Abstract: Soil respiration is a key component in the carbon cycle of forest ecosystems, which can be divided into soil microbial respiration and root respiration. Effects of nitrogen addition and understory removal on soil microbial respiration and root respiration would be different. In this study, we investigated the effects of nitrogen addition (N⁺) and understory removal (U^-) on soil respiration and its components in a Pinus sylvestris var.mongolica plantation in Horqin Sandy Land during the growing season of 2018. The results showed that soil microbial respiration was the main contributor to soil respiration in this ecosystem, with a contribution rate of 85.53%. The contribution rate of root respiration was smaller, being 14.47%. During the growing season, the average values of soil respiration were 1.59 (CK), 1.73 (N⁺), 1.66 (U^-), and 1.89 (N⁺U^-) μmol C·m^-2·s^-1. Nitrogen addition had a positive effect on soil respiration during the early and late stages of the growing season, while understory removal showed a positive effect during vigorous growth season. Under the effects of nitrogen addition+vegetation removal, soil respiration rate increased significantly, indicating that nitrogen addition+vegetation removal had synergistic effects on soil respiration. Soil microbial respiration rates were 1.36 (CK), 1.45 (N⁺), 1.44 (U^-) and 1.52 (N⁺U^-) μmol C·m^-2·s^-1, without variation among different treatments. Both nitrogen addition and understory removal reduced the temperature sensitivity of soil microbial respiration. The R₁₀ of root respiration changed more greatly than that of the microbial respiration under each treatment, indicating that root respiration responded more sensitive to each treatment than soil microbial respiration. Under the influence of soil water content, soil respiration and soil temperature showed different changes, indicating that soil water content is the limiting factor for soil respiration in Pinus sylvestris var. mongolica plantation in Horqin Sandy Land. Our results help understand carbon cycling and its modeling of sandy Pinus sylvestrisvar. mongolica plantation.

Key words: growth characteristics, Porites lutea, sea surface temperature, east sea area of Hainan Island.