Microbial Ecology of Flooded Rice Soils

Abstract

More than 90% of the rice fields in Asia are cultivated with the soil submerged during most of the growth period. Wetland rice fields are of three types: irrigated, shallow-water rain fed, and deep-water rain fed. Management practices in an irrigated rice field include: (1) leveling the land and constructing levees to impound water; (2) puddling of wet soil; (3) maintenance of 5–10 cm of standing water during rice growth; (4) draining and drying at harvest of rice; and (5) re-flooding after an interval of from a few weeks to as long as 8 months. In shallow-water rain-fed fields, the soil is submerged as in the irrigated fields, but the rice crop is frequently subjected to drought or to flooding deeper than the height of rice. In deep-water rice fields, the rice is sown on dry land before the onset of the monsoon, and the depth of the floodwater gradually increases as the rice grows, sometimes reaching 2-5 m. Near the harvest, the floodwater is gradually drained. Rain-fed rice fields in monsoon Asia may be subjected to severe desiccation during the dry season.