Soil organic carbon stabilization in soils derived from contrasting parent materials in the Brazilian Amazon

Soils store two to three times more carbon than the atmosphere and three to four times more than terrestrial vegetation, yet they are increasingly degraded worldwide, with important implications for climate regulation and ecosystem services. Among the controls on soil organic carbon (SOC) stabilization and persistence, parent material (largely determined by geology) is a key driver. We investigate SOC stabilization mechanisms in Amazon forest soils developed from contrasting parent rock types, including sedimentary, plutonic, metamorphic, and volcanic substrates, and include a subset of pasture samples to evaluate mineral controls on carbon dynamics under land-use change. We will analyze 120–160 soil samples collected by the National Forest Inventory (IFN–SFB) along a transect spanning 70°W to 56°W in longitude and 12°S to 9°S in latitude, in a region with an average precipitation of ~1,900 mm yr^-1. Samples will undergo physical organic matter fractionation, and the mineral-associated organic matter fraction (MAOM) will be characterized using X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and sequential wet-chemical extractions to quantify reactive, silicate-associated, and pyritic Fe. Carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N) will be measured in both SOC fractions to infer carbon sources and nitrogen dynamics. Statistical analyses, including ANOVA, Tukey's HSD (or nonparametric equivalents), linear regression, and ordination methods, will be used to compare mineralogical controls on SOC. We hypothesize that variation in parent material across the Amazon shapes the balance between pedogenic Fe (oxyhydr)oxides and aluminous clays, with Fe-rich substrates promoting stronger SOC stabilization than Fe-poor, sand- or clay-dominated substrates.