This dataset is used for our paper ‘Symbiosis revisited: phosphorus and acid buffering stimulate N2 fixation but not Sphagnum growth’ in Biogeosciences. It contains background data on Sphagnum in the Ilperveld field site, the Netherlands and on content of peat monoliths from deep (20 cm) soil layers from Ilperveld. These peat monoliths and Sphagnum mosses were transported to mesocosms in the water bath lab in Nijmegen in September 2013. Two species of Sphagnum (S. palustre and S. squarrosum) were applied to the peat and a flux of treatment solution was added to the mesocosms during 10 weeks. Treatments were a full factorial design of phosphorus (P) and bicarbonate (HCO3) fluxes in surface water. After 10 weeks (in February 2014) surface water nutrient concentrations were measured and an incubation experiment was done on N2 fixation activity in Sphagnum. Also Sphagnum performance data were obtained. All can be found in this data set.