Using proximal soil sensors and fuzzy classification for mapping Amazonian Dark Earths

Mats Söderström,Jan Eriksson,Lilian Rebellato,Denise Pahl Schaan,Suzana Romeiro Araújo,Christian Isendahl,Per Stenborg

doi:10.23986/afsci.8195

Abstract

We tested if hand-carried field proximal soil sensing (PSS) can be used to map the distribution of anthropogenic Amazonian Dark Earths (ADE). ADE soils are rich in archaeological artefacts, nutrients, organic matter and carbon in the very stable form of pyrogenic carbon, also referred to as black carbon or biochar. To test the capacity of PSS to detect signature ADE properties we measured electrical conductivity (ECa), magnetic susceptibility (MSa) and gamma ray data by transect sampling and compared these readings, using fuzzy classification, with datasets on chemical soil properties from a 28 ha large study area located on the Belterra Plateau of the Lower Amazon in northern Brazil. Results indicate that ECa and MSa measurements were good indicators of ADE signatures, but that the gamma radiation sensor was less useful in the deeply weathered soils. PSS and fuzzy classification can be used for rapid field mapping of ADE for both agricultural and archaeological purposes.

Highlights

The potential of Amazonian Dark Earths (ADE) as a valuable resource for agriculture and as a key source for understanding the prehistory of the Amazonian cultural landscape has been recognized for some time (Sombroek1966, Smith 1980)
A important land use practice contributing to ADE formation is ‘slash and char’ agriculture in which burning with reduced supply of oxygen transforms part of the biomass to pyrogenic carbon through pyrolysis, supplemented by the addition of other kinds of organic material associated with long-term concentrated settlement
ADEs are rich in archaeological artefacts, nutrients such as phosphorus (P), calcium (Ca) and magnesium (Mg), organic matter and carbon (C) in the very stable form of pyrogenic carbon, and is a unique agricultural and archaeological resource distinct from the strongly weathered soils that dominate in the Amazon region

Summary

Introduction

The potential of Amazonian Dark Earths (ADE) as a valuable resource for agriculture and as a key source for understanding the prehistory of the Amazonian cultural landscape has been recognized for some time (Sombroek1966, Smith 1980). ADEs are rich in archaeological artefacts (e.g. ceramics and lithics), nutrients such as phosphorus (P), calcium (Ca) and magnesium (Mg), organic matter and carbon (C) in the very stable form of pyrogenic carbon, and is a unique agricultural and archaeological resource distinct from the strongly weathered soils that dominate in the Amazon region. While organic matter normally decomposes rapidly under tropical climatic conditions, pyrogenic carbon is resistant to disintegration and increases the long-term nutrient-holding capacity of the soil. Terra mulata is primarily distinguished from surrounding natural soil by darker colour of the surface horizon due to higher C content and occurrence of pyrogenic carbon, and is commonly interpreted as resulting from less intensive agriculture (Sombroek 1966, Woods and McCann 1999)

Methods

Results

Conclusion