Gradualism in Acacia dealbata Link invasion: Impact on soil chemistry and microbial community over a chronological sequence Pablo Souza-Alonso * , Alejandra Guisande-Collazo, Luís Gonz  alez Department of Plant Biology and Soil Science, University of Vigo, 36310 Vigo, Spain article info Article history: Received 17 January 2014 Received in revised form 15 October 2014 Accepted 25 October 2014 Available online 8 November 2014 Keywords: Acacia dealbata Chronosequence of invasion Soil nutrients Plant invasion Enzymatic activities Soil microorganisms Diversity and function abstract 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 Ca 2þ ,K þ and Mg 2þ contents showed irregular trends during the different periods of invasion. In addition, soil enzymatic activities of acid phosphatase, b-glucosidase, urease and N-acetyl glucosami- nidase 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. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction The loss and change of aboveground biodiversity in the composition of plant species is the most evident effect of plant invasions (Reinhart et al., 2005; Brewer, 2008; Hoyos et al., 2010; de Abreu and Durigan, 2011). However, plant invasions can modify soil structure and chemical composition (Ehrenfeld, 2003; Reinhart and Callaway, 2006; Chac on et al., 2009; Yang et al., 2010; Castro-Díez et al., 2012; Novoa et al., 2014) and soil microorganism structure (Hawkes et al., 2006; Broz et al., 2007; Lorenzo et al., 2010a) and function (Castro-Díez et al., 2009; Dasonville et al., 2011; Elgersma and Ehrenfeld, 2011). Recently, authors such as Dasonville et al. (2011) asserted that alien plant species can create novel niches by modifying native conditions; mainly in the storage and release of C and N. The presence of plant invaders generally modifies ecosystem litter inputs, both quantitatively and qualitatively (Ehrenfeld, 2010). Alteration in organic matter inputs also produces changes in decomposer community composition (http://www.sciencedirect. com/science/article/pii/S0038071705003184Parmelee et al., 1989), which affects the physiological capacity of the microbial commu- nity (Waldrop et al., 2000). Therefore, decomposition processes are strongly regulated by litter characteristics of the dominant plant species in an ecosystem (Hoorens et al., 2003). Reciprocally, belowground soil organisms, such as fungal and bacterial com- munities, show important feedback with plants (Van der Putten et al., 2007), influencing the relative abundance of plant species within a community (Klironomos, 2002) and contributing to soil processes like litter decomposition and nutrient mineralization, mainly via their enzymatic activity (Sinsabaugh, 2010). There is a general consensus among invasive ecologists about the noxious effects produced by the entrance of foreign plant species into the ecosystem functioning. However, information about the time elapsed since the introduction of the invader is generally not provided. Consequently, in studies concerning inva- sive processes, information about changes in the impact over time is rarely available (Ehrenfeld, 2003). Some authors have found that changes in soil properties as C or N contents and microbial Abbreviations: SOM, soil organic matter; AP, acid phosphatase; BG, b-glucosi- dase; Ur, urease; PPO, polyphenol oxidase; GAP, glycine aminopeptidase; NAGase, N-acetyl glucosaminidase; pNP, paranitrophenol phosphate; pNA, paranitroaniline. * Corresponding author. Tel.: þ34 986 812 594; fax: þ34 986 812 556. E-mail address: souza@uvigo.es (P. Souza-Alonso). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio http://dx.doi.org/10.1016/j.soilbio.2014.10.022 0038-0717/© 2014 Elsevier Ltd. All rights reserved. Soil Biology & Biochemistry 80 (2015) 315e323