Trade-offs and synergies of plant traits co-drive efficient nitrogen use in intercropping systems

Nitrogen (N) enrichment increases aboveground net primary productivity, but causes a loss of plant diversity and a decline in ecosystem stability. Diverse systems that enhance the N-use efficiency help achieve sustainable development goals. Functional traits have been used to predict ecosystem functions in diverse systems. However, few studies considered both the effect of functional traits and evaluated the relationship between traits and N acquisition in intercropping. The aim of this study was to evaluate the relationships between functional traits resulting from interspecific interactions and determine relationships between functional traits and N acquisition of intercropping systems. Based on a four-year N-manipulation field experiment, we measured 10 functional traits related to light interception and nutrient-use efficiency in different systems (maize (Zea mays L.), peanut (Arachis hypogaea L.), soybean (Glycine max L. Merrill.) monocultures, and maize/peanut and maize/soybean intercrops) at the pre-tasseling stage of maize, calculated the crude protein yield and aboveground N content at harvest time. The maize grain N concentration was increased by N application, and it increased by intercropping in legumes. Both maize/soybean and maize/peanut intercropping systems increased crude protein productivity. Synergies among plant functional traits of maize allowed maize to establish its dominance, which resulted in greater N acquisition and crude protein yield of intercrops. Trade-offs among plant functional traits (between root traits and shoot traits of legumes) show strategies of legumes in response to interspecific competition. The specific leaf area of the legume was a key trait and affected interspecific interactions. This N-use advantage of intercropping was directly influenced by the aboveground functional traits of the system (community-weighted mean values of functional traits), but indirectly affected by belowground functional traits of the system. Our study highlights greater N uptake of the intercropping system and suggests that intercropping can be used to increase the quality of crop products via enhancing crude protein production. The synergy and trade-offs in plant functional traits together enhance efficient N use in the intercropping systems. These findings help to better understand the underlying mechanisms that determine efficient resource use in diverse cropping systems and have implications for the sustainable management of food-production systems.