Soil organic carbon and morphology as affected by pine plantation establishment in Minas Gerais, Brazil

Reforestation with Pinus sp. trees is often associated with changes in soil physical and chemical soil properties, including depletion of soil organic carbon (SOC). In some cases, leaching of Fe compounds complexed by organic compounds can also occur, bleaching soil colors and perhaps further affecting SOC dynamics. This work aimed to compare SOC concentrations and stocks, as well as soil Fe compounds, under pine plantation and native semideciduous forests in Brazil. The hypotheses tested were that, under pine stands, SOC concentration and stocks decrease, and bleaching caused by Fe mobilization through the profile also affects SOC dynamics. Thus, paired plots consisting of neighboring pine plantations and native forests were sampled in triplicate to a 1-m depth in four different clayey soils (1 Oxisol and 3 Ultisols). Soils were characterized by mineralogical, chemical and physical analyses, SOC content by dry combustion, extraction of Fe compounds (total and amorphous oxides, and organically-bound) and also micromorphology. Soil order influenced how pine plantation affected soil structure, as bulk density increased in the Oxisol but generally decreased in the Ultisols. SOC concentrations and stocks reached 8.5% in the 0–5 cm layer and 309 Mg ha−1 to a 1-m depth, respectively), which are high values compared with the Brazilian literature. Although SOC concentrations were significantly lower in subsurface layers of two soils under pines, after correction for changes in bulk density, only one Ultisol in the midslope position showed SOC stocks significantly lower (by ca. 56 Mg ha−1) to a 0–40 cm depth, but not to a 1-m depth. Changes in Munsell colors indicative of bleaching or Fe mobilization were noted in all soils, and could be ascribed to consistent and simultaneous changes in amorphous Fe oxides and SOC in a midslope Ultisol, demonstrating podzolization in that site. In addition, soil thin sections suggested that Fe compounds were removed from the general matrix of peds in the Ultisols under pines, and concentrated in other areas of the same peds, which can explain changes in color dissociated from vertical changes in concentration of Fe forms. In summary, pine plantations in the studied area seem more prone to conserve SOC than in other cooler or drier regions in Brazil, although changes associated with Fe mobilization can occur in some Ultisols in midslopes, as reported elsewhere in sandier soils under cooler climates.