Drying responses of microbial litter decomposition and associated fungal and bacterial communities are not affected by emersion frequency

Leaf-litter decomposition is driven by microbial and invertebrate communities and is likely to be severely affected by climate change and increased water abstraction and diversion. The occurrence, frequency, and severity of drying events are increasing in rivers worldwide. These events are a major threat to aquatic microbial communities and their contribution to riverine biogeochemical processes. A growing body of evidence suggests a dramatic reduction in litter processing rates and microbial activity during emersion. We hypothesized that leaf-litter decomposition would be less affected by high than low emersion frequency (characterized by longer emersion periods between immersion events, assuming a constant emersion duration). We exposed Alnus glutinosa L. litter to high-, medium-, and low-frequency emersion-immersion cycles in the Albarine River (eastern France). After 28 d of incubation, we quantified leaf mass loss and the biomass, extracellular enzyme activity (cellobiohydrolase), and structure (molecular fingerprint) of the fungal and bacterial communities that developed on the leaves. Leaf-litter decomposition was negatively affected by drying, but we found no effect of emersion frequency. Cellobiohydrolase activity and fungal and bacterial biomass did not vary with emersion frequency, and microbial biomass was dominated by fungi in all treatments at the end of the incubation period. Fungal and bacterial community structures were relatively insensitive to emersion frequency. Emersion duration had a larger effect on microbial community composition and leaf-litter decomposition than emersion frequency. We found no evidence of a link between the composition of microbial communities and their enzymatic activity or their ability to decompose leaf litter. Our findings suggest some degree of functional redundancy in microbial communities affected by drying stress during early stages of leaf-litter decomposition.