Drought stress impacts soil microbial nutrient limitation more strongly than O3 pollution

Summer drought conditions generally coincide with exposure to high ground-level concentrations of ozone (O3). These two stressors compromise the availability of water, energy, and nutrients to vegetation and microbial communities present within the soil. However, relatively little is known regarding the responses of rhizosphere and non-rhizosphere soil microbial nutrient acquisition to conditions of drought and O3 exposure. Here, we used ecoenzymatic stoichiometry modeling and vector analyses to decipher the resource constraints on rhizosphere and non-rhizosphere soil microbial metabolic limitation within the soil of hybrid poplars exposed to two watering regimens (well-watered or reduced water of 40 %) and two chronic O3 treatments (charcoal-filtered air or ambient air +40 ppb of O3). Results show that, simulated drought stress resulted in stronger effects on the structure of soil microbial communities and associated enzymatic activity relative to exposure to elevated O3 levels. Exposure to these two stressors induced a microbial phosphorus (P) limitation response, with these limitation effects being more pronounced in the rhizosphere soil relative to non-rhizosphere soil. Structural equation modeling revealed that drought-associated declines in soil water availability and pH values were the primary factors influencing this microbial P limitation. Reduced soil water levels resulted in the exacerbation of microbial P limitation as a consequence of the inhibition of P mineralization and availability of C and N, whereas declining soil pH values had the opposite effect, alleviating microbial P limitations by reducing soil alkalinity and thereby aiding in the dissolution of the insoluble P present in alkaline soil. These results emphasize the complex responses of rhizosphere and non-rhizosphere soil microbial metabolic activity to elevated O3 levels and drought stress, providing invaluable insight into the importance of elemental stoichiometry-driven microbial metabolic variability of nutrient cycling activity at the root-soil interface in O3-polluted and drought-exposed ecosystems. •Soil microbes were limited by P availability in this alkaline soil.•Drought stress was the main factor affecting soil enzymes and multifunctionality.•Drought-induced the reduction in soil moisture led to microbial P limitation.•Drought-induced the reduction in soil pH alleviated microbial P limitation.•Elevated O3 effects failed to affect soil microbial nutrient limitation.