Assessing nitrogen fixation in mixed- and single-species plantations of 'Eucalyptus globulus' and 'Acacia mearnsii'

Mixtures of 'Eucalyptus globulus' Labill. and 'Acacia mearnsii' de Wildeman are twice as productive as 'E. globulus' monocultures growing on the same site in East Gippsland, Victoria, Australia, possibly because of increased nitrogen (N) availability owing to N₂ fixation by 'A. mearnsii'. To investigate whether N2 fixation by 'A. mearnsii' could account for the mixed-species growth responses, we assessed N₂ fixation by the accretion method and the 15N natural abundance method. Nitrogen gained by 'E. globulus' and 'A. mearnsii' mixtures and monocultures was calculated by the accretion method with plant and soil samples collected 10 years after plantation establishment. Nitrogen in biomass and soil confirmed that 'A. mearnsii' influenced N dynamics. Assuming that the differences in soil, forest floor litter and biomass N of plots containing 'A. mearnsii' compared with 'E. globulus' monocultures were due to N₂ fixation, the 10-year annual mean rates of N₂ fixation were 38 and 86 kg ha⁻¹ year⁻¹ in 1:1 mixtures and 'A. mearnsii' monocultures, respectively. Nitrogen fixation by 'A. mearnsii' could not be quantified on the basis of the natural abundance of 15N because such factors as mycorrhization type and fractionation of N isotopes during N cycling within the plant confounded the effect of the N source on the N isotopic signature of plants. This study shows that 'A. mearnsii' fixed significant quantities of N₂ when mixed with 'E. globulus'. A decline in δ15N values of 'E. globulus' and 'A. mearnsii' with time, from 2 to 10 years, is further evidence that N2 was fixed and cycled through the stands. The increased aboveground biomass production of 'E. globulus' trees in mixtures when compared with monocultures can be attributed to increases in N availability.