We investigated the soil chemical environment of coastal peat mires with a Picea glehnii forest and Sphagnum community in Ochiishi, north-eastern Japan. We tested the hypothesis that the soil chemical environment in an ombrotrophic peat mire is affected by the vegetation and the consequent chemical modification of precipitation by plants. We measured the chemical properties of peat pore water and precipitation both in the P. glehnii forest and the Sphagnum community and found that the forest acidifies the peat and accumulates sea salt deposited by precipitation. Picea glehnii grows in soils that have a higher salt concentration than the Sphagnum communities. We measured a stem flow salt concentration that was higher than the bulk deposition. The salt concentration in precipitation and peat pore water decreased with increasing distance from the coastline. These results imply that atmospheric sea salt deposition is effectively trapped by the P. glehnii canopy, and it then contributes to the accumulation of salt and the consequent acidification within the peat. Stem-flow water was more acidic than bulk deposition or throughfall within the P. glehnii, a forests. This means that acids are dissolved while precipitation runs through on the bark of P. glehnii, and then the acids are transferred and accumulate in the peat. The chemical modification of precipitation by P. glehnii makes the soil chemical environment more acidic than the Sphagnum community that usually makes the soil environment acidic in a peat mire ecosystem.