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© iForest -Biogeosciences and Forestry IntroductionArbuscular mycorrhizal fungi (AMF) play a critical role in the early establishment of land plants (Redecker et al. 2000). They can improve the host plants growth (Wu et al. 2011), improve plant nutrient and water uptake, enhance plant tolerance to environmental stresses, including metal tolerance, and are abundant in the plant rhizosphere interface (Smith & Read 2008, Xu et al. 2008, Barea et al. 2011. AMF are important micro-organisms which contribute to plant diversity and ecosystem functions (Van der Heijden et al. 1998). About 80% of higher plants have associations with arbuscular mycorrhizae (Smith & Read 2008). These associations lead to better phosphorous (P) absorption (Rooney et al. 2011) and the fungi can supply up to 80% of P and 25% of nitrogen (N) to the host plants (Marschner & Dell 1994). In turn, the fungi are highly dependent on their host plants for carbon nutrition (Smith & Read 1997). P is an important element which is actively supplied to host plants by AMF. The amount of P influx into colonized roots can be three to five times higher than non-mycorrhizal roots (Smith et al. 2003).Some soil characteristics have significant influences on the development of mycorrhizal fungi in roots, such as potassium (K), nitrogen (Treseder 2004), pH (Isobe et al. 2007, and compaction (Waltert et al. 2002), as well as climatic conditions and host plant effects (Kivlin et al. 2011). Increasing soil acidity or alkalinity was also a detrimental factor to AMF sporulation in field soils (Isobe et al. 2007). AMF also affect soil enzymes (Huang et al. 2009, Wu et al. 2011 by increasing activities such as dehydrogenase, phosphatase and urease (Huang et al. 2009).Wild service tree (Sorbus torminalis L. Crantz) is widely distributed across Europe, north-western African forests, and southwestern Asia (Nicolescu et al. 2009). It is also a rare, protected tree species found in the northern forests of Iran at elevations between 170 to 2700 m a.s.l., where it can reach up to 34 m in height (Pourmajidian 2000). Different species of the genus Sorbus are found in association with both arbuscular mycorrhizal and ecto-mycorrhizal fungi (Raspe et al. 2000).In this study, we hypothesized that AMF colonization of specific plants can vary significantly between sites with different soil characteristics. A comprehensive study was conducted to determine how AMF colonization of a wild service tree (Sorbus torminalis) was influenced by soil nutrients and enzyme activities at three different sites. A better understanding of AMF relationships with the wild service tree and its role in nutrient absorption and soil enzymes will enable better management of the tree stands in terms of nutrient uptake and more vigorous tree growth. Materials and methods Study sites and samplingThe study was conducted in a natural mixed forest of northern Iran consisting of Fagus orientalis Lipsky, Carpinus betulus L., and Acer cappadoeicum Gled along with a minor population of Sorbus torminalis. According to...