Effect of invasive Acacia dealbata Link on soil microorganisms as determined by PCR-DGGE

Acacia dealbata Link is an Australian woody legume that has become a serious environmental problem in Northwest Spain where it forms dense monospecific patches modifying the structure of different native ecosystems and threatening native aboveground biodiversity. In spite of the dramatic changes observed in the vegetation of invaded sites little is known about the consequences of invasion for soil microorganisms. To investigate the effect of A. dealbata invasion on the structure of soil fungi and bacteria communities, samples were taken from invaded and non-invaded areas from three different ecosystems in Northwest Spain: pine forest, shrubland and grassland. In each ecosystem type, soil samples were taken in areas of native vegetation, areas invaded by A. dealbata and in the transition zone between native and invaded vegetation. Soil microorganisms were analyzed in the different samples by PCR-DGGE using general primers for eubacteria and fungi. Soil analyses were also performed to evaluate the effect of A. dealbata invasion on soil fertility.The invasion by A. dealbata consistently increased soil N, C, organic matter and exchangeable P content in the three studied ecosystems. A clear effect of the invasion on the overall structure of microorganism communities was only observed in the shrubland where soil fungal communities in the invaded and transition areas clustered together and apart from the native soil. Significant differences in soil microorganisms richness and diversity between invaded and not invaded soils were only found in the grassland. Grassland invasion by A. dealbata lead to a significant increase of bacterial richness and to a significant reduction in fungal richness and diversity. Our results show that although the changes on soil chemistry due to A. dealbata invasion are consistent among the studied ecosystems, the effect on soil microorganisms depends on the ecosystem type affected by the invasion.