Air pressure as a driver of plant-specific microbial responses in the rhizosphere.

In response to climate change, plants in mountain regions are shifting their distribution ranges upward, exposing them to novel abiotic conditions such as reduced atmospheric pressure. While these changes are likely to affect plant physiology, their impact on plant-associated microorganisms in the rhizosphere has not yet been investigated. In this study, we used the terraXcube Ecotron facility to experimentally discriminate air pressure from other elevation-related factors like humidity and temperature, and to assess its influence on the rhizosphere microbiota of three plant species: a grass (Brachipodium rupestre), a forb (Hieracium pilosella), and a legume (Trifolium pratense). Plants were grown under controlled environmental conditions at four simulated elevations (260, 1500, 2500, and 4000 m a.s.l.), corresponding to pressure levels of 98, 85, 75, and 62 kPa, respectively. Microbial biomass and activity were significantly influenced by air pressure, but in a plant-specific manner. In addition, air pressure also led to notable and plant-specific shifts in the community composition of prokaryotes and, to a lesser extent, fungi. Redundancy analysis identified air pressure as a central predictor of these rhizosphere community shifts. Notably, no correlations were detected between microbial community composition and morphological and physiological plant traits, suggesting that air pressure should directly affect microorganisms, independently of plant-mediated effects. This study demonstrates that even under constant temperature and humidity, air pressure alone can restructure rhizosphere microbial communities, highlighting a critical yet often overlooked driver of plant-microbe dynamics during uphill range shifts. Whether such alterations in the rhizosphere microbiota ultimately enhance or impair soil chemistry, plant health, and ecosystem functioning remains an important question for future research.