Microbial and bryospheric photosynthesis of boreal peatlands have peatland-type-specific responses to long-term drying.

The impacts of drying on bryospheric photosynthesis are poorly understood. Utilising a 20-yr-long experiment, we quantified the effects of long-term water level drawdown (WLD) on links between bryospheric photosynthesis, microbial community composition, decomposition, and environmental variables. The community structure of photoautotrophic microbes was investigated using metabarcoding and quantitative polymerase chain reaction. Microbial photosynthesis was measured as photosynthesis efficiency (φPSII) and maximum electron transport rate through photosystem II (ETR). Bryospheric photosynthesis was measured using an infrared gas flux analyser. Chlorophyta, Cyanobacteria, and Ochrophyta were dominant photoautotrophic phyla in all study areas, but communities had site-specific WLD responses. WLD increased microbial and bryospheric photosynthesis in the poor fen. ETR was promoted by soil phosphorus concentration, φPSII, and Chlorophyta abundance, and bryospheric photosynthesis by Cyanobacteria abundance, a deep water table, soil sulphur concentration, and decomposition. In undrained areas, high abundance of Cyanobacteria promoted soil nitrogen concentration and, therefore, photosynthesis. In WLD areas, these connections were lost, but bryospheric photosynthesis benefited from WLD directly. Our study confirms that photoautotrophic microbes, especially Cyanobacteria and Chlorophyta, are important contributors to bryospheric photosynthesis in pristine boreal peatlands. Furthermore, increased bryospheric photosynthesis after drying may offset carbon loss from increased decomposition, but this depends on the site characteristics.