High soil moisture triggers negative plant–soil feedbacks in a tropical forest

Abstract Plant‐microbial interactions help stabilize coexistence and shape ecological communities. These interactions are sensitive to abiotic conditions, but how they may be affected by environmental change is unknown. Variation in water availability may be particularly important as it impacts both plant and microbial physiology. Local water availability naturally fluctuates in terrestrial ecosystems, and climate change is predicted to accentuate this variation. For example, environments that experience seasonal rainfall, such as Central American tropical forests, are expected to experience drier dry seasons and wetter wet seasons. To explore the consequences of altered moisture regimes for plant‐microbial interactions, we conducted a plant–soil feedback experiment in central Panama in which we inoculated seedlings from six shade‐tolerant tree species with the soil microbial communities associated with either conspecific or heterospecific adults or with sterilized soil and grew them under two different soil moisture regimes: 50% or 100% of total soil water holding capacity (WHC). At 50% WHC, seedling survival and growth were unaffected by the source of soil inoculum. At 100% WHC, seedling survival was reduced in the sterile soil treatment and total biomass was highest for seedlings grown in heterospecific soil. Soil moisture modified the strength of plant–soil feedbacks. Plant–soil feedbacks were negative at high soil moisture (100% WHC) but disappeared at low soil moisture (50% WHC). Our work extends a growing body of research on moisture effects on plant–soil feedbacks in the North American prairie to the American tropics, and shows that plant–soil feedbacks and their implications for stabilizing coexistence are context dependent and sensitive to soil moisture in tropical seedlings. As a result, phenomena that reduce soil moisture, such as climate change or forest fragmentation, could reduce diversity in seedling assemblages by weakening negative plant–soil feedbacks. Read the free Plain Language Summary for this article on the Journal blog.