Agriculture Reference
In-Depth Information
phosphorylation electron transport. Many aerobic microorganisms use NO
3
-
as electron
acceptor to derive energy from organic compounds when oxygen tension is low
(heterotrophic denitrification), and, in this process, N
2
O is an obligatory intermediate.
Microbial nitrification is the oxidation of ammonium to nitrite and nitrate and, in most
soils, autotrophic bacteria are responsible of this process, even though some studies suggest
heterotrophic nitrifiers may also contribute to nitrification and N
2
O production (Schimel et
al., 1989; Anderson et al., 1993). N
2
O is not an obligatory intermediate of nitrification
process, since when O
2
supply is limited in soil (denitrification by nitrifiers), autothrophic
bacteria can produce nitrous oxide by enzymatic reduction of nitrite. Autothrophic
ammonium-oxidizing bacteria such as
Nitrosomonas europea
can use NO
2
-
as an alternative
electron acceptor under anaerobic conditions, thus reducing NO
2
-
to N
2
O by the nitrite
reductase enzyme (Firestone & Davidson, 1989; Groffman, 1991). The production of N
2
O
may also be caused by other microorganisms, e.g., during dissimilatory reduction of nitrate to
ammonium, nitrate reduction to nitrite, and nitrate assimilation (Smith & Zimmerman, 1981;
Bleakley & Tiedje, 1982). The mechanism of N
2
O production by these bacteria and their role
in N
2
O production in soil require extensive study.
Typical key soil factors affecting the two microbial processes are: pH, temperature, total
and mineral nitrogen content, labile organic matter availability, water content, soil aeration,
redox potential. It is fundamentally necessary to distinguish between soil production and soil
emission, since the N
2
O produced by the two microbial processes does not necessarily leave
the soil, depending on soil physical characteristics. Nitrification and denitrification can be
active simultaneously in soil, since it is a complex and heterogeneous system with aerobic
and anaerobic microsites, which are not homogeneously distributed, and, consequently, N
2
O
can be evolved via both these processes (Nielsen et al., 1996; Abbasi & Adams, 2000). At the
same time, in microsites where anaerobic conditions are extreme, denitrification process can
utilize N
2
O produced by the same process or by nitrification, thus reducing the gas emission
from soil. Davidson (1991) presented a simplified model (hole in the pipe model) to describe
the processes affecting production and emission of N
2
O from soils. The model identifies three
levels of control. Level I is represented by all factors affecting the rates of nitrification and
denitrification; level II is for N
2
O emissions depending on how soil physical-chemical
parameters affect the ratio of end-products via both processes and on how fast; level III
accounts for N
2
O diffusing to atmosphere through the soil gaseous phase.
Typical soil managements, such as nitrogen fertilization and irrigation, are responsible
for large N
2
O fluxes. The great application of N fertilizers in traditional croplands, such as
NO
3
-
, NH
4
+
, NH
4
NO
3
and urea, is a key controller of microbial processes involved in N
2
O
evolution from soil (Bremner & Blackmar, 1980; Duxbury et al., 1982; Dambreville et al.,
2006). Eichner (1990) listed the following factors affecting fertilizer-derived N
2
O emissions:
(1;
management factors
) fertilizer type, application rate, application technique, application
timing, tillage system, use of other chemicals, crop type, irrigation, and residual N and C from
crops and fertilizer and (2;
environmental factors
) temperature, precipitation, soil moisture
content, SOC content, soil O
2
status, soil porosity, soil pH, freezing and thawing cycles, and
microorganism abundance and activity.
Bouwman (1996) summarized international research results on the release of N
2
O from N
fertilizers, and developed a generalized fertilizer induced emission (FIE) of 1.25% of the
amount of N fertilizer applied. The fraction of applied N actually emitted as N
2
O varies
widely on a site-specific basis. Even if the 2006 IPCC Guidelines for National Greenhouse
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