Soil solutions at 15 cm and 60 cm under a mature broadleaf and Korean pine mixed forest (>200 years old) at Changbai mountain, northeast China, were sampled using porous ceramic cups from July 2006 to October 2008, to study the effects of nitrogen (N) addition on concentrations and fluxes of dissolved organic matter and inorganic N. The soil net N mineralization and wet atmospheric deposition of N and dissolved organic carbon (C) under forest canopy were also measured. The addition of N sources such as (NH4)2S04, NH4Cl and KN03 at rates of 2.25 and 4.5 g N/m2 each year tended to increase concentrations and fluxes of inorganic N and dissolved organic N in soil solutions at 15 cm and 60 cm depths and soil net N mineralization, and it reduced leaching losses of soil dissolved organic C. The concentration ratios of dissolved organic C to dissolved organic N and special UV absorbance values in the soil solutions at both depths were mostly smaller under the N-fertilized forest plots than under non-fertilized plots. Soil net N mineralization under the N-fertilized forest plots can contribute to the leaching losses of inorganic N from the soil. Our observations indicate that N inputs to temperate forest floors can affect the status of N and C processes m underlying forest soils.