Lamellae formation processes in tropical soils in southeastern Brazil

• The genesis of lamellae was studied in Brazilian tropical soils.
• Lamellae of upper slope positions originated by illuvial processes.
• Medium slope lamellae originated by degradation of the Bt by clay eluviation.
• The process of Bt degradation is an important mechanism of lamellae formation.
• Lamellae formed by this process should be also interpreted as pedogenic.

The pedogenic genesis of lamellae is commonly reported as involving clay illuviation. However, some pedogenic lamellae in tropical soils may be formed by a different process involving the degradation of a continuous Bt horizon and an intense loss of clay. In the present work, we investigated soils along a toposequence containing lamellae formed both by illuviation and Bt degradation to better understand the pedogenic processes and the environmental controls associated with the genesis of lamellae in tropical soils. Soils were described and sampled along a hillslope in the São Pedro region, São Paulo state, and submitted to physical (particle size), chemical (pH and exchangeable cations), and micromorphological analyses. In the Upper Segment of the studied hillslope, dominated by low gradients, the soil is deep, reddish, and contains a sequence of 12 to 20 lamellae/interlamellae. An examination of thin sections revealed abundant illuvial features formed by moderately to strongly oriented clay bridges between sandy particles in the lamellae, supporting an illuvial origin. In the Middle Segment of the hillslope, characterized by higher gradients, the soil is shallower and yellowish, presents some mottles, and contains a sequence of 2 lamellae/interlamellae. Field descriptions showed curved links between these lamellae and the underlying Bt as well as isolated spots similar to lamellae within interlamellae. The lamellae, Bt, curved links, and spots present similar micromorphological characteristics and only a few illuvial features, which suggest a genetic relationship between lamellae and the Bt horizon and minimize the importance of clay illuviation. The origin of these lamellae is related to the degradation of Bt and clay eluviation by subsurface lateral flows, developed mainly in the contact between Bt and the overlying, more permeable horizons. This process seems to be an important way of lamellae formation under tropical climate and should thus be incorporated into the term pedogenic lamellae, which is usually associated in the international literature with lamellae formed by clay illuviation.