Climate Change Mitigation - Climate Resilient Agriculture for Ensuring Food Security - page 298

inhibition of CH 4 production and emission by an
application of single superphosphate and a
smaller inhibition by an application of rock phos-
phate. They attributed this inhibitory effect to the
high PO 4 2− content of the P fertilizers. Nitrifi cation
inhibitors (thiourea, sodium thiosulfate, and
dicyandiamide) inhibited the CH 4 emission activ-
ity of fl ooded rice fi eld soil (Bronson and Mosier
1994 ).
Rath et al. ( 1999 ) found that the subsurface
application of urea super-granules was margin-
ally effective in reducing the CH 4 fl ux relative to
that in untreated control plots. Bronson and
Mosier ( 1994 ) reported that N fertilizers inhibit
methanotrophic microorganisms in soils.
Generally, fertilizers with an ammoniacal form of
N (NH 4 + -N) increase CH 4 emissions.
In principle, three different causes have been
suggested for the inhibitory effect of nitrogenous
fertilizers, especially NH 4 + -N fertilizers, on CH 4
oxidation which results in increased emissions of
CH 4 :
• An immediate inhibition of the methanotro-
phic enzyme system
• Secondary inhibition through the NO 2 - pro-
duction from methanotrophic NH 4 + oxidation
(Megraw and Knowles 1987 )
• Dynamic alteration of microbial communities
of soil (Powlson et al. 1997 )
(i) Advantages
• Crop growth and yields are stimulated,
while emissions are reduced compared to
fertilizers without mitigation potential.
(ii) Disadvantages
• Fertilizers with higher mitigation poten-
tial may cost more.
• Economics and mitigation potential.
According to Wassmann and Pathak ( 2007 ),
rice production without organic amendments
demonstrated the technical feasibility of reducing
emissions at relatively low costs.
The addition of phosphogypsum is an effi -
cient strategy to reduce emissions. Its actual
costs varied from US$ 1.5 to 2.5 per t CO 2 e
saved in the Philippines and China, respectively,
and the reduction potential is approximately 1 t
CO 2 e ha −1 . However, the relative cost for phos-
phogypsum application in Haryana (India) was

higher (US$5 per t CO 2 e saved), and the reduc-
tion potential was 0.29 t CO 2 e ha −1 .

13.1.5.9 Electron Acceptors
According to Lueders and Friedrich ( 2002 ),
methane emissions from paddy fi elds can be
reduced by the addition of electron acceptors to
stimulate microbial populations that compete
with methanogens. Under ferrihydrite amend-
ment, acetate was consumed effi ciently (<60 M),

and a rapid but incomplete inhibition of
methanogenesis occurred after 3 days.
Methanogenesis can be suppressed by the
supplementation of alternative electron acceptors
such as Fe (III) or sulfate, when electron donors
for respiratory processes become limiting
(Achtnich et al. 2005 ). This mitigation strategy is
based on the thermodynamic theory which pre-
dicts that the energetically more favorable elec-
tron acceptor will be utilized fi rst under
substrate-limiting conditions (Zehnder and
Stumm 1988 ). Microorganisms which can reduce
the energetically more favorable electron accep-
tor [e.g., nitrate, Fe (III), sulfate] will outcompete
those using a less favorable electron acceptor
(e.g., CO 2 ).
Functional shifts can occur within a rice fi eld
soil microbial community by supplementing
alternative electron acceptors in the form of fer-
rihydrite and gypsum, and thereby respiratory
processes other than methanogenesis are pro-
moted. Under gypsum addition, hydrogen was
rapidly consumed to low levels (~0.4 Pa), indi-
cating the presence of a competitive population
of hydrogenotrophic sulfate-reducing bacteria
(SRB). This was paralleled by a suppressed activ-
ity of the hydrogenotrophic RC-I methanogens as
indicated by the lowest SSU rRNA quantities.
Full inhibition of methanogenesis only became
apparent when acetate was depleted to non-
permissive thresholds (<5

μ

M) after 10 days.
The enhanced activity of FRB (Ferric iron-
reducing bacteria) and SRB (sulfate-reducing
bacteria) resulted in almost complete inhibition
of methanogenesis under conditions of limiting
substrate and non-limiting electron acceptor
availability. Considering the electron uptake
potential of eight electrons per CO 2 and SO 4 2 and

μ