Fungicide and warming interact to reduce soil ecosystem functioning.

Soil microbial communities are essential to the functioning of their ecosystems, providing vital services such as plant growth promotion and bioremediation. However, intensively managed environments such as agricultural soils are increasingly challenged with multiple harsh environmental stressors from the compounding effects of climate change and conventional agricultural practices. Understanding how soil communities will respond to environmental stress along multiple axes is key for predicting how environmental change will impact terrestrial soil ecosystems. We combine experimental evolution with community phenotyping, 16S rRNA gene sequencing, fungicide resistance assays, and plant experiments, to show that fungicide and warming temperatures together drive a synergistic reduction in soil community respiration rates, metabolic capacity and Hordeum vulgare plant shoot biomass, which would have been missed by looking at stressor effects independently. Combined warming and fungicide stress caused widespread loss of metabolic activity, particularly for carbohydrates and carboxylic acids, highlighting impairment of community function. Fungicide-only treated soil isolates had a 16-fold increase in fungicide resistance, but only in the absence of warming-suggesting that warming constrained fungicide adaptation. Together, our findings show that the number of stressors (rather than the nuances of individual stressors) may be key for predicting soil microbial community responses to environmental change across multiple axes.