| In this paper, open path eddy covariance system and automated observing systems of gradient weather were used to determine the characteristics of CO2 flux and its response mechanism to environmental factors in a 13-year-old Chinese fir plantation ecosystem in Huitong of 2008. At the same time, the carbon absorption capacity of system was estimated. The study contributed to the research of carbon cycle of Chinese fir plantation and the understanding of carbon sink of forests in China, especially the southern forest plantations. The results showed that:1. CO2 flux had obvious diurnal and yearly variations. First of all, the average diurnal variation was basically the same in each month and the value of CO2 flux was negative in the daytime as well as positive in other times, which showed that the system was a carbon sink in daytime and a carbon source in nighttime. Secondly, diurnal cumulative value of CO2 flux floated between -19.2 and 10.7 mg·m-2·d-1. Thirdly, the carbon sink capacity of system in growing season was significantly larger than non-growing season. The maximum of negative value was about -0.5 to -0.7 mg·m-2·s-1 in vigorously growing season and between -0.3 and -0.5 mg·m-2·s-1 in the rest months. Significantly, the forest absorbed more CO2 in sunny days. The maximum and average negative value were -0.6 mg·m-2·s-1 and -0.3 mg·m-2·s-1 in sunny days at daytime while the values were -0.4 mg·m-2·s-1 and -0.1 mg·m-2·s-1 in cloudy days.2. The total amount of NEE, RE, GEE were respectively -313.4,958.7,-1272.0 g C·m-2·a-1 of Huitong Chinese fir plantation ecosystem in 2008. The seasonal variation of GEE was basically consistent with NEE. The monthly GEE were all negative, and the maximum is June (-157.7 g C·m-2·month-1) while the minimum is January (-48.1 g C·m-2·month-1). Differently, the monthly RE varied as a "single peak" curve in the year and came to minimum in January (38.5 g C·m-2·month-1) whereas reached its maximum in July (117.8 g C·m-2·month-1). On day scales, both NEE and RE had a strong linear relationship with GEE whereas all the correlation of NEE, RE and GEE were linear on month scale.3.CO2 flux was mainly affected by photosynthetic active radiation (PAR) in the daytime and fitted the Michaelis-Menten model well. With the increase of PAR, the carbon sink was increased until the value of PAR reached to 1200μmol·m-2·s-1. The response of CO2 flux to PAR was different at different levels of air temperature and water vapor pressure deficit (VPD). Besides, the correlation of day-time CO2 flux and PAR was more obvious than common days. Evenmore, it would conductive to CO2 flux saturated when the temperature was about 25℃or the VPD neared to 1.5 kPa. In addition, the diurnal CO2 flux and the air temperature or VPD fitted a quadratic curve model.4. At nighttime, especially in sunny days, the respiration of ecosystem had a strong correlation with soil temperature and soil water content (SWC). Night-time CO2 flux showed an exponential relationship with soil temperature at 5 cm depth, and the flux increased exponentially with the increasing of soil temperature. The relationship was more obvious with the average at different soil temperature ranges. In the month scale, the respiration of ecosystem was related to SWC as a quadratic parabola opening downward and the correlation coefficient was higher at different SWC gradient.5.NEE, RE and GEE were effected by air temperature, VPD and SWC obviously on month scale. For one thing, both GEE and RE with air temperature showed significant linear correlation while NEE and air temperature related as a quadratic curve. For another thing, the relationships of monthly NEE, RE, GEE and VPD were quadratic while the correlation of the three ones and SWC showed as a quadratic parabola.
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