NUTRIENT RETENTION BY MULTIPOND SYSTEMS - MECHANISMS FOR THE CONTROL OF NONPOINT-SOURCE POLLUTION

The processes of the multipond system in an experimental agricultural watershed located in southeastern China were studied during a 2-yr per iod (1994-1995), with the purpose of the research being the reduction of nonpoint nutrient pollution at its sources. The mechanisms studied included water storage capacity, sedimentation, denitrification, and r emoval of nutrients by the harvest of macrophytes from ponds and ditch es. The results showed that the retention of both water and nutrients depended on the water storage capacity of the ponds, the total pond vo lume, rainfall, surface runoff, and irrigation amounts. For the years of 1994 and 1995, the water retention rate was 85.5%, while the nutrie nt retention rate reached 98.1 and 97.8% for total N (TN) and total P (TP), respectively. Sediment deposit was another important mechanism. The average sedimentation rate was 30.0 mm yr(-1) (from 1985-1995). Fo r the whole multipond system (35 ha), the average retention amounts re ached 9800 kg of N and 2800 kg of P by sediment accumulation per year. The results demonstrated that denitrification in ponds and ditches wa s an important mechanism for removing N from the watershed. The highes t possible rate was more than 0.17 mg N g(-1) soil during the summer s eason. The results suggested that the multipond system, which kept wat er in balance, benefited the water, nutrient, and sediment recycling i n the terrestrial ecosystem, as web as helped to reduce agricultural n onpoint pollution at its sources. Therefore, the multipond system, wit h its low cost in construction and maintenance, is recommended as a go od practice both for the control of nonpoint pollution at its sources and for sustainable agricultural development.