this site offers the opportunity to test if Nitrogen fixation by legume trees impacts the dynamics of native SOM mineralization by measuring the soil organic carbon isotope composition (δ13C) of the different soil organic matter fractions. We postulated that if native soil organic matter mineralization is higher under eucalypt plantations than in the presence of acacia either alone or in mixture, the oldest, more stabilized soil organic matter fraction, will be less 13C depleted when acacia is present.