SIGNIFICANCE OF NUTRIENTS UPTAKE MECHANISMS IN CROPPING SYSTEMS

摘要

Management of nutrients uptake and management in cropping systems is increasing it importance. Although the concept of sustainability originated mainly from pressures of environmentalists in the developed countries, concerns on long-term viability ofcropping systems have implications across all scales of farming and in all agroecological zones. In developed countries, adoption of certain features of cropping systems from the Green Revolution era, basically .depending of heavy use of chemical fertilizer, has systematically displaced local practices and knowledge, which were inherently sustainable and environmentally friendly. Nitrate contamination issues recently uncovered in north central Europe, the US and several countries in Asia were resulted from what seemed to be optimal economic fertilization practices (Fedkiw, 1991). Researchers estimate that 20% of the N that humans are putting into watersheds is consistently getting into rivers. Situation is getting even worse. In Japan, 670,000t of N fertilizer is applied annually to agricultural field, which equals to the application of 130kg N ha~(-1) to whole agricultural field. In addition to this, 700,000t of N in animal manure is also applied mainly to agricultural fields. This heavy N application is causing nitrate contamination of ground water and watershed surrounding agro-ecosystems. In future, 690,000t of various forms of wastes from human activities is expected to be applied to agricultural field. To lessen the nitrate pollution, technologies which can improve inorganic and organic N use efficiencies based on the good understandings on N dynamics in cropping systems and N acquisition mechanisms of crops have to be developed.