Organic carbon rich-soils in the brazilian semiarid region and paleoenvironmental implications

Soil organic carbon (SOC) contents are commonly high in the mountainous tropical soils with wet climates in Southeastern and Southern Brazil, but high SOC soils in the Brazilian semiarid zone are poorly known. The Borborema Plateau represent a complex core of folded and uplifted crystalline rocks with the highest altitudes in the Brazilian semiarid. The present study objetived to describe unusual SOC-rich soils from the semiarid Borborema Plateau, discussing past and present, soil-forming processes, and landscape evolution. We described and characterized 11 Late Quaternary soils in the Borborema Plateau, emphasizing chemical and physical properties, soil carbon stock, and water behavior, allowing us to know about the soil diversity of the semiarid highlands. Five Umbrisols and one Cambisol were described in concave slopes and summits above 1,000 m, under relicts of semideciduous forest and Rupestrian Grassland vegetation; downslope, we found Planosols, Leptosols, Regosols, Luvisols, and Arenosols in partially dissected pediments below 600 m, representing typical soils of the Caatinga (Dry forest). Borborema highland soils with high SOC are acidic, dystrophic, moderately developed, whereas soils from the Caatinga range from moderately acid to moderately alkaline, hypereutrophic, and poorly to moderately developed. The Umbrisols and Cambisols have a mean SOC content of up to 94% higher than typical Caatinga soils. Water retention and availability are similar between acidic soils with high SOC and typical dry forest soils. Radiocarbon analysis of humic fraction of soil indicated the dominance of C3 plants since the start of pedogenesis of Umbrisols, at ~ 6,900 cal yr BP. The stabilization of high amounts of organic carbon in highland semiarid soils was attributed to P, acidity, and fine particles. These small relicts of soils with high SOC represent hotspots of SOC stocks in the Brazilian semiarid, and they protect "islands" of relict vegetation from past wetter Late Quaternary paleoenvironments.