Abstract:Shrub fertile island′ is a common feature worldwide in arid regions, and it greatly influences the structure and function of the arid ecosystems. However, the shrub fertile island′ is species dependent. To examine the species effect on this phenomenon, soils were sampled from canopied area, vertically projected limit of crown margin and interspace between shrubs in Tamarix spp. and Haloxylon ammodendron Bge. communities at an oasis-desert ecotone in south Junggar Basin. Soil samples were analyzed for texture, pH, soil organic matter (SOM), total nitrogen (TN), available nitrogen (AN), calcium carbonate (CaCO3) and total phosphorus (TP). Results indicate that (1) fertile islands′ develop both under Tamarix and H. ammodendron canopies, with the differences of soil properties between islands′ and interspaces decreasing with depth; (2) there are gradients in biologically-limiting nutrients from islands′ toward interspaces, whereas the biologically-unlimited elements are more or less or equally distributed in islands′ relative to interspaces; (3) in terms of the spatial scale, islands′ of Tamarix spp. may extend out of the canopied area while islands′ of H. ammodendron are relatively smaller and more shallow than that of Tamarix spp.; (4) spatial sizes of the islands′ for different soil indices vary even for the same shrub species. In conclusion, Tamarix islands′ have more nutrients enriched, larger area and greater depth in comparison to that of H. ammodendron. These differences are greatly due to the different morphologies of the two species, with the hemispheroidal crowns of Tamarix spp. more efficiently capturing and maintaining litter. As a result, the positive feedbacks among higher litter input, soil structure and nutrient storage make the accelerated biogeochemical cycling reach a greater spatial extent under Tamarix spp. To thoroughly clarify the development mechanism and role of the shrub ‘fertile island’ in the arid ecosystem, however, is of great challenge and needs further work.