In this study

the isotope spectrometer was used to observe the nighttime CO

2

concentration and δ

13

C during the vigorous growth period of Platycladus orientalis plantation in Beijing mountainous area. The difference of nocturnal respiration δ

13

C 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 δ

13

C 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 δ

13

C 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 δ

13

C 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 δ

13

C 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 δ

13

C 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 δ

13

C was 2.06‰~7.03‰ higher than the aboveground δ

13

C. Stepwise regression analysis showed that the nocturnal respiration δ

13

C 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 δ

13

C 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 δ

13

C. Through the study of night respiration δ

13

C

it was possible to evaluate more comprehensively the carbon balance of for

est ecosystems in Beijing mountainous area

and provide a scientific basis for regional forest ecosystem management.