The global population is expected to reach 9.1 billion by 2050, requiring a 70% increase in food production compared to 2005. However, climate change, including drought, threatens vegetation around the world. It has been suggested that soil microbial communities can improve plant productivity and survival but the problem is more complex than simple application of plant “probiotics” to the soil. A major knowledge gap is how soil microbial communities alter plant physiological responses to drought, and whether they have a positive or negative impact on plant drought tolerance. To address this knowledge gap, we examined plant physiological response to drought as a function of soil microbial community composition. We hypothesized that microbial community composition will affect the following plant functions: germination, growth, drought tolerance, photosynthesis, stomatal conductance, growth, and wilting. To test these hypotheses, seeds of a fast-growing C4 grass, blue grama (Bouteloua gracilis) were planted in sterilized sand (the control treatment) or sterilized sand inoculated with soil microbial communities from 15 geographically distinct New Mexico soils exhibiting functional differences in carbon cycling. After substantial growth (14 weeks), drought was imposed. Plant physiological measurements were taken before, during, and after drought.
Results/Conclusions
Our results show that one month after planting, germination and shoot height were significantly greater in inoculated plants compared to controls. During the initial stage of drought, inoculated plants were more productive (indicated by greater growth and rates of photosynthesis) than controls because the inoculated plants maintained higher soil moisture. This delayed the onset of drought but also caused inoculated plants to have lower tissue drought tolerance than controls. As drought persisted and soil moisture declined to zero, inoculated plants were more susceptible to drought as indicated by significantly lower stomatal conductance, greater wilting, and faster mortality compared to controls. These data suggest that soil microbes promote B. gracilis’ opportunistic growth strategy—i.e., fast plant growth when water is available and dieback when water is scarce. In this system, microbes enhanced plant productivity and dampened drought stress over short timescales, but accelerated mortality and increased plant susceptibility to drought over long timescales. This work provides an unexpected insight into plant-microbe-soil interactions.