Humus form in ecosystems of the Atlantic Forest, Brazil

Abstract In order to describe the humus forms present in different ecosystems of the Atlantic Forest (Brazil), a morpho-functional criterium was adopted. Samples were collected from eight sites: three sites in the Tabuleiro Forest, four sites in the Restinga, and one site in Tijuca Forest. Soil samples were sorted in L, F, and H horizon and the A horizon. A horizon was analysed for pH, total C and N, available P, and exchangeable bases. Our results include the recognition of an A interface horizon beneath the holorganic layer as well as aggregates 2–10 mm in diameter in the A horizon. Accumulation of organic matter was found to be related to both nitrogen content in litterfall and soil type. In the Tabuleiro Forest Tropical Mesotrophic Mull, Tropical Oligotrophic mull and Eumoder humus forms were found. Here the soil types, Ultisol and a Spodosol, explained in part the variation in humus form. In the Restinga, a wide range of humus forms was also observed including Moder–Mull, Dysmoder, Mesotrophic Mull and Eumoder; humus form was found to be related to the C:N ratio of the litterfall or to the soil substrate. At the Tijuca Forest, beneath an holorganic layer typical of a Moder (presence of the H horizon), an A horizon with a low C:N ratio typical of Mull was encountered. From the eight sites described, the Atlantic Forest biome can be said to be characterised by a great diversity of humus forms, which is a reflection of the complex environmental conditions found there.