The aim of this book is to present, in depth, updated information on soil and microbial processes involved in mixed plantations of Eucalyptus and N2-fixing species, especially Acacia mangium, focusing on Forestry, Soils, Biology, Ecosystem Services and Sustainability. The potential of substituting chemical N fertilizer by a consortium of leguminous species that fix atmospheric nitrogen is an interesting solution for a more sustainable, economically and environmentally sound forest system. Among the main topics, we present reference topics on soil microbiology, as biological nitrogen fixation, the role of mycorrhiza in mixed plantations, bio-indicators of soil quality, and plantgrowth promoting bacteria with biotechnological potential. Here we discuss Ecosystem services and ecological benefits of these systems, the invasive potential of A.mangium, as well as the regulations and perspectives of land use policies for mixed forests and their role in the sustainability of the system.