Root traits and soil legacies drive species competition outcomes

Abstract Plant competition can be affected by plant functional traits but also by differences in fitness mediated by soil microbes. Climatic conditions such as drought further influence plant competition. Yet little is known about how soil microbes and drought interact with plant species that have distinct root traits and how this influences plant competition outcomes. We grew three plant species that co‐occur in temperate grasslands in China ( Stipa krylovii , Artemisia frigida , Agropyron cristatum ) in monocultures and mixtures and subjected the plant combinations to five soil inocula (root‐associated soil of S. krylovii , A. frigida, A. cristatum , an equal mixture of the three root zone soils and sterilized soil) as well as to a drought treatment. The relative change in plant biomass was used to determine plant competition outcomes. The three species exhibited clear differences in competitive abilities with A. cristatum > S. krylovii > A. frigida , and soil inocula or the drought treatment did not change the order. The relative yield (RY) of plants was affected by soil inocula, drought and plant arrangement. The strongest competitor, A. cristatum , with high total root length, root surface area and root volume experienced more negative biotic feedback, and drought enhanced the magnitude of these negative effects. On the contrary, the most inferior competitor, A. frigida , with high specific root length tended to have neutral or positive biotic feedback, and drought had no effect. Furthermore, the RY and fitness difference (reflected as the competitive ability in the mixture) of the three species were differentially influenced by root traits and plant–soil feedback. RY of A. cristatum could be predicted by the feedback effect in the mixture, and the fitness difference was mainly related to root traits. Both RY and fitness differences of A. frigida (the weakest competitor) could be predicted by root trait differences and feedback effects. Differences in root traits were the best predictors of the intermediate competitor S. krylovii . Our study shows that competition outcomes of co‐existing species depend on root traits and species‐specific PSF effects in mixture. Future work should examine the mechanisms that explain how plant competition and soil microbial heterogeneity act in conjunction with climate change in influencing plant coexistence. Read the free Plain Language Summary for this article on the Journal blog.