A novel sequential chemical extractions protocol to access mineral phases stabilizing organic matter in Amazonian Dark Earths (Holocene)

Abstract

The association of organic matter with pedogenic minerals plays a crucial role in the stabilization of soil organic matter (SOM). To evaluate the contribution of different mineral phases to mineral-organic associations, the clay fractions of both small (<500 μm) and large (>500 μm) soil aggregates from Amazonian Dark Earths (ADEs) and reference soils (Ref) across five sites in the Brazilian Amazon were subjected to of sequential chemical extraction: hydroxylamine (HL), sodium dithionite (DT), sodium pyrophosphate (PYR), and NaOH. The extraction residues were analyzed by X-ray diffraction, thermogravimetry, and dry combustion for carbon (C) concentration. On average, PYR extracted 58 % of the C in both ADE and Ref soils, indicating that this extracted C predominantly corresponds to fractions associated with individual or poorly polymerized Fe and Al octahedral groups. The sequential extraction protocol, incorporating dithionite before PYR, facilitated an enhanced separation of mineral-organic association compartments. Among the minerals analyzed, kaolinite and hematite of low crystallinity were found to be more significant in mineral-organic associations compared to their high crystalline counterparts. Despite the presence of abundant small crystal-sizeof goethite in the samples, hematite was more effective in organo-mineral interactions. In the Amazonian environment, mineral-organic associations in the clay fraction occur indistinctly in ADE and adjacent Refs soils lacking an anthropic A horizon. These specific organic-mineral associations are likely responsible for the reduced decomposition rates and the sustained high levels of SOM in ADEs for over thousands of years since their anthropic formation (Holocene), even under the humid tropical climate.