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The degree of tree-crop competition for nitrogen (N) and its effect on fertilizer-use efficiency and N movement were examined in a jujube tree (Zizyphus jujuba Mill.)/wheat (Triticum aestivum Linn.) agroforestry system during 2011–2012. Assessment of competition was accomplished via the installation of an experiment with five imbedding depths (20, 40, 80, 120 and 200 cm) of 15N-enriched CO(NH2)2 and two configuration patterns (sole and intercropped treatment). The percentage of N derived from fertilizer (NDFF) and fertilizer-use efficiency (UFN) were determined using 15N-enriched CO(NH2)2 (5.22% atom enrichment) applied at 8 g N per plant. Land equivalent ratios of the jujube/wheat intercropping system reached maximum ratios of 1.45 for grain (fruit) yield and 1.67 for biomass in 2011 and 1.38 for grain (fruit) yield and 1.59 for biomass in 2012. The alley cropping yield decreased significantly compared to sole-cropped plants in both years, as did the biomass of intercropped wheat in 2012. This study showed a significant difference in nitrogen acquisition between intercropping and the expected value for the various depths in both years. The %NDFF of the two plants showed significant differences between intercropping and the expected value at various depths in both years, and the intercropped %NDFF was higher than the expected value. The total UFN data, which represented an increase in response to intercropping, indicate that plants in the intercropping treatment had a higher UFN than the expected value. The UFN for jujube trees in all treatments was lower than that for wheat, indicating less uptake and utilization of N fertilizer. Management interventions, particularly those that increase fertilizer in the shallow layer may be critical for the success of this system.

Planting trees near crops can diversify the landscape, provide pollinators for crops, and allow predators to control insect pests. However, trees can also compete with crops for light, water, and nutrients. Field experiment at the Shihezi Oasis in northern Xinjiang Province assessed the effects of distance on photosynthetic characteristics and yield of wheat in a jujube tree (Zizyphus jujuba Mill.)/wheat (Triticum aestivum L.) agroforestry system. The experiment used two treatments: a control of sole-cropped wheat (D0) and a jujube tree/wheat agroforestry system. The agroforestry system included the different distances of 90, 110, and 130 cm (D1, D2, and D3, respectively) between the wheat and the jujube trees. Photosynthetic characteristics, leaf area index (LAI), the number of stems, and yield components of wheat were determined. The net photosynthetic rate (Pn) and the stomatal conductance (Gs) of the D0, D2, and D3 during the filling stage were significantly higher than the D1. No significant difference in transpiration rate (Tr) and intercellular CO2 concentration (Ci) was noted between sole-cropped wheat and agroforestry system. From the tillering to ripening stages, significant differences were observed in stem, tiller number, and dry matter accumulation per plant, and in LAI, which ranked D0 > D3 > D2 > D1. D0 and D3 plants had significantly higher numbers of spikes, grains per spike, and weight of thousand grains than D1 and D2 plants. The crop yield was ranked as D0 > D3 > D2 > D1, with significant differences among D0, D1, D2, and D3. In conclusion, wheat yield increased as the distance from jujube trees increased in this jujube tree/wheat intercropping system.

Effects of tree shading on crops within an agroforestry system in Hetian oasis regions in northwestern China are poorly understood. Measurements of light intensity, light use efficiency (LUE), light extinction coefficient, leaf area index (LAI), crop yield and biomass were studied in determining the effects of tree shading on crop yield in a jujube (Zizyphus jujuba Mill.)/wheat (Triticum aestivum L.) intercropping system. The treatments comprised (1) sole-cropped wheat, (2) 6-year-old sole jujube trees, (3) 8-year-old sole jujube trees, (4) 10-year-old sole jujube trees, (5) intercropping of wheat/6-year-old jujube trees, (6) intercropping of wheat/8-year-old jujube trees and (7) intercropping of wheat/10-year-old jujube trees. We found that the land equivalent ratios (LERs) of all three jujube tree/wheat treatments were larger than one and this indicated that plant growth factors were used more efficiently by the intercrop than by sole crops and the land use advantages of the intercropping systems were significantly greater than monoculture crops. Because of shading by trees, there was a decrease in intercropped wheat LAI, growth and yield, and the fruit yield of the intercropped jujube trees also decreased to a certain degree. The light interception of intercropped wheat was significantly decreased by different tree ages up to a certain point. The LUE results ranged from 1.68 to 1.31 and 1.13 g DM MJ [photosynthetically active radiation (PAR)]−1, indicating that LUE can be significantly affected by increasing tree age. Compared to intercropping 8-year-old jujube trees, a higher light extinction coefficient was found in intercropping 6- and 10-year-old jujube trees.

