Evidence for a Water-Uptake Independent Effect of Plant on Soil Microbiota Under Different Water Regimes

The on-going climate change impacts soil water distribution and availability, ranging from highly limited during droughts to excessive during rainfalls. While effects of water availability were documented for soil microbiota and plant growth, it remains unclear if the soil water content (SWC) directly impacts microbiota or indirectly, via plant presence and its own response to water fluctuations. Uncoupling direct and indirect effects is challenging, given the effect of plant on SWC via water-uptake and transpiration. To identify a potential water uptake-independent effect of plant, we studied the response of soil microbiota in two contrasted soils exposed to three water levels, either under the presence or absence of Brassica juncea (drought-sensitive, water-logging-tolerant) or Brachypodium distachyon (drought-tolerant, water-logging-sensitive). Using an automatic high-throughput plant watering system, we accurately maintained SWC levels to compensate evapotranspiration, enabling direct and indirect effect comparison. Plant traits were measured and the bacterial and fungal communities with or without plants were analysed. We highlighted cases of indirect effects of the plant on the microbiota under varying SWC, independently of plant water uptake. These effects were likely instigated by the physiological state of the plant due to SWC, and were dependant on the plant species and the soil considered. While bacterial community was more sensitive to SWC than fungal community, we found that the fungal community in the clayey soil was directly affected by SWC, and could opportunistically interact with a drought sensitive plant like Brassica juncea to alleviate SWC effects.