Sedimentary facies drive the development of organic-rich Podzols from a tropical barrier island

Abstract

Dissolved organic matter (DOM) retention in deep soil is an important process of carbon sequestration, especially for Podzols, which are soils with an accumulation of organic matter and/or oxides in the subsoil. Although DOM is immobilized at the soil subsurface by the saturation of organic complexes with metals (Fe and Al) during podzolization (i.e. the process of Podzol formation), such a chemical mechanism may have a minor effect in soils poor in Fe and Al. Here, we leverage the negligible content of organic-complexed Fe and Al in three Podzol profiles from Ilha Comprida (Southeastern Brazil) to determine the particle size distribution and sedimentary structures inherited from the parent material that enhances podzolization. The investigated Podzols have unique sequences of sedimentary facies, i.e., a body of sediments with a specific combination of mineral assemblages, fabrics, grain size, and depositional or penecontemporaneous structures, both biological and physical, reflecting specific depositional processes. Our results indicate that podzolization via DOM retention increases in response to two hydrologic mechanisms: (i) capillary break at the interface of sedimentary facies with a mean sand size of 2.6 ± 0.3 phi (small pores) over a layer of a mean sand size of 1.9 ± 0.2 phi (large pores); and (ii) physical barrier for vertical water percolation at the abrupt textural transition between sandy-over-clayey sedimentary facies. Both mechanisms influence the formation of the Bh and Bhm horizons. We suggest that studies on organic carbon dynamics and soil classification efforts should recognize the prowess of sedimentary facies and the contact between them in retaining substantial amounts of organic matter in deep soil.