Abstract: Nitrogen (N) addition can impact root activities and microbial metabolism, thus altering soil respiration rate. In this study we assessed the effects of N addition on soil CO₂ flux in a Keerqin sandy grassland during July and September. The results showed that N addition significantly influenced the soil respiration rate. Compared with unfertilized plots (2.18 μmol·m^-2·s^-1), the respiration rate of Naddition plots was increased by 15.2% and the increment of respiration rate in the daytime (17.1%) was higher than that at night (14.4%). N addition significantly increased soil respiration from July to September. The monthly variation in soil respiration firstly rose then declined, and peaked on the 18th July (3.36 μmol·m^-2·s^-1). N addition increased the temperature sensitivity of soil respiration and the basal soil respiration, with Q₁₀ values as 3.27 and 3.30, and R_10 values as 0.42 and 0.54 μmol·m^-2·s^-1 in Naddition and unfertilized treatments, respectively.

Key words: Korean pine broad-leaved forest, selective cutting disturbance, soil phosphorus, Hedley phosphorus fractionation, phosphorus availability.