Impact of rice cultivation systems

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Rice is a unique crop in that it is highly adaptable and can be grown in very diverse environments (Yoshida, 1981). The ecosystems within which rice is grown may be characterized by seasonal change of temperature, rainfall pattern, depth of flooding and drainage, and by the adaptation of rice to these agroecological factors. In addition, the degree of water control available is a useful tool to classify rice ecosystems because it characterizes management design for improving productivity (Huke and Huke, 1997) and also the conditions for CH4 emissions (Sass et al, 1992; Yagi et al, 1996; Wassmann et al, 2000b). The major rice ecosystems are classified as irrigated, rainfed, deepwater and upland. Irrigated rice can be further subdivided into continuously and intermittently flooded systems, and rainfed rice into regular, drought-prone and deepwater systems, according to the flooding patterns during the cultivation period.

The available database indicates that the CH4 emission per unit area and season follows the order: continuously flooded irrigated rice >intermittently flooded irrigated rice >deepwater rice >regular (flood-prone) rainfed rice >drought-prone rainfed rice (Table 8.1). Upland rice is not a source of CH4, since it is grown in aerated soils that never become flooded for any significant period of time. However, this ranking only provides an initial assessment of the emission potentials that can locally be superseded by crop management favouring or lowering actual emission rates (Wassmann et al, 2000a, 2000b). The flooding pattern before the cultivation period significantly influences the emission rates (Fitzgerald et al, 2000; Cai et al, 2003). Differences in residue recycling, organic amendments, scheduled short aeration periods, soils, fertilization and rice cultivars are major additional causes for variations of CH4 fluxes in rice fields. Various organic amendments incorporated into rice soils, either of endogenous (straw, green manure etc.) or exogenous origin (compost, farmyard manure etc.), increases CH4 emissions (Sch├╝tz et al, 1989; Yagi and Minami, 1990; Sass et al, 1991). The impact of organic amendments on CH4 emissions depends on type and amount of the applied material which can be described by a dose response curve (Denier van der Gon and Neue, 1995; Yan et al, 2005). Lowest CH4 fluxes are recorded in fields with low residue recycling, multiple aeration periods, poor soils and low fertilization with resulting poor rice growth and low yields. The source strength of rainfed rice is most uncertain because of its high variability in all factors controlling CH4 emissions.

Table 8.1 Ratio of the areas of irrigated rice fields subject to various water regimes

Country

Continuous flooding

Single drainage

Multiple drainage

Source

India

0.30

0.44

0.26

ALGAS report3

Indonesia

0.43

0.22

0.35

ALGAS report

Vietnam

1

0

0

ALGAS report

China

0.2

0

0.8

Li etal, 2002

Japan, Korea, Bangladesh

0.2

0

0.8

Assumed to be the same as China

Other monsoon Asian countries

0.43

0.22

0.35

Assumed to be the same as Indonesia

Other countries

0.3

0.44

0.36

Assumed to be the same as India

Note: ' ALGAS = Asia Least Cost Greenhouse Gas Abatement Strategy. www.adb.org/REACH/algas.asp, the website of the Asian Development Bank Source: Yan et al (2009)

Reports were downloaded from (ADB).

Note: ' ALGAS = Asia Least Cost Greenhouse Gas Abatement Strategy. www.adb.org/REACH/algas.asp, the website of the Asian Development Bank Source: Yan et al (2009)

Reports were downloaded from (ADB).

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