Agriculture Reference
In-Depth Information
C
6
H
12
O
6
3 CO
2
+ 3CH
4
.
This transformation requires successive actions by a sequence of populations of micro-
organisms that degrades complex molecules into simpler compounds: (1) hydrolysis of
biological polymers into monomers (glucides, fatty acids, amino acids) by an aerobic, or
facultatively, or strictly anaerobic microflora; (2) acetogenesis from the previous metabolites
by a syntrophic or homoacetogenic microflora; (3) methanogenesis from the simple
compounds that can be used by methanogens (in particular H
2
, CO
2
and acetate ion) which
constitutes the last step of the methanogenic fermentation. Methanogenesis, requires strict
anaerobiosis and low oxydo-reduction potentials (Eh < -200 mV).
Methanotrophy in soils is known in two forms. The first form is called high affinity
oxidation, and occurs at CH
4
concentrations close to that of the atmosphere (< 12 ppm). It
appears to be ubiquitous in soils that have not been exposed to high NH
4
+
concentrations
(Topp & Hanson, 1991). High affinity oxidation is estimated to contribute 10% of total CH
4
consumption (Topp & Pattey, 1997). The second form of oxidation is a low affinity oxidation
occurs at CH
4
concentrations higher than 40 ppm. It is considered as methanotrophic activity
in the strictest sense (Jones & Nedwell, 1993). Methane oxidation in methanogenic
environments (ricefields, peat soils, landfills, etc.) is a low affinity type.
Methanotrophs use CH
4
as carbon and energy source. Oxygen availability is the main
factor limiting their activity.
Different environmental conditions and factors affect the two microbial processes. Gas
diffusion and its consequentual relationship to the oxydo-reduction level and CH
4
transfer.
This factor is in turn affected by water content and soil texture. In addition, the typical
conditions affecting the microbial activities: temperature, pH, Eh, substrate availability,
physicochemical properties of soils.
Lowland rice cultivation represents the only major source of CH
4
from established
cropping systems; about 40 Tg year
-
1
are emitted from rice soils worldwide (Sass et al.,
1999).
The effect of N fertilization on net soil-atmosphere CH
4
emission occurs by means of
different actions, even if its effect on CH
4
emissions from rice fields is not well understood
(Zuo et al., 2005) and data from literature are sometimes contradictory, depending by the
nature of the fertilizer and the quantity applied (Lindau, 1994). Urea application in rice fields
is usually reported as a fertilization practice increasing CH
4
emission by increasing rice
productivity and soil pH resulting from urea hydrolysis (Wang et al., 1992; Lindau & Bollich,
1993). Bufogle et al. (1998) and references therein reported that CH
4
emissions were higher
in ricefields fertilized with urea than those fertilized with ammonium sulfate, since in extreme
anaerobic condition sulfate-reducing bacteria compete with methanogenic bacteria. Under
such conditions, electron acceptors other than CO
2
, especially nitrate and sulfate, may cause
bacterial competition which is unfavourable to methanogens and decrease CH
4
production/emission. H
2
and acetate are preferentially used by sulphate-reducing bacteria,
sulfate application generally reduces methanogen activity. Ammonium sulfate was frequently
reported to significantly reduce (30 to 60%) CH
4
flux from ricefields (Bronson et al., 1997;
Cai et al., 1997). Wang et al. (1992) explained that the negative effect on CH
4
production of
sulfate addition with N fertilizer was not associated with an increase in soil Eh, so a direct
inhibitory effect of sulfate on methanogenesis was assumed. Moreover, indirect ammonium
inhibition for methanogenesis, and consequent CH
4
production, was hypothesized to be
Search WWH ::
Custom Search