Gradualism in Acacia dealbata Link invasion: Impact on soil chemistry and microbial community over a chronological sequence

Acacia dealbata Link, a leguminous tree native of Australia, has become a major problem due to its invasiveness throughout the world. However, little is known about its impact over time. In this study, we have explored the impact of A. dealbata on soil nutrients and on soil microbial community function and structure in 4 mixed invaded forest sites in NW Spain, in a chronosequence of invasion: (1) a minimum of 25 years; (2) an average of 15 years; (3) an average of 7 years and (4) less than 3 years. pH significantly diminished over time as organic matter increased. Soil nutrients were progressively altered under A. dealbata; total C, N and P invariably increased as different periods of invasion time also increased, whilst Ca2+, K+ and Mg2+ contents showed irregular trends during the different periods of invasion. In addition, soil enzymatic activities of acid phosphatase, β-glucosidase, urease and N-acetyl glucosaminidase increased significantly, and soil basal respiration enhanced over the sequence of the invasion. DGGE analyses suggested variations in the structure of microbial and fungal communities over the whole assessed period due to A. dealbata presence. This is the first time that chronological sequences have been included to investigate the impact of A. dealbata invasion. Our results show that the initial dominance of A. dealbata and its negative impact on soil and microbial parameters cannot be recovered even long periods after the invasion.