Non-linear impacts of Eucalyptus plantation stand age on soil microbial metabolic diversity

Although it is generally accepted that planting exotic plant species alters metabolic function of soil microbial communities, its temporal dynamic is often ignored when evaluating ecological effects of associated land use changes. To investigate the dynamic impacts of successive Eucalyptus planting on carbon metabolic activities of soil microbial communities, we studied community-level physiological profiles of soil microbial communities in different generations of Eucalyptus plantations. We studied community-level physiological profiles of soil microbial communities, using the Biolog (TM) Ecoplates incubation, in adjacent first (G1), second (G2), third (G3), and fourth (G4) generation Eucalyptus plantations that were, respectively, aged 3, 8, 14, and 19 years in Guangxi province, southern China. We used the 'space-for-time substitution' approach to investigate the impact of stand age of exotic Eucalyptus plantations on carbon metabolic diversity and activities of soil microbial communities. For each Eucalyptus plantation generation, three experimental plots were randomly selected. In each plot, one composite soil sample from 0 to 10 cm in depth was obtained for the analyses. Single carbon source utilization varied with Eucalyptus plantation stand age. Among preselected 31 carbon sources, utilization of 17 carbon sources changed significantly, which was best described by a quadratic function (ten carbon sources) and an exponential function (seven carbon sources). As a result, cumulative averaged metabolic activity and metabolic diversity of soil microbial communities showed quadratic and exponential changes relative to Eucalyptus plantation stand age. The order of cumulative averaged carbon metabolic activity and metabolic diversity were G1 > G4, G3 > G2 and G1 > G2 > G3, G4 (p < 0.05), respectively. The factors contributing to carbon source utilization structure of soil microbial communities for different stand ages of Eucalyptus plantations were shrub richness, soil organic carbon content, microbial biomass carbon, C-to-N ratio, and N-to-P ratio. Eucalyptus plantation stand age has inconsistent non-linear impacts on two aspects of soil microbial metabolic function: (1) quadratic impacts on carbon metabolic efficiency and (2) exponential impacts on carbon metabolic diversity. The decreasing carbon metabolic diversity has no significant impact on carbon metabolic efficiency during successive Eucalyptus plantings. The results show that the importance of assessing long-term impacts of land use changes on soil microbial communities from exotic plantations by quantifying multi-aspect non-linear changes on soil microbial metabolic function.