Soil C and Al availability in tropical single and mixed-species of Eucalyptus sp. and Acacia mangium plantations

The Brazilian forestry sector has expanded to the Cerrado biome regions, where the soils are highly acid, with a high exchangeable Al content and a general nutritional deficiency. Nevertheless, the levels of soil organic matter (SOM) under the forest plantings are generally high and may play an important role in the complexation of Al in the soil solution. The objective of this study was to evaluate the effect of the introduction of Acacia mangium in monospecific Eucalyptus spp. plantations on the soil C content and its effect on the soil Al availability. The study was conducted at four experimental sites in Sao Paulo and Minas Gerais States in Brazil. We evaluated three treatments: monospecific stands (pure) of Acacia mangium and Eucalyptus and a mixed stand (constituted by Acacia mangium and Eucalyptus in 1: 1 proportion), with four replicates at each site. The soil total C and the Al content in the soil solution (Als), the exchangeable Al (Ale) and the organically complexed Al (Al-OM) were determined at the soil depths of 0-5 and 5-10 cm. The introduction of Acacia in Eucalyptus plantations (mixed stand) significantly increased the soil C content by approximately 10% only at one of four sites studied, which was probably because the land was used as pasture before the introduction of the forest plantation and the because of high N input in a Eucalyptus plantation. Despite little improvement in the soil C content and consequently in the Al complexed by the soil organic matter, the introduction of Acacia in the Eucalyptus stands increased the Al availability by approximately 13% due to the reduction in the soil pH.