Abstract:In this study, the isotope spectrometer was used to observe the nighttime CO2 concentration and δ13C during the vigorous growth period of Platycladus orientalis plantation in Beijing mountainous area. The difference of nocturnal respiration δ13C of different components of the P. orientalis ecosystem was analyzed, and its response to environmental factors was explored. The results showed that when using the Keeling plot method to fit the δ13C released by the nocturnal respiration in the P. orientalis plantation from July to September, the fitting accuracy improved from 0.43 to 0.82 and the error decreased from 0.54~0.99 to 0.50~0.82 after the atmospheric stability was used as the screening condition. The variation range of δ13C was (-28.76±0.51) ‰~(-25.18±0.59) ‰, which showed a trend of increasing first and then decreasing. The above-ground variation range of the aboveground arborvitae branch δ13C was (-33.16±1.08) ‰~(-26.82±0.18) ‰, and showed an increasing trend, reaching its maximum at the end of July, and tended to be stable in September. The underground variation of δ13C in the underground soil was (-28.55±0.17) ‰~(-21.39±0.37) ‰, and showed an increasing trend, reaching the maximum in July, as well as the variation was the largest, and the change was stable in September. The relationship between δ13C in each month and time was quadratic function. During the vigorous growth period, the night soil respiration flux accounted for 54.77% of the ecosystem respiratory flux, and the underground δ13C was 2.06‰~7.03‰ higher than the aboveground δ13C. Stepwise regression analysis showed that the nocturnal respiration δ13C was significantly affected by atmospheric humidity. The contribution rate of the atmospheric humidity to the variation of carbon isotope value of each component was more than 60%. The underground δ13C was not affected by the soil temperature. Except for atmospheric humidity, the atmospheric temperature and soil temperature and humidity had relatively balanced effects on the aboveground δ13C. Through the study of night respiration δ13C, it was possible to evaluate more comprehensively the carbon balance of forest ecosystems in Beijing mountainous area, and provide a scientific basis for regional forest ecosystem management.