Agriculture Reference
In-Depth Information
urease
2NH
4
þ
þ
HCO
3
þ
OH
;
CO NH
2
ð
Þ
2
þ
3H
2
O
!
ð
11
1
Þ
:
3
2
O
2
!
NH
4
þ
þ
NO
2
þ
2H
þ
þ
H
2
O
ð
11
2
Þ
;
:
1
2
O
2
!
NO
2
þ
NO
3
:
ð
11
3
Þ
:
Depending on conditions such as cation-exchange capacity (CEC), texture class
(clay content), pH, and soil temperature, NH
4
+
can also be converted to NH
3
, which
is lost to the atmosphere (Byrnes
2000
).
Thus, it is observed that not every compound applied to crops as fertilizer is
absorbed by plants. Losses may occur through leaching, evaporation, fixing, or even
by erosion (Aarnio and Martikainen
1995
; Resende
2002
). The average nitrogen
content absorbed by crops, particularly in tropical regions, is between 50 and 70 %
(Finck
1992
; Mortvedt et al.
1999
). It means that an average of 60 % of this nutrient
is lost after application, causing part of investment in fertilization not to return as
expected. The loss of nitrogen through volatilization of NH
3
to the atmosphere is
the main problem responsible for the low efficiency of urea applied to the soil
surface, which may lose up to 80 % of applied nitrogen (Tabatabai and Bremner
1972
; Wang et al.
2004
). In addition to the economic losses, there are also
environmental damages because the excess of those nutrients in the soil can lead
to a contamination of both water and atmosphere, depending on the applied
material. Thus, the efficient use of agrochemicals is essential and strategic to obtain
maximum crop yields and to minimize the economic and nutrient losses to the
environment.
The nitrogen cycle, as for other elements, is normally in equilibrium. However,
this cycle can be driven by agricultural or industrial activities or even by natural
causes. This often results in undesirable accumulation of intermediates; many of
them show certain toxicity. When the nitrogen cycle becomes unbalanced, there are
often ecological problems. For example, excess NH
4
+
and oxidized nitrogen com-
pounds promote algal blooms, which in conjunction with other problems cause
oxygen depletion. The NO
2
and NO
3
ions, at certain levels, may be toxic to
humans and have been associated with various diseases. Nitrogen compounds are
also one of the causes of acid rain (Kuenen and Robertson
1988
; Liu et al.
2013
).
Another problem, originated from the imbalance of the nitrogen cycle in soil, is
the release of gases such as N
2
O and NO, which can also cause environmental
problems. In the soil, these gases are produced mainly by denitrification process,
that is, the conversion of NO
3
to N
2
, but this process may produce NO and N
2
Oas
intermediates (Eq. (
11.4
)), in the presence of specific enzymes. This process
primarily occurs under anaerobic conditions, but can also occur in partially anaer-
obic conditions (anaerobic sites in the soil), which indicates that during the nitri-
fication process, N
2
O can also be formed (Bremner
1997
; Kool et al.
2011
).
