Agriculture Reference
In-Depth Information
accomplished by nitrates produced by its nitrification (Conrad & Rothfuss, 1991; Jugsujinda
et al., 1995). On the other hand, ammonium is inhibitory for methanotrophy, reduces CH
4
reoxidation resulting in an increase of net soil emission (Conrad & Rothfuss, 1991).
Compared with urea, it has been confirmed that the advantage of using ammonium sulfate
reduced CH
4
emission by 50
-
60% (Kimura et al., 1992; Lindau, 1994).
N fertilizers containing nitrate can also reduce CH
4
production/emission since nitrate
application causes a competition for H
2
between denitrifying bacteria and methanogens,
favoring denitrifying bacteria. Nitrate is active also in the reduction of CH
4
emission from
soil by increasing soil Eh, since it is an oxidant (Jugsujinda et al., 1995; Roy & Conrad,
1999).
Methane consumption in soil, in contrast to CH
4
production, is broadly affected by
agricultural N use. Methanotrophic bacteria capable of consuming atmospheric CH
4
are found
in most aerobic soils, including arable lands, making the uptake of CH
4
globally important.
The global soil sink of CH
4
has been estimated to be about 30 Tg CH
4
year
-
1
, corresponding
to the same magnitude as the annual atmospheric increase of CH4 (about 37 Tg CH
4
year
-
1
)
(Robertson et al., 2013). In natural terrestrial ecosystems, CH
4
uptake is limited by its rate of
diffusion in soil microsites and by methanotrophic activity (von Fischer et al., 2009).
Diffusion is regulated by some physical factors: moisture, temperature, soil structure and CH
4
concentration in the bulk soil atmosphere. Some authors reported that agricultural
management reduced soil CH
4
oxidation approximately 70% or more (Mosier et al., 1991;
Robertson et al., 2000).
The mechanism responsible of this suppression is largely related to soil N availability as
affected by N fertilizers, and other N inputs (Steudler et al., 1989; Suwanwaree & Robertson,
2005). Ammonium is known to competitively inhibit CH
4
mono-oxygenase, the principal
enzyme responsible for oxidation at atmospheric concentrations. For this reason ammonium
and urea usually inhibit atmospheric CH
4
oxidation, contrary to the nitrate (Le Mer & Roger,
2001). In addition there is a transfer of the CH
4
oxidizing activity towards nitrification and the
toxicity of NO
2
-
produced (Le Mer & Roger, 2001). Nitrite, the end product of
methanotrophic ammonia oxidation, was found to be a more effective inhibitor of CH
4
consumption than ammonium (Schnell & King, 1994).
E
FFECT OF
N
F
ERTILIZATION ON
S
OIL
N
2
O
E
MISSION
N
2
O is a greenhouse gas with a global warming potential 298 times greater than CO
2
whose emission to the atmosphere is mainly from global soils. The increase of N
2
O in the
atmosphere is due to the human alterations of the global N cycle, with 24% of annual
emissions produced by agricultural soils and the application of N fertilizer (Bouwman, 1996
and 2002a; IPCC, 2007). Soils are a key source of N
2
O emissions to the atmosphere
contributing to about 53% of the global anthropogenic emission (Denman et al., 2007) and are
directly related to the combined effect of climate, crop management and soil physical-
chemical characteristics affecting microbial driven processes.
Soil N
2
O production is the biogenic product of microbial processes of denitrification and
nitrification, as affected by physical-chemical characteristics of soil. Bacterial denitrification
is a respiratory reduction of nitrate and/or nitrite to gaseous NO, N
2
O and N
2
, coupled to
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