Crop cultivar mixtures stabilize productivity, partly via facilitation, when conditions are less benign

Context of problem: Crop cultivar mixtures can increase and stabilize productivity; however, their effects on year-to-year temporal stability are positive in some cases, and neutral or negative in others. This inconsistency is not understood, in part because the mechanisms underlying the effects of diversity on stability are not clear, and in part because we do not understand the role of the abiotic environment on the functioning of crop mixtures. Objective of research question: To test whether maize cultivar mixtures increase maize productivity and temporal yield stability compared to monocultures, in benign vs. less benign conditions, our objective was to explore the role of facilitation in the overall effect of crop diversity. Methods: We performed two field experiments comparing mixtures of up to eight cultivars to monocultures over five years at Zhangye with irrigated conditions and with higher organic matter, total nitrogen, available phosphorus and available potassium, and over four years at Lishu with rainfed conditions without irrigation, and with lower soil fertility. Water supply at Zhangye was 18.5% higher than at Lishu and the water supply at Lishu was dependent on precipitation with greater year-to-year variation. Results: At the Lishu site, productivity was 18.3% lower than at the Zhangye site, and the temporal stability of aboveground biomass for cultivar mixtures was 31.5% higher than that of monocultures. This increase in temporal stability in mixtures corresponded to 6.9% greater productivity overall and 48.1% less variation in productivity than in monocultures at the Lishu site. Complementarity effects were positive and facilitation was much more important at the Lishu site. Temporal stability of aboveground biomass was also positively correlated with complementarity effects (P < 0.05) and facilitation (P < 0.01). There was no evidence of enhanced stability or facilitation at the Zhangye site. Conclusions: Our findings emphasize that cultivar mixtures enhance ecosystem functioning under less benign conditions with lower productivity, in part via facilitation, but not under more productive benign conditions. Implications or significance: Our study highlights that increasing crop cultivar diversity may better respond to more abiotically stressful and fluctuating conditions, and provides insight into why crop cultivar mixtures vary in their effects on stability.