Formation of black carbon rich 'sombric' horizons in the subsoil - A case study from subtropical Brazil

Author(s)

Chiapini, Mariane ; Schellekens, Judith ; Calegari, Marcia Regina ; de Almeida, Jaime Antonio ; Buurman, Peter ; de Camargo, Plinio Barbosa ; Vidal-Torrado, Pablo

• Document type: Journal article
• Source: Geoderma; v. 314, p. 13-pg., 2018-03-15.

Abstract

The formation of sombric horizons (dark horizons in the subsoil) is still not understood. In order to improve our understanding of the formation of sombric horizons we studied these soils in southern Brazil from various perspectives. The lateral configuration and grain size distribution excluded the possibility that the sombric horizon is a paleosol covered by an eolian deposit or colluvium. Micromorphology showed intense biological activity, indicating strong bioturbation. The absence of clay and OM coatings indicates that the sombric horizon in the study area was not formed by illuviation. Considering changes with depth, phytoliths and delta C-13 isotopes clearly showed that the OM from the sombric horizon had a larger contribution from grasses, while a larger contribution from black carbon (BC) was evidenced by the molecular composition. A larger contribution from grasses and BC both correspond to drier climatic conditions. However, similar depth trends for delta C-13 and PAHs were found in the reference profile (without sombric horizon), in agreement with climatic change but not explaining the different morphology. The molecular composition and C/N ratio showed that the profiles differed in degree of decomposition, with the OM in the soils that contained a sombric horizon being more decomposed than that in the reference profile. The sombric horizon is thus a remnant of an earlier phase of soil formation under a drier climate, which is made visible by differences in decomposition of OM related to subsequent more humid conditions. Stronger decomposition in the profiles with a sombric horizon may be related to better drainage, explaining their occurrence in the highest positions within the landscape and suggesting a topographic control. (AU)

• FAPESP's process: 13/03953-9 - Organic matter dynamics in tropical peatlands (Diamantina, Brasil) - development of molecular proxies to reconstruct environmental changes
• Grantee: Judith Schellekens
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• FAPESP's process: 15/03577-2 - Pedogenesis and molecular characterization of sombric horizons in soils in Tijucas do Sul region - PR
• Grantee: Mariane Chiapini
• Support Opportunities: Scholarships in Brazil - Master
• FAPESP's process: 14/23969-0 - Soil organic matter dynamics and its relation to the environment: molecular chemistry (pyrolysis-GC/MS) to understand soil formation in tropical soils
• Grantee: Pablo Vidal Torrado
• Support Opportunities: Regular Research Grants