Appraisal of the crop-rotation effect of rhizobial inoculation on potato cropping systems in relation to soil bacterial communities Darine Trabelsi a, * , Haroun Ben Ammar b , Alessio Mengoni c , Ridha Mhamdi a a Laboratory of Legumes, Centre of Biotechnology of Borj-Cedria, BP 901, Hammam-Lif 2050, Tunisia b Technical Centre of Potatoes, Saida, 2031, Tunisia c Department of Evolutionary Biology, University of Firenze, via Romana 17, I-50125 Firenze, Italy article info Article history: Received 13 February 2012 Received in revised form 17 May 2012 Accepted 19 May 2012 Available online 5 June 2012 Keywords: Crop rotation Common bean Inoculation Potato Rhizobia Soil communities T-RFLP abstract Non-legume crops grown in rotation with legumes usually have reduced N-fertilizer requirement, which has both economic and environmental benefits. In this study, we aimed to assess the indirect effect of Phaseolus vulgaris inoculation with two indigenous rhizobia strains on potato growth promotion and disease control in relation to inoculation effects on soil bacterial communities. T-RFLP profiling demonstrated that inoculation significantly increased the phylotype richness of the bacterial commu- nities at the end of P. vulgaris life cycle. A significant difference in richness between simple and dual inoculation was found. Effects on bacterial structure are clearly sensed with both inoculants. Various bacteria like Halomonas, Arthrobacter, Rahnella, Actinobacterium and Frankia were enhanced by inocu- lation irrespective of the inoculant type. However, other bacteria like Clostridium, Bacillus, Steno- trophomonas and Xanthomonas were enhanced by rhizobial mono-inoculation only and not by co- inoculation with both strains. Some bacteria may potentially behave like plant growth promoting rhi- zobacteria (PGPR) towards potatoes grown in rotation with common bean, as indicated by the 32% increase observed in potato yield, and also by the 56% decrease in potato wireworm infection. Therefore, rhizobia inoculation may contribute to the rotational benefits of legumes in potato cropping systems not only by providing fixed nitrogen, but also by increasing microbial diversity and structure, potentially stimulating plant growth promoting rhizobacteria and enhancing disease control. However, these effects depend largely on inoculant formulation. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Explaining factors that drive the diversity, distribution and abundance of species is one of the central themes in ecology. While understanding the diversity, distribution and abundance of large organisms is already a difficult task (Gaston, 2000). This task is considerably more complex for microbial ecologists, considering the small size and staggering diversity and abundance of microor- ganisms (Torsvik et al., 1990; Curtis et al., 2002; Gans et al., 2005). Several factors have been put forth to explain patterns of soil microbial community composition, including spatial variation, soil type, nematodes and vegetation (Wardle et al., 2004; Martiny et al., 2006). In vegetated areas, it is supposed that soil resources for microbial growth are mainly controlled by plant inputs through the deposition of litter and root exudates (Wardle et al., 2004). However, factors other than plants have been also reported as drivers of microbial community composition. For instance, abiotic and biotic soil factors, such as pH, salinity and biofertilizers, were reported as being the most important in influencing microbial community composition (Fierer and Jackson, 2006; Lozupone and Knight, 2007; Trabelsi et al., 2009, 2011). Symbiotic N 2 -fixation by legumes is generally believed to be the dominant source of biological nitrogen input in the biosphere. Nitrogen chemicals account for as much as 30% of the total fertil- izers needed for agricultural crops (Tran et al., 2006). With the increasing cost of chemical fertilizers and concern about environ- mental pollution, the role of biological nitrogen fixation in sustainable agriculture, has to be elucidated more in deep. The benefits of inoculation may also be mediated to the non-legume crops grown in rotation by increasing yields and reducing disease infestation, probably through stimulating microbial diversity and activities (Sturz and Kimpinski, 2004). Potato is one of the most important crops in the world. Like for most agricultural crops, large amounts of nitrogen inputs are * Corresponding author. Tel./fax: þ216 79325948. E-mail address: trabelsi_darine2@yahoo.fr (D. Trabelsi). Contents lists available at SciVerse ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2012.05.013 Soil Biology & Biochemistry 54 (2012) 1e6