Causes of Climate Change - Climate Resilient Agriculture for Ensuring Food Security - page 24

produced in the rumen by fermentation, practices
that speed the passage of feed from the rumen
can also reduce methane formation.
Methane is also formed in soil through the
metabolic activities of a small but highly specifi c
bacterial group called “methanogens.” Their
activity increases in the submerged, anaerobic
conditions developed in the wetland rice fi elds,
which limit the transport of oxygen into the soil,
and the microbial activities render the water-
saturated soil practically devoid of oxygen. The
upland, aerobic soil does not produce methane.
Water management, therefore, plays a major role
in methane emission from soil. Altering water
management practices, particularly mid-season
aeration by short-term drainage as well as alter-
nate wetting and drying, can greatly reduce meth-
ane emission from rice cultivation. Improving
organic matter management by promoting aero-
bic degradation through composting or incorpo-
rating into soil during off-season drain period is
another promising technique.

to the soil. There are also indirect emissions
attributed to soil leaching of N 2 O and atmo-
spheric deposition of nitrogenous compounds
from agricultural activities.
A large amount of N 2 O is also emitted from
microbial denitrifi cation of solid waste from live-
stock, primarily cattle. The amount released depends
on the size of the animal, the amount of nitrogen in
the waste, and the method of managing the waste.
Nitrous oxide has the capacity to absorb and
reemit approximately 310 times the amount of
heat compared to carbon dioxide - it has 310
times the global warming potential (GWP) of
carbon dioxide. One kilogram of N 2 O is therefore
310 kg CO 2 e. Nitrous oxide is released during the
production and use of manufactured fertilizers.
The diagram below illustrates the chain of events
in nitrogen cycling (Fig. 2.6 ).
Two chemical reactions in the soil produce
N 2 O. The process of nitrifi cation can take place
whereby microorganisms in the soil transform
ammonium (NH 4 +) to nitrate (NO 3 −). N 2 O is a by-
product of nitrifi cation. A second process, denitrifi -
cation, involves the transformation of nitrate
(NO 3 −) (from the nitrifi cation process or from the
application of nitrate fertilizers) to nitrogen gas
(N 2 ) and which N 2 O is again a by-product.
Because nitrogen is such an important part of
agricultural systems, the production of N 2 O can-
not be eliminated from farming systems. However,
N 2 O production (by nitrifi cation and denitrifi ca-
tion) needs to be controlled and reduced. This can
be achieved through certain methods of manure
and slurry application and storage, attention to
accurate nutrient budgeting to achieve better
nitrogen utilization in crop and grassland produc-
tion systems, and attention to drainage and soil
management. High N 2 O emissions can occur
when clover leys and other crop residues are
plowed in if no growing crop is present to take up
the nitrogen that is released, this again needs to be
considered and managed carefully.

2.3.4 Nitrous Oxide (N 2 O)

Nitrous oxide is a powerful greenhouse gas pro-
duced by soil cultivation practices, especially the
use of commercial and organic fertilizers, fossil
fuel combustion, nitric acid production, and bio-
mass burning. Forests, grasslands, oceans, soils,
nitrogenous fertilizers, and burning of biomass
and fossil fuels are the sources of N 2 O while it is
removed by oxidation in the stratosphere. Soil
with a contribution of about 65 % is the major
contributor to the total N 2 O emission. The main
processes that cause emission of N 2 O are soil cul-
tivation, fertilizer and manure application, and
burning of organic materials and fossil fuels.
About 75 % of agricultural emissions of N 2 O
have been attributed to nitrogen fertilizer applica-
tion to soils, including direct emissions from syn-
thetic fertilizers, biological fi xation in crops, and
crop residues. This in turn depends on the type of
fertilizer that is used, how and when it is used,
and the methods of tilling that are followed.
Contributions are also made by leguminous
plants such as beans and pulses that add nitrogen

2.3.5 Chlorofl uorocarbons (CFCs)

CFCs are synthetic compounds entirely of indus-
trial origin used in a number of applications, but