NUTRIENT AND SEDIMENT RETENTION IN ANDEAN RAISED-FIELD AGRICULTURE

RAISED-FIELD agriculture was widespread throughout Central and South A merica in prehispanic times1,2. In this system of agriculture, crops a re cultivated on a series of raised beds, which are separated from one another by deep, water-filled channels. In some regions, rehabilitati on of the raised fields is now underway, largely because this practice leads to fertile soils, adequate water supply and protection from fro st and therefore to substantially higher yields than more conventional methods3,4. Here we report analyses of water quality in the channels alongside rehabilitated raised fields in the vicinity of Tiwanaku, on the Bolivian side of the Lake Titicaca basin (Fig. 1). We find that hi gh concentrations of nitrate, available phosphate and turbidity declin e significantly as the water flows through the raised-field channels. Water flowing through control sites shows no significant change. Reten tion of nutrients and suspended sediments in the channels helps to mai ntain soil fertility and reduces pollution of down-stream waters. Thus it seems there are environmental benefits in rehabilitating raised fi elds, which complement and help sustain the economic benefits demonstr ated previously3,4.