The functioning and productivity of pre-Columbian raised fields (RFs) and their role in the development of complex societies in Amazonian savannas remain debated. RF agriculture is conducted today in the Congo Basin, offering an instructive analogue to pre-Columbian RFs in Amazonia. Our study of construction of present-day RFs documents periodic addition of organic matter (OM) during repeated field/fallow cycles. Field investigations of RF profiles supported by spectrophotometry reveal a characteristic stratigraphy. Soil geochemistry indicates that the management of Congo RFs improves soil fertility for a limited time when they are under cultivation, but nutrient availability in fallow RFs differs little from that in uncultivated reference topsoils. Furthermore, examination of soil micromorphology shows that within less than 40 years, bioturbation almost completely removes stratigraphic evidence of repeated OM amendments. If Amazonian RFs were similarly managed, their vestiges would thus be unlikely to show traces of such management centuries after abandonment. These results call into question the hypothesis that the sole purpose of constructing RFs in pre-Columbian Amazonia was drainage. Recent research in Amazonia has revealed that centuries of human occupation before European arrival have in many sites profoundly altered soils and landscapes 1,2. The widespread presence of Terra preta do indio and similar Anthrosols, collectively termed Amazonian Dark Earths (ADEs) 3,4 , and tree species composition altered by human activities 5-7 , belie previous perceptions of 'pristine' Amazonian forests. Seasonally inundated savanna regions of Amazonia offer other striking examples of human-transformed landscapes: large areas of pre-Columbian wetland raised fields (RFs). These are elevated earthen structures of varying size and shape, from round mounds < 2 m in diameter to platforms up to 100 m broad and hundreds of meters long, on which farmers grew flood-intolerant crops 8. RFs were an important component of subsistence systems for people living in these environments 8,9. However, soils of RF landscapes have received much less attention than the ADEs that are widespread in forested Amazonia, and how RF soils were managed remains poorly understood 8,10. Knowledge on the functioning of RF soils is crucial for a better understanding of the societies that depended on them. It could also be useful in conceiving sustainable ways to use similar environments today 11. In Amazonia, as elsewhere, one key function of RFs was to provide well-drained ground for crops 8,9,12-14. But whether making RFs in Amazonian savannas conferred other benefits has been much debated, and the predominant opinions have shifted over time. Early research on Amazonian RFs was inspired by comparisons
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