A pot experiment using root separation technique was conducted to further understand the effect of root interaction played in intercropping system under different nitrogen levels. The results showed that root interaction and increasing nitrogen application increased the green leaf area per plant and chlorophyll content of soybean, but their effects gradually decreased with increasing nitrogen fertilization level. Root interaction and increasing nitrogen application can improve photosynthetic characteristics of soybean, but root interaction only had a significant effect under low nitrogen level. The number of bacteria, fungi, actinomycetes and Azotobacteria was also obviously affected by root interaction and nitrogen fertilization, and the number of Azotobacteria presented a changing trend of first increased and then decreased with increasing nitrogen fertilization level. Root interaction and increasing nitrogen application improved soybean yield and its components, but their effects gradually decreased with increasing nitrogen fertilization level. The root activity of soybean was obviously affected by root interaction, and was significantly positively correlated with green leaf area per plant, chlorophyll content, photosynthetic rate and economic yield per plant. Our results indicate that the advantage effect of root interaction and increasing nitrogen application will be partially inhibited with an increasing nitrogen fertilization level.

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