An ancient method of raised-field farming now being revitalized on the plains surrounding South America's Lake Titicaca offers environmental lessons for modern agriculture, according to an aquatic scientist at the University of California, Davis. The system of raised, cultivated beds separated by deep channels of water successfully filters out nutrients and sediment, making them available for crops and preventing downstream water pollution, reports Heath J. Carney, a researcher in environmental studies. Carney's findings appeared this month in the journal Nature. "What is great about this system is that here we have something that is pre-Columbian, and the more you look at it, the more ecological wisdom you see," said Carney of the raised-field method of agriculture. "These practices retain and recycle essential nutrients and organic matter, while minimizing erosion. As a result, sediment and associated nutrients don't cause the downstream and nearshore pollution observed in other parts of the lake." Carney and colleagues conducted their study on the Bolivian side of Lake Titicaca -- by volume Latin America's largest lake -- which straddles the border between Peru and Bolivia. Water flows from the Andean foothills across broad, grassy plains into Lake Titicaca. Plains people, whose culture flourished from 700 A.D. to 1000 A.D., originally developed the system of raised fields, according to Carney. They intercepted stream and spring water for irrigation in the canals of the raise-field systems, then released it to flow into the lake. These same fields were cultivated until about 800 years ago, he said. Rehabilitation of the raised-fields on the Bolivian side of the lake began about 15 years ago after archaeologists began to examine the area. The raised-field system of cultivation provides the local subsistence-level potato farmers with fertile soils, adequate water supply and frost protection, Carney said. Data gathered by his research team helps explains explain why the method is so successful. Over a period of 14 months, the research team analyzed water quality in two raised-field sites and two conventional fields devoted to dry-land farming with little or no irrigation. They monitored the concentration of the nutrients nitrate and phosphate in the water and in the aquatic vegetation, as well as the amount of sediment suspended in the water. Samples were taken monthly during the wet season and every other month during the dry season. The analyses indicated that the nutrients are taken up by aquatic plants found in the raised-field channels. Farmers harvest these aquatic plants and spread them on the raised beds, using them as organic fertilizer. The nutrient-rich sediment is likewise recovered from the channels and spread on the raised fields during the dry season, thus maintaining the canal network and improving the planting beds. While raised-field farming offers important benefits in terms of nutrient flow and cycling, Carney stressed that the type and extent of these environmental benefits depend on the location of the raised fields, local hydrological conditions and agricultural management practices. Ideally, such raised fields should be located in flat plains near a lake shore, so that they can intercept and retain both nutrients and sediments before the water enters the lake, he said. Collaborating on the study were Charles R. Goldman of UC Davis, Michael W. Binford of Harvard University, Alan L. Kolata of the University of Chicago and Ruben R. Marin of Universidad Mayor de San Andres in Bolivia. Funding was provided by the U.S. Department of State, the National Science Foundation, the Environmental Protection Agency and UC Davis.