Effects of nitrogen addition on dissolved N2O and CO2, dissolved organic matter, and inorganic nitrogen in soil solution under a temperate old-growth forest

Soil solutions at 15 cm and 60 cm depths under a Korean pine and broadleaf mixed forest (> 200 years old) at Changbai mountain, northeast China, were sampled using porous ceramic suction cups from July 2006 to October 2007, to study the effects of nitrogen (N) addition on the concentrations of dissolved nitrous oxide (N2O) and carbon dioxide (CO2), dissolved organic matter and inorganic N. The actual concentrations of dissolved N2O and CO2 in soil solutions could be obtained using a combination of continual three phase equilibrations and gas chromatography. The dissolved CO2 concentrations in soil solutions at 15 cm and 60 cm depths varied from 4.3 to 15.5 and from 3.5 to 18.3 μg CO2–C ml− 1, respectively, and dissolved N2O concentrations at both depths varied from 1.8 to 34.9 and from 2.5 to 99.3 ng N2O–N ml− 1, respectively. The addition of N sources such as (NH4)2SO4, NH4Cl and KNO3 at a rate of 4.5 g N/m2 each year tended to decrease concentrations of dissolved organic carbon (DOC) in soil solutions at 60 cm depth in 2007, which was contrary to the increase in dissolved CO2 concentrations under N-fertilized forest plots. However, the N addition did not give an obvious effect on the concentrations of dissolved CO2 and DOC in soil solutions at 15 cm depth. There was an increase in concentrations of NH4+–N, NO3−–N and dissolved organic nitrogen (DON) of soil solutions under N-fertilized forest plots. Rainfall after thawing in spring could promote the accumulation of dissolved N2O in soil solutions at 15 cm and 60 cm depths, particularly at 60 cm depth under the (NH4)2SO4 added plots, due to mineralization of DON and nitrification. All the tested properties of soil solutions at 15 cm and 60 cm depths were well correlated. Among these properties, the dissolved N2O concentrations of soil solutions at both depths were better correlated with the DON concentrations at 60 cm depth, and the dissolved CO2 concentrations at 15 cm depth with the DOC concentrations at both depths. Hence, both DOC and DON can contribute to the formation of dissolved CO2 and N2O in the soil solutions at varying depths under N-fertilized forest plots, respectively. Our observations strongly indicate that N inputs to temperate forest floors can affect the status of N and carbon processes in underlying forest soils.