中文摘要：

理论上,土壤呼吸通量的量值可以通过观测土壤呼吸CO2扩散速率（δc/δt）计算得到.但是为获得δc/δt,通常须允许土壤呼吸箱内CO2浓度升高,因此,如何估算外界大气CO2浓度条件下的δc/δt是土壤呼吸观测技术的关键,关系到观测结果的准确性.通常δc/δt的估算会受土壤表层大气CO2扩散梯度（即土壤呼吸箱内CO2扩散梯度和大气CO2浓度昼夜变化）的影响.目前,线性回归方法是土壤呼吸观测中估算δc/δt的基本方法.然而,常用的线性回归方法会低估δc/δt,而指数回归方法则可以准确地估算δc/δt.夜间δc/δt的变化与大气CO2浓度之间存在非常明显的负相关关系.夜间土壤表层大气CO2扩散梯度的减小导致线性回归方法明显低估δc/δt.δc/δt的昼夜变化过程存在明显的非对称性现象,而指数回归方法可以更好地描述δc/δt昼夜变化的非对称性响应.

英文摘要：

Aims Soil CO2 flux is driven primarily by the CO2 diffusion gradient across the soil surface. Ideally, the soil CO2 flux measurement should be made without affecting the diffusion gradient across the soil surface. With the closed chamber system, the soil CO2 diffusion rate （δc/δt） is required to estimate the soil CO2 flux. To obtain the δc/δt, the chamber CO2 concentration must be allowed to rise. Consequently, the δc/δt will be affected by the CO2 diffusion gradient across the soil surface because of the decreased CO2 diffusion gradient in the soil chamber. Additionally, the δc/δt will also be affected by the diurnal variation of the CO2 concentration across the soil surface. Our objective was to compare linear and exponential fitting methods to estimate δc/δt. Methods Currently, the δc/δt is commonly estimated using linear fitting regression. Instead of using the linear fitting method, an exponential fitting method is used to fit the time series of chamber CO2 concentration adopted in the LI-8100 automated soil CO2 flux system. Important findings The δc/δt estimated from the linear slopes was consistently underestimated as compared to that from exponential initial slopes. Nighttime δc/δt was significantly negatively correlated with soil surface CO2 concentration, suggesting that the decreased CO2 diffusion gradient across the soil surface strongly influences the δc/δt. For the closed-chamber method, linear curve fitting significantly underestimated the δc/δt rate during the nighttime. These results demonstrated the importance of estimating the δc/δt at ambient soil surface CO2 concentration. The response of the δc/δt to air temperature exhibited significant asymmetry characteristic, showing that it is a better way for exponential fitting to make long-term and continuous soil CO2 flux measurement to elucidate the magnitudes and processes of soil CO2 flux in the typical terrestrial ecosystem.