EFFECT OF THE SYSTEM OF RICE INTENSIFICATION (SRI) ON ...

The degree of GHGs emissions will to a large extend depend on what type of rice cropping systems
is used. Irrigated/continuously flooded systems will pose the largest risks. Flooded rice fields with
anaerobic conditions will produce methane from decomposition of organic matter anaerobically
(Wassmann et al., 2000) and emit CH4 through rice plants (roots and stems) as either diffusion or
ebullition (Neue, 1993). Methane is found in smaller amounts in the atmosphere than CO2 but one
molecule of methane will trap as much heat as 30 molecules of CO2 (Neue, 1993), although Yue et
al. (2005) state that the Global Warming Potential (GWP 3 ) of CH4 is 23.
Nitrous oxide (N2O) is emitted when N is added to agricultural soils, in forms of e.g. fertilizers,
through volatilization of e.g. NH3, leaching, run-off and redeposition (Zou et al., 2009) and though
it is found in smaller concentrations than CO2 in the atmosphere the GWP of N2O is 296 times those
of CO2 (Li et al., 2009). N2O in soils is derived from production of nitrification and denitrification
processes (Zou et al., 2005) and is favored by aerobic conditions. CO2 derived from soil is produced
from respiration from microbes, roots and from bulk soil respectively (Xu et al., 2008).
3.2 The effect of organic amendments and chemical fertilizers on GHG
emissions
Organic amendments or chemical fertilizers will pose various and different affects on GHGs
emissions from soils. Lou et al., (2007) conducted studies on CO2 and N2O emissions from irrigated
rice fields and concluded that when rice straw was applied and incorporated in the soils the
emissions of CO2 and N2O were much higher than fields where straw had been removed and only
rice roots were left. This is because straw contains more cellulose than roots and roots contain more
lignin than straw and will therefore decompose slower than straw (Lou et al., 2007). Irrigated rice
soils amended with rice straw emit more than twice as much CH4 compared to soils amended with
mineral fertilizers (Yagi & Minami, 1990) and soils without any amendments (Watanabe et al.,
2005). However disposal of rice straw long time before a rice crop is initiated will decrease
methane emissions (Yan et al., 2009). Yan et al. (2009) estimated that if all single and double
cropping rice growing areas would apply their straw off the season, methane emissions could be
decreased with 4.1 Tg a - 1 per year. Yan et al. (2009) develop a theoretical map of south East Asia
(figure 2) illustrating potential mitigation possibilities (drainage and application of straw off the
growing season) and their effects per year on the emission of methane.
3 GWP is a set of tools comparing a given GHG and how much heat it will capture compared to CO2 (Li et al., 2009)
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