the weeks following planting and before the crop is large enough to assimi-
late large quantities of soil inorganic N (Drury et al., 2012). Slow-release fertilizer products may
also be considered by producers when they do not have time to apply N later in the growing season
because of other farming operations and/or because of wet soil conditions (Drury et al., 2012).
Soon et al. (2011) evaluated regular and polymer-coated urea for N losses. Their results showed
that N
2
O emissions were between 1.5 and 1.7 times greater from regular urea than polymer-coated
urea.
The loss of N can be reduced by adopting techniques that can reduce nitrification. Shoji et al.
(2001) reported that in Colorado, N
2
O emissions were very high in the first 3 weeks, following urea
application to barley. These authors further reported that when the nitrification inhibitor (DCD) or
polyolefin-coated urea was applied, the emissions were considerably lower during these 3 weeks
and then increased to similar levels as regular urea after this time period. The net effect relative to
regular urea was an 81% reduction in N
2
O emissions with DCD and a 35% reduction with coated
urea. Polymer-coated urea is used to reduce N losses from the soil via leaching or denitrification
by delaying urea hydrolysis until later in the growing season thereby reducing the rate of ammo-
nium and nitrate formation until the plants are larger and have bigger root system and consequently
greater N uptake capacity (Drury et al., 2012).