A field study was conducted at Hetian oasis, southern Xinjiang, northwestern China, to investigate root distribution and morphology affected by interspecific interactions between jujube trees (Ziziphus jujuba Mill.) and cotton plants (Gossypium hirsutum L.). The treatments comprised (1) sole cotton, (2) 5-year-old sole jujube trees, (3) 7-year-old sole jujube trees, (4) 9-year-old sole jujube trees, (5) intercropping of cotton/5- year-old jujube trees, (6) intercropping of cotton/7-year-old jujube trees, and (7) intercropping of cotton/9-year-old jujube trees. Roots in each plot were sampled vertically by auger to a depth of 100 cm at 20-cm intervals and horizontally up to 150 cm away from the base of the trees at 30-cm intervals. The results showed that all jujube/cotton intercropping systems exhibited advantages of intercropping, with a land equivalent ratio > 1. The roots of both intercropped cotton and jujube had lower root length density (RLD) and root diameter (RD) at all soil depths compared with those of sole-cropped cotton and jujube trees. Interspecific competition with jujube trees altered the root development and morphology of the cotton plants. The roots of the trees spread underneath the crop and the roots of intercropped cotton tended to have a shallower distribution in the soil profile. A greater distance from the trees resulted in less influence on the RLD and RD of intercropped cotton. Older jujube trees had more developed roots and consequently higher RLD and RD. These findings may partly explain the interspecific competition effects in jujube tree/cotton agroforestry systems. However, the mechanisms underlying the root interactions between cotton and jujube require further investigation.

The objective of this experiment was to learn more about root morphology and spatial distribution in a specify walnut (Juglans regia L.)/wheat (Triticum aestivum L.) intercropping system. The effects of intercropping on aboveground yield and land equivalent ratio (LER) were also determined. The experiment was conducted at Hetian in the southern part of the Xinjiang Uyghur Autonomous Region. The results showed that walnut/wheat intercropping reduced the aboveground yield of both component species. However, the LER of the intercropping system averaged 1.62. This meant that intercropping increased land use efficiency. Intercropping significantly reduced the root length density, root surface area density, and root diameter (RD) of both component species compared with sole-cropping. In contrast, intercropping significantly increased the specific root length of walnut. Root competition between species was the greatest in the 0–40 cm soil depth. Root competition caused uneven distribution of RD and specific root length. The plasticity of root morphology and spatial distribution is key for adapting to competition and maximizing water and nutrient uptake in walnut/wheat agroforestry systems.

