Benefactor and allelopathic shrub species have different effects on the soil microbial community along an environmental severity gradient S. Hortal a, * , F. Bastida b, c , J.L. Moreno b , C. Armas d , C. García b , F.I. Pugnaire d a University of Western Sydney, Hawkesbury Institute for the Environment, Hawkesbury Campus, Locked Bag 1797, Penrith, NSW 2751, Australia b Centro de Edafología y Biología Aplicada del Segura, Consejo Superior de Investigaciones Científicas, Campus Universitario de Espinardo, P.O. Box 164, E-30100 Murcia, Spain c Department of Agroforestry Technology and Science and Genetics, School of Advanced Agricultural Engineering, Castilla La Mancha University, Campus Universitario s/n, Albacete, Spain d Estaci on Experimental de Zonas  Aridas, Consejo Superior de Investigaciones Científicas, Carretera de Sacramento s/n, E-04120 La Ca~ nada de San Urbano, Almería, Spain article info Article history: Received 3 March 2015 Received in revised form 5 May 2015 Accepted 7 May 2015 Available online 21 May 2015 Keywords: Retama sphaerocarpa Thymus hyemalis Facilitation Interference Semiarid ecosystems abstract Patches where shrubs have either positive or negative effects on their understory plant community are common in arid ecosystems. The intensity and balance of these effects change along environmental severity gradients but, despite the major role of soil microbes in plant interactions, little is known about the differences among soil microbial communities under these species and their possible influence on such contrasting shrub effects. We hypothesized that microbial communities associated to benefactor and allelopathic shrubs would differ among them and that differences would increase with environ- mental severity. To test these hypotheses we characterized soil microbial biomass, activity and com- munity composition under a benefactor shrub species, Retama sphaerocarpa, an allelopathic shrub species, Thymus hyemalis, and in bare soil among plants (gaps) at three sites along an environmental severity gradient. Shrubs promoted an increase in soil bacterial diversity, being bacterial communities associated to benefactor shrubs, allelopathic shrubs and gaps different in composition. Microbial enzy- matic activity and biomass increased under shrubs and under more mesic conditions; nonetheless, they were highest under benefactor shrubs at the most arid site and under allelopathic shrubs at the less severe site. Compared to gaps, the presence of shrubs induced changes in microbial activity and com- munity composition that were larger at the most severe site than at the less severe site. Along the gradient, benefactor shrubs enhanced the abundance of bacterial groups involved in organic matter decomposition and N fixation as well as plant pathogens, which could contribute to Retama's outstanding positive effects on understory plant biomass and diversity. Plant patches mitigate the effects of extreme conditions on associated plant and soil microbial communities and promote soil biodiversity and ecosystem functioning in arid ecosystems, with shrubs actively selecting for specific microbial groups in their understory. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Shrubs in dry environments grow often in patches, which pro- mote ecosystem heterogeneity (Goberna et al., 2007). Most shrub species have significant effects on their understory plant commu- nities, ranging from negative to positive (Pugnaire et al., 2004). Patches where shrubs facilitate establishment and growth of other species are common in these systems (Pugnaire et al., 2011; McCluney et al., 2012). Benefactor shrubs provide shade and pro- mote large modifications in understory soil by increasing moisture (Prieto et al., 2011), organic matter content (Pugnaire et al., 2004), microbial biomass, activity and abundance of some microbial groups (Goberna et al., 2007; Hortal et al., 2013). They therefore constitute hotspots of microbial activity, driving many ecosystem processes (Austin et al., 2004; Goberna et al., 2007; Rodriguez- Echeverria et al., 2013). Interference processes where shrub species negatively affect the establishment and growth of other plants under their canopy have * Corresponding author. Tel.: þ61 245701925. E-mail address: s.hortalbotifoll@uws.edu.au (S. Hortal). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio http://dx.doi.org/10.1016/j.soilbio.2015.05.009 0038-0717/© 2015 Elsevier Ltd. All rights reserved. Soil Biology & Biochemistry 88 (2015) 48e57