Nitrogen fixation in soils and canals of rehabilitated raised-fields of the Bolivian altiplano

Raised-field agricultural systems were exploited extensively by pre-Columbian Andean civilizations and now are used less extensively by contemporary Bolivian and Peruvian farmers as a method for farming the perennially wet soils of the intermontane pampas of the Andean high altiplano. The raised-field agricultural systems are linear or semi-linear raised planting platforms interspersed with shallow, parallel canals filled with water. The elevated planting platforms raise the critical rooting zone of crops such as potatoes, quinoa, or barley above perennially saturated soils. A hypothesized advantage of the raised-field system is that agricultural production can be increased by exploiting the ability of bacteria and Azolla fililculoides to fix nitrogen in the canals adjacent to the planting platforms. To test this hypothesis, nitrogen fixation was estimated in situ in the water column of canals, in the soils of planting platforms, in the soils of a dry hillside field, and in the sediments and water column of marshes and totora beds. Nitrogen fixation was significantly greater in the canals of the raised-field systems and in marshes compared to planting platforms and the hillside field. These data support the theory that pre-Hispanic agricultural prosperity of the Andes, as well as other regions of Central and South America, was partially supported by extensive development of raised-field agriculture and probable exploitation of nitrogen fixation.