Root plasticity and interspecific complementarity improve yields and water use efficiency of maize/soybean intercropping in a water-limited condition

Intercropping maize and soybean is renowned for improving crop production and resource use efficiencies. Interspecific competition and complementarity with respect to root plasticity is essential knowledge for understanding the mechanisms of overyielding and optimizing intercropping species selection. We conducted a three-year field experiment (2017–2019) to quantify land and water productivities in relation to above- and below-ground interspecific interactions, root growth and distribution under different nitrogen rates in maize/soybean intercropping. The land productivity in terms of land equivalent ratio (LER) in maize/soybean intercropping was 1.10 across all years and N rates. The yield increase in intercropped maize was mainly gained from an 45% increase in kernel numbers, while the yield loss of intercropped soybean was caused mainly by an 35% decrease in the pod numbers compared to sole cropping. The system level water use efficiency, defined as water equivalent ratio (WER) was also 1.10. Compared with sole stands, intercropped maize consumed more water during the vegetative stage, but intercropped soybean took up more during the reproductive stage. That indicated a temporal complementarity of water use in the intercrop, which benefited maize kernel formation and partially offset the negative shading effect of soybean grain filling. Soybean showed a marked increase in root length density (RLD). Compared to the produced aboveground biomass (DM) in the intercrop, the intercropped soybean invested more assimilates into root than shoot, as defined as root plasticity, the RLD/DM ratio of soybean in the intercrop was 76% more than sole system. However, the intercrop did not change root plasticity of maize. The overlap of maize and soybean roots, i.e. interspecific interaction interface, occurred mainly within the position between two border rows and at first soybean row. Under interspecific competition, soybean in the intercropping created both temporal and spatial differentiation for water uptake, which might be a key reason for enhancing intercropping land and water productivities. Our results contribute to understanding the mechanism of interspecific interaction for maximizing land and water productivities in rain-fed intercropping.