A Congo Basin ethnographic analogue of pre-Columbian Amazonian raised fields shows the ephemeral legacy of organic matter management

Leonor Rodrigues,Doyle Mckey,Tobias Sprafke,Bernard G Barthès,Stéphen Rostain,Joseph Yoka,Isabelle Bertrand,Carine Bokatola Moyikola,Marion Comptour

doi:10.1038/s41598-020-67467-8

Abstract

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.

Highlights

The functioning and productivity of pre-Columbian raised fields (RFs) and their role in the development of complex societies in Amazonian savannas remain debated
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[5,6,7], belie previous perceptions of ‘pristine’ Amazonian forests
Construction of RFs is described in detail by Comptour et al.[11] and summarized here

Summary

Introduction

The functioning and productivity of pre-Columbian raised fields (RFs) and their role in the development of complex societies in Amazonian savannas remain debated. If Amazonian RFs were managed, their vestiges would 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. 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]. In most Amazonian RF systems, there is no permanent aquatic compartment that could have supplied such resources, and their usually nutrient-poor s oils[18,19,20] would not have allowed the permanent cultivation that characterizes the chinampas, whose soils have both alluvial and volcanic inputs

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