Biodiversity and Conservation 10: 1933–1947, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. Functional diversity and community structure of microorganisms in rhizosphere and non-rhizosphere Canadian arctic soils L. TAM 1 , A.M. DERRY 2 , P.G. KEVAN 1 and J.T. TREVORS 1,∗ 1 Department of Environmental Biology, University of Guelph, Ontario, Canada, N1G 2WL; 2 CW405, Department of Biological Sciences, University of Alberta, Edmonton, Canada, AB T6F 2E9; ∗ Author for correspondence (e-mail: jtrevors@uoguelph.ca; fax: +1-519-837-0442) Received 12 July 2000; accepted in revised form 3 January 2001 Abstract. Functional diversities of microorganisms in arctic soil samples at three incubation temperatures were assessed using sole-carbon-source-utilization (SCSU). Soil samples from four sites were collected from the rhizosphere and non-rhizosphere soils. Microorganisms were extracted from samples and in- oculated into ECO-Biolog plates and incubated at 4, 10 and 28 ◦ C. Calculations of Shannon–Weaver diversity and Shannon–Weaver evenness were based on the substrate utilization in the Biolog plates. Shan- non–Weaver diversities (H ′ ) in rhizosphere samples were significantly greater ( ¯ x H ′ = 3.023 ± 0.197; P < 0.005) than in non-rhizosphere samples ( ¯ x H ′ = 2.770 ± 0.154). Similarly, the evenness (E) of the inoculated microbial cells exhibited significant differences (P< 0.005) between the rhizosphere and non-rhizosphere soil samples ( ¯ x E = 0.880 ± 0.057 for soils with rhizosphere; ¯ x E = 0.807 ± 0.044 for non-rhizosphere samples). Higher microbial diversity and evenness were observed in samples incubated at 4 ◦ C than at 28 ◦ C [least significant difference (lsd) = 0.29], and evenness indices were higher in rhizosphere samples than in non-rhizosphere soils incubated at all three temperatures (lsd = 0.02). Principal component analysis (PCA) of the multivariate data set differentiated the soil samples on the relatively gross scale of mi- crobial communities isolated from rhizosphere and non-rhizosphere soils at all three temperatures. Key words: Arctic, biodiversity, Biolog, Canada, community structure, microorganisms, non-rhizosphere soil, rhizosphere soil Introduction Soil microbial diversity is essential for long-term ecosystem sustainability (Staddon et al. 1998a,b). Soil microorganisms fulfil essential ecosystem functions, such as biogeochemical cycling, decomposition of plant, animal and microbial wastes, and of pollutants; in addition they participate in symbiotic relationships with higher or- ganisms (Kunc 1994; Staddon et al. 1997; Marilley et al. 1998). A number of studies have investigated changes to structure and functional diversities of microbial commu- nities from a variety of soil environments such as the differences between rhizosphere and non-rhizosphere zones (Bachmann and Kinzel 1992; Tedla and Stanghellini 1992; Young et al. 1995; Thompson et al. 1992; Kim et al. 1999) and functional microbial di- versity in arctic soils (Derry et al. 1999a). Soil microbial populations can respond to the