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Intercropping Persian walnut (Juglans regia L.) and wheat (Triticum aestivum Linn.) in northwest China is receiving increasing attention because it offers potential advantages for improving land-use efficiency, increasing economic returns to farmers and increased sustainability in crop production. We carried out a two-year field intercropping experiment, using a system composed of young Persian walnut trees with wheat to determine the root competitive interactions between the two species, aboveground biomass and the economic yield of each component of the intercropping system at a spacing of 4 x 6 m in Hetian old oasis, south Xinjiang province, northwest China. Roots were sampled by auger in the Persian walnut tree/wheat intercropping system and in the sole wheat and sole walnut systems, down to a depth of 1 m in the soil profile. The results show that the roots of the trees spread under the crop and that the roots of intercropped wheat tended to have a shallower distribution in the soil profile. The roots of both intercropped walnut and wheat had a lower root length density and root diameter and a higher specific root length at all soil depths than those of sole walnut trees and wheat in 2012. Although root competition belowground led to decreases in yield and biomass of both crops, intercropping of Persian walnut and wheat improved the utilization of plant growth resources (LER > 1) as compared with sole crops. Tree height, diameter at breast height and mean radius of crown of intercropped trees in both intercropping years had lower values when compared with the monoculture. We conclude that interspecific interactions occurred largely as below-ground competition between the species investigated. Competition for soil resources was the primary limiting factor for wheat productivity. Further research is needed to examine the dynamic processes of species interactions in intercropping systems with trees of increasing age.