Border row effects on the distribution of root and soil resources in maize–soybean strip intercropping systems

The cause of yield increase in crop strip intercropping is often inseparable from the boundary effect, and the distribution of roots and nutrients in the below-ground has an important influence on plant growth. However, previous studies on the boundary effect of intercropping have mainly focused on the above-ground part, and the state of the below-ground part is unknown. So we investigated the distribution of roots, water and NO3—N in the belowground part of the border row(adjacent rows of two crops in intercropping) in a maize and soybean strip intercropping system at different plant growth stage. There were three treatments in this experiment: maize monocrop, soybean monocrop and maize–soybean intercrop (four rows of maize and four rows of soybean). The distribution of maize roots extended beyond and below the border with the soybean rows and was concentrated in the 0–20 cm soil layer, while the growth of intercropped soybean roots was inhibited by maize. Intercropping increased the soil water content (SWC) in soybean rows at the podding stage and in maize rows at the drumming stage, and intercropping had a greatly effect on SWC of the system as a whole. Intercropping also increased the soil NO3—N content (SNC) in the intercropping system at the podding stage and had a negative effect at the drumming stage, and the effect of SNC in the intercropped maize rows was wider, while for intercropped soybean it was limited to the adjacent rows of soybean. The effect of intercropping on SWC and SNC occurred mainly in the 20–60 cm soil layer. Intercropping increased the grain weight of maize and decreased the grain weight and pods per soybean plant. Overall, the root system of maize in the border rows in the intercropping pattern extended into the soybean rows, intercropping increased soil water and soil nitrate-N levels in maize during the critical yield formation period, resulting in increased maize yields, while soybean was negatively affected in the intercropping pattern resulting in reduced yields, and we have calculated that intercropping maize and soybean has advantages in terms of land productivity and water use.