线性和指数回归方法对土壤呼吸CO2扩散速率估算的影响

 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(əc/ət)计算得到。但是为获得əc/ət，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əc/ət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əc/ət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əc/ət的基本方法。然而，常用的线性回 归方法会低估əc/ət，而指数回归方法则可以准确地估算əc/ət。夜间əc/ət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əc/ət。əc/ət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əc/ət昼夜变化的非对称性响应。

EFFECTS OF LINEAR AND EXPONENTIAL FITTING ON THE INITIAL RATE OF CHANGE IN CO2 CONCENTRATION ACROSS THE SOIL SURFACE

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 (əc/ət) is required to estimate the soil CO₂ flux. To obtain the əc/ət, the chamber CO₂ concentration must be allowed to rise. Consequently, the əc/ət 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 əc/ət 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 əc/ət.Methods Currently, the əc/ət are 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-8 100 automated soil CO₂ flux system. Important findings The əc/ətestimated from the linear slopes was consistently underestimated as compared to that from exponential initial