Formation of soil diversity in the mountainous tropics and subtropics: Rocks, time, and erosion

Abstract

The diversity of soils and complexity of soil cover in humid tropics and subtropics, especially in the mountain regions, cannot be fully explained by climatic control over soil formation. The objective of the research was to investigate the role of time and erosion processes as factors controlling soil diversity in humid tropical and subtropical mountains that differ in the characteristics of the parent rocks. The study is based on the concept of two main pathways of the weathering of silicate rocks, depending on whether phyllosilicates are present or lacking in their composition. This has a direct relationship to thickness and properties of the weathered mantle. A chronosequence of soils on river and marine terraces developed on weathered chloritized diabase porphyrites were studied in humid subtropics of the South Caucasus, on the eastern coast of the Black Sea, Georgia. On a terrace sequence with sand and pebble marine sediments that ranges in age from 1000 to 500 000 years, Regosols, Ferralic Cambisols, Haplic Nitisols and Stagnic Acrisols are defined as consecutive stages of time-dependent soil formation. An erosional soil sequence in the hilly-mountain area of this region shows “regressive soil development”, from Stagnic Acrisols to Regosols/Leptosols. The erosion truncates relatively thick pre-erosion soils and regolith layer-by-layer; the exposed deep horizons play the role of parent rock for soil formation. In addition, soil sequences traced across hillslopes composed of serpentinite, ultrabasic and sedimentary rocks devoid of phyllosilicates have been studied in tropical regions of western Cuba. A mosaic of Ferralsols, Cambisols and Leptosols was formed under erosional impact on pre-erosion shallow soils and regolith. Time-dependent and erosion-dependent regularities of pedogenesis play an essential role in soil diversity and usually complicate initial soilscape, leading to formation of soils typical for extra-tropical/subtropical conditions.