Red clover productivity under drought: Are soil microbes a burden or a treasure?

Drought-induced changes in plant traits are well-studied, e.g. building smaller and thicker leaves, reducing stomatal density or increasing root uptake. However, the extent to which these changes depend on plant-soil interactions remains unclear. Mutualistic soil microbes like arbuscular mycorrhizal fungi (AMF) and Rhizobium are known to enhance plant productivity, water and nutrient uptake and stress tolerance. Therefore, alterations in soil microbial loads are expected to affect not only plant productivity but also plant responses to drought through changing plant traits. In this study, we investigated the effects of plant-soil interactions on the productivity and traits of red clover (Trifolium pratense L.) under repeated drought. We conducted a pot experiment with two treatments: water treatment (wet versus drought) and microbial reduction by soil steam sterilization (native versus sterilized soil). We found that plants in native soil showed lower productivity and plant traits associated with slow-growing strategies (i.e. small and coarse leaves, lower stomatal density and higher root mass). However, the reduction of soil microbial load by soil steam sterilization increased plant productivity under wet conditions and led to plant traits associated with fast-growing strategies. Drought in sterilized soil decreased productivity, promoted earlier wilting and resulted in the development of plant traits associated with a more slow-growing strategy. Furthermore, roots became longer and thinner and rhizobial nodulation decreased, demonstrating the reduction of mutualists. Hence, our results show that soil microbes trigger plant traits, which suggests that they play an important role in plant responses to drought, in maintaining plant productivity and in prolonging plant vitality.