History matters: Heterotrophic microbial community structure and function adapt to multiple stressors

Ecosystem functions in streams (e.g., microbially mediated leaf litter breakdown) are threatened globally by the predicted agricultural intensification and its expansion into pristine areas, which is associated with increasing use of fertilizers and pesticides. However, the ecological consequences may depend on the disturbance history of microbial communities. To test this, we assessed the effects of fungicides and nutrients (four levels each) on the structural and functional resilience of leaf‐associated microbial communities with differing disturbance histories (pristine vs. previously disturbed) in a 2 × 4 × 4‐factorial design (n = 6) over 21 days. Microbial leaf breakdown was assessed as a functional variable, whereas structural changes were characterized by the fungal community composition, species richness, biomass, and other factors. Leaf breakdown by the pristine microbial community was reduced by up to 30% upon fungicide exposure compared with controls, whereas the previously disturbed microbial community increased leaf breakdown by up to 85%. This significant difference in the functional response increased in magnitude with increasing nutrient concentrations. A pollution‐induced community tolerance in the previously disturbed microbial community, which was dominated by a few species with high breakdown efficacies, may explain the maintained function under stress. Hence, the global pressure on pristine ecosystems by agricultural expansion is expected to cause a modification in the structure and function of heterotrophic microbial communities, with microbially mediated leaf litter breakdown likely becoming more stable over time as a consequence of fungal community adaptions.