Pedogenesis of soils with accumulation of organic carbon in the subsurface horizons in a saline lake in the Pantanal wetland of Nhecolândia, Brazil

Abstract

The Nhecolândia region is formed by sandy sediments, coming mainly from the Taquari river basin, with soil sodification and the accumulation of organic matter in the subsurface being the main pedogenic processes observed in this environment. This research aimed to study the genesis of soils with spodic features in the subsurface horizons in an alkaline environment in the saline lagoons of the Pantanal of the State of Mato Grosso do Sul, Brazil. Three trenches were excavated, and the soils were morphologically described and classified according to Brazilian Soil of Classification System in Espodossolos and WRB in Arenosols. In addition, deformed samples from all soil horizons were sampled and used in physical, chemical, mineralogical and micromorphological analyses. In all profiles, the sandy texture predominates (sand ≥879 g kg−1) the colors vary from light olive-brown on the surface horizons to grayish-brown-very dark on the subsurface horizons, with hues ranging from 2.5Y to 5Y. In the profiles, the occurrence of the E horizon was observed, followed by the subsurface horizon with cementation, forming ortstein. The profiles showed high pH values (6.8–10.2), electrical conductivity (EC) > 4 dS m−1, and high saturation for Na+ (up to 60.6%), characterizing them as saline-sodic soils. In the soils around the saline lagoon, it was observed the accumulation of Fe (Fe2O3), Al (Al2O3) and dissolved organic carbon (DOC) in the horizons with spodic features. Among the humic substances, the humin fraction presented the highest levels, mainly in the B horizons, with variations from 0.1 to 6.2 g kg−1. In the thin sheets, coatings and fillings were observed, consisting of organo-ferruginous colloidal material of brown to opaque color with monomorphic characteristics, covering and binding the quartz grains. Alkaline soil conditions in saline lakes contribute to the dissolution of clay minerals and surface organic matter, and later, to the dispersion of dissolved organic compounds together with Al2O3, Fe2O3, and amorphous silica, forming subsurface horizons rich in organic matter, Al, neoformed clay minerals and ortstein cemented horizons.