Re-examining the Amazonian Dark Earth Phenomenon: Biochar

Abstract

This thesis presents three linked research projects. The first and second research chapters (Ch. 2 & 3) exhibit results from studies examining biochar material properties. The third and final research chapter (Ch. 4) provide two unique findings; 1) that there are mineralogical differences between Amazonian Dark Earth and Brazilian Oxisol soil profiles, and 2) that iron mineralogy does affect soil microbial respiration rates. These chapters are associated with each other through hypotheses surrounding Amazonian Dark Earth (ADE) pedogenesis. Biochar (a subset of black carbon materials) is often cited as the key factor for explaining the observed enhanced fertility of ADE soils when compared to natural occurring surrounding Brazilian Oxisol soils. Biochar is often researched to understand how the effects observed in ADE soils may be applied elsewhere. Data presented in chapters 2 & 3 of this thesis, however, raise questions regarding its soil enhancing properties. Data presented in chapter 4 provide evidence for how a previously overlooked factor in Amazonian Dark Earth soils, iron mineralogy, could potentially affect additional soil properties including soil microbial respiration rates. Differing soil microbial rates with time will alter carbon sequestration rates and soil fertility. The fundamental conclusion of this thesis is that the data collected here supports the suggestion that ADE soils should be reexamined, with a focus on the iron mineralogical differences found between ADE and Brazilian Oxisol soils.