The jujube tree (Zizyphus jujuba Mill.)/wheat (Triticum aestivum L.) agroforestry system is frequently used in China’s Xinjiang Province. The system improves land-use efficiency and increases economic returns. A field experiment was conducted at the Hetian oasis in southern Xinjiang Province to investigate the relationship between root distribution and interspecific interaction between the two intercropped species. The study included seven treatments: sole-cropped 5, 7, or 9-year-old jujube trees (treatments 1–3); 5, 7, or 9-year-old jujube trees intercropped with wheat (treatments 4–6); and sole-cropped wheat (treatment 7). To determine vertical root distribution, soil cores were collected in 20-cm increments from the 0 to 100-cm soil depth. The cores were collected at horizontal distances of 30, 60, 90, 120, and 150 cm from the jujube rows. The results showed that the land equivalent ratios were >1 for the three jujube/wheat intercropping systems. This indicated that these systems were advantageous compared with sole cropping. Tree height, breast height diameter, and mean crown radius were less in the intercropped treatments than in the corresponding sole-cropped treatments. Intercropping reduced the root length densities (RLDs) and root diameters (RDs) of both jujube and wheat at all soil depths. The RLD and RD of 9-year-old jujube trees were greater than those of the 5- and 7-year-old trees, which indicated that the root systems of the 9-year-old trees were more developed. Wheat root growth was inhibited more by older jujube trees than by younger ones. In conclusion, jujube tree/wheat intercropping can be practical and beneficial in the region. However, the mechanisms involved in the belowground interspecific interactions are still unknown. Additional research is needed to provide optimal management strategies and technologies for jujube/wheat intercropping.

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 × 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.

A field study was conducted at Hetian, southern Xinjiang, northwest China, to investigate root morphology as affected by interspecific interactions between jujube (Ziziphus jujuba Mill.) and wheat (Triticum aestivum L.). The treatments comprised (1) sole wheat, (2) 3-, 5- and 7-year-old sole jujube trees, and (3) intercropping of wheat/3-, 5- and 7-year-old jujube trees. Roots were sampled by auger in each plot down to 100 cm depth at 20 cm intervals in the soil profile and horizontally up to 150 cm away from the base of the trees at 30 cm intervals. All jujube/wheat intercropping systems had advantages of intercropping with a land equivalent ratio (LER) >1. There were significant differences in the contours of both root length density (RLD) and root diameter (RD) in intercropped wheat and jujube in the vertical and horizontal direction at corresponding soil depths but the RLD and RD of the 7-year-old jujube/wheat intercropping system were less influenced by intercropping in this respect than 3- and 5-year-old jujube intercropped with wheat. The roots of both intercropped wheat and jujube had smaller RLD, RD and larger specific root lengths (SRLs) at corresponding soil depths than did sole wheat and jujube. The older the jujube the larger were the SRL values of intercropped wheat and the smaller the RLD and RD of intercropped wheat. The greater the distance from the jujube the less influence there was on the RLD, SRL and RD of intercropped wheat and jujube and the greater the distance from the jujube the smaller was the SRL of intercropped wheat and the greater the RLD and RD of intercropped wheat (but still less than the monoculture wheat). The older the jujube the more developed were the jujube roots so that the smaller the SRL of jujube the bigger the RLD and RD of jujube. Jujube tree roots showed a mainly downward trend and extended laterally 150 cm from the trees resulting in the roots of the jujube trees and the wheat having niche overlap at a soil depth of 20–40 cm. The mechanisms underlying the thinner roots of wheat and jujube require further investigation.

Even though agronomists have considered the spatial root distribution of plants to be important for interspecific interactions in agricultural intercropping, few experimental studies have quantified patterns of root distribution and their impacts on interspecific interactions in agroforestry systems. A field experiment was conducted to investigate the relationship between root distribution and interspecific interactions between intercropped jujube tree (Zizyphus jujuba Mill.) and wheat (Triticum aestivum Linn.) in Hetian, south Xinjiang province, northwest China. Roots were sampled by auger in 2-, 4- and 6-year-old jujube tree/wheat intercropping and in sole wheat and 2-, 4- and 6-year-old sole jujube down to 100 cm depth in the soil profile. The roots of both intercropped wheat and jujube had less root length density (RLD) at all soil depths than those of sole wheat and jujube trees. The RLD of 6-year-old jujube intercropped with wheat at different soil depths was influenced by intercropping to a smaller extent than in other jujube/wheat intercropping combinations. 6-year-old jujube exhibited a stronger negative effect on the productivity of wheat than did 2- or 4-year-old jujube and there was less effect on productivity of jujube in the 6-year-old system than in the 2- or 4-year-old jujube trees grown in monoculture. These findings may partly explain the interspecific competition effects in jujube tree/wheat agroforestry systems.