Historical development

Single-crop irrigated rice systems in Asia date back several thousand years. Double cropping became common in the longer Yangzi River region about 1,000 years ago and triple cropping probably started in the 14th century (Greenland 1997). Naturally occurring sedimentation, nutrient inflow by irrigation, organic residues, biological N: fixation, and carbon assimilation by floodwater flora and fauna played an important role in securing the sustainability of these traditional irrigated rice systems (Greenland 1997). Around 1.000 years ago, rice yields in China and Japan were about 1 t ha~' and it took several hundred years to increase them to 2 t ha"1 (Greenland 1997).

The breeding and widespread adoption of high-yielding varieties in the 1960s led to a rapid intensification in the tropical lowlands of Asia. The release of semidwarf short-duration varieties such as IR8 (1966), IR20 (1969), and IR36 (1976) triggered investments in irrigation infrastructure and allowed farmers to grow two to three rice crops per year. The new varieties had a short growth period and more efficient biomass partitioning, were short-statured and lodging-resistant, and responded well to fertilizer N additions. Tillage and management intensity increased and soils remained submerged for longer periods. The use of external inputs such as fertilizers, water, energy, and pesticides increased and the diversity of rice varieties used in the irrigated systems decreased. By growing two or three short-duration crops per year, each at a higher yield level than before 1965, annual crop nutrient removal increased five- to sevenfold compared with the pre-Green Revolution period. The use of N:-fixing green manure ceased in many rice areas because its main purpose of providing N was replaced by cheap and less laborintensive mineral N. To facilitate land preparation for the next crop, fanners started to cut the entire crop and remove or burn the straw (Uexkuell and Beaton 1992). The average grain yield of irrigated rice reached 4.9 t ha~' in 1991 (Cassman and Pingali 1995a).

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