Maize cultivar mixtures increase aboveground biomass and grain quality via trait dissimilarity and plasticity

Despite being visually cryptic, conspecific cultivar mixtures increase diversity and can increase productivity in agricultural systems. However, the mechanisms underlying this increase are unclear. In natural systems, differences in traits among species and among ecotypes correspond with diversity-increased productivity, but whether these attributes are important in cultivar mixtures is still unclear. To explore how cultivar mixtures affect agroecosystem productivity and the roles of trait dissimilarity, we conducted a four-year (2017–2020) field experiment in Northeast China with maize. The experimental treatments included eight single cultivar monocultures, four two-cultivar mixtures, one four-cultivar mixture, one six-cultivar mixture and one eight-cultivar mixture. Experiments were arranged in a randomized complete block design with three replications. Across the four years, all cultivar mixture combinations increased aboveground biomass and crude protein in grain by 7.0% and 4.1%, respectively, compared to the average of the eight monocultures. Grain yield, aboveground biomass, and grain crude protein in overyielding cultivar combinations increased by 2.0–9.0%, 13.0–20.0% and 4.8–10.7%, respectively, compared to the average of the component monocultures of each mixture over four years. We calculated positive complementarity effects for grain yield, aboveground biomass and grain crude protein. All selection effects of cultivar mixtures were negative or neutral. There were strong positive correlations between complementarity effect and functional trait dissimilarity in cultivar mixtures, based on grain yields (P< 0.010), aboveground biomass (P< 0.001) and grain crude protein (P< 0.050), whereas there were no such positive relationships between selection effects and functional trait dissimilarity. Moreover, we found higher levels of trait dissimilarity for shoot diameter, leaf area and specific leaf area in cultivar mixtures than in monocultures. Also, the shoot diameter, leaf area and specific leaf area of specific cultivars were higher in mixtures than in monocultures. Our results indicated that niche complementarity derives from functional trait dissimilarity, and strong complementarity effects correlate with greater functional trait dissimilarity in cultivar mixtures. These findings provide evidence that plasticity in traits, rather than only inherent traits, contributed to the diversity effects, and the greater functional trait dissimilarity among cultivars in mixtures can increase crop productivity via increasing cryptic diversity in agroecosystems. Our results link changes to plant aboveground traits in cultivar mixtures, which shed light on yield advantages in diverse agroecosystems.