Soil morphology related to hydrology and degradation in tropical coastal podzols (SE Brazil)

Morphology of podzol profiles can be used to improve our understanding of soil formation and degradation processes. The morphology of a podzol chronosequence was studied for a 1800 m long cliff perpendicular to the coastline at the southern coast of the Island Ilha Comprida (São Paulo, Brazil). The large variation in hydrological conditions with time (surfaces with ages from 6000 to 300 yrs BP) and space (topography) allowed studying the relation between podzol morphology and drainage conditions, both during soil formation and degradation. On the oldest two surfaces, the podzols formed under poorly drained circumstances. Uplift during their formation resulted in (1) very deep Bh horizons and (2) degradation of the upper B horizon as a result of improved drainage. On the younger surface most podzols were poorly drained, but well-drained podzols and transitions occurred in beach ridges. In the cliffs on the southern coast of the island, improved drainage of the poorly drained podzols resulted in degradation of the B horizon. This degradation exhibits various forms, including (1) complete removal of the B horizon down to an underlying clay layer, (2) degradation of the B horizon along root channels with preferential vertical water transport, (3) degradation of the upper B horizon resulting in a diffuse EB transition, and (4) areas of organic matter (OM) depletion related to microbial activity, sometimes combined with burrowing. Type (2) degradation may result in a convoluted EB transition, the formation of islands of B horizon in the deepening E horizon, and formation of E horizon tongues into the Bh. Recent (post degradation) OM bands were found in E and B horizons. A thorough study of podzol morphology can be used to reconstruct several stages of their formation and degradation in relation to drainage conditions.