Introduction - The Biogeochemistry of Submerged Soils - page 3

Table 1.2 Rice ecosystems in the main rice-producing countries in Asia
Harvested area (kha)

Irrigated Rainfed lowland Flood-prone Upland Total

WS a DS a 0-30 b 30-100 b

India 15 537 4123 11 985 4447 1364 5060 42 516

China 20 490 9146 1990 0 0 499 32 125

Indonesia 2963 2963 2872 1006 2 1209 11 015

Bangladesh 351 2267 3271 2873 1220 697 10 679

Thailand 274 665 6382 1778 342 203 9644

Vietnam 1630 1630 1963 651 177 322 6373

Myanmar 1812 1386 2033 478 362 214 6285

Philippines 1175 1029 911 341 0 165 3621

Pakistan 2125 0 0 0 0 0 2125

Cambodia 140 165 1069 349 152 24 1899

Nepal 706 24 406 166 118 68 1488

Korea, Rep. of 776 0 326 0 0 1 1103

Sri Lanka 377 251 213 26 0 0 867

Total 49 211 24 003 34 056 12 131 3737 8853 13 1991

a Wet/dry season.
b Depth of floodwater (cm).
Definitions of ecosystem types:
Irrigated . Grown in levelled, bunded fields with good water control. Crop is transplanted or direct seeded in
puddled soil, and a shallow floodwater is maintained on the soil surface so that the soil is predominantly anoxic
during crop growth.
Rainfed lowland . Grown in level to gently sloping, bunded fields that are flooded for at least part of the cropping
season. Water depths exceed 100 cm for no more than 10 consecutive days. Crop is transplanted in puddled soil or
direct seeded on puddled or ploughed dry soil. During season soil alternates between oxic and anoxic conditions
of variable duration and frequency.
Flood-prone . Distinguished from rainfed lowland rice by extent and duration of flooding. Fields are flooded to at
least 100 cm and often much more for at least 10 consecutive days in the growing season. Crop is transplanted
in puddled soil or direct seeded on ploughed dry soil; soil may alternate between oxic and anoxic conditions
during season.
Upland . Grown in level to steeply sloping fields that are rarely flooded. No effort is made to impound water as
for other rice ecosystems. Crop is direct seeded on ploughed dry soil or dibbled in wet, non-puddled soil.
Source : IRRI (2002). Reproduced by permission of IRRI.

'ecosystems' distinguished by land and water characteristics and adaptations of
the rice plant to them. These are defined in Table 1.2 together with their extents
in the main rice-producing countries in Asia.

1.2 BIOGEOCHEMICAL CHARACTERISTICS

Wetlands are intermediate between upland systems and true aquatic systems, both
in terms of their hydrologies, being intermittently to permanently flooded, and
in terms of their biogeochemistries, being sources, sinks and transformers of