Characterization of organo-mineral interactions in Terra Preta de Índio

Mineral-organic associations are considered key factors in the permanence of soil organic matter (SOM), but information about the performance of this mechanism in Amazonian Dark Earth (ADE) ("Terra Preta de Indio") is still scarce. Since these anthropic soils are being assumed in recent years as a model of sustainability, understanding the organic matter accumulation process in ADE might be useful for establishing strategies that nowadays can optimize carbon sequestration. The aim of this study was to evaluate the relative contribution of the mineral-organic association to the chemical recalcitrance mechanism in the stabilization of organic matter in ADE soils. For this, five ADE's soils were compared with five respective reference soils in the Amazon region: (i) Lago Grande and (ii) Caldeirão in Iranduba-AM; (iii) Maguari in Belterra-PA; (iv) Teotônio and (v) Santa Paula in Porto Velho-RO. Analyzes for the quantification of carbon and nitrogen in the soil and its physical fractions, and characterization of the mineral-organic association through sequential chemical extraction and scanning transmission X-ray microscopy (STXM) coupled to near edge X-ray absorption fine structure spectroscopy (NEXAFS) were applied to the clay fraction of both soils. The ADEs presented average accumulations of 31% carbon and 44% nitrogen more than the reference soils. The silt fraction of most Amazonian sites showed higher accumulations of carbon and nitrogen, and also the presence of aromatic structures, indicating greater humification and stability of the SOM. However, the clay fraction of ADE had accumulations of 36% carbon and 49% nitrogen higher than the reference soil, in addition to the similarity of the C:N ratio of this fraction in both soils, indicating that the contribution of SOM stabilization by mineral-organic association may have a primary role in relation to chemical recalcitrance. The ADE presented extractions of iron, aluminum, silicon and the mineralogy of the clay fraction similar to the reference soil, however, a greater presence of carboxylic, phenolic and O-alkyl carbon. From the sequential extraction it was possible to create a model of the mineral-organic association in ADE to better understand the dynamics of carbon accumulation and the contribution of this mechanism in the permanence of organic matter in these anthropic soils. (author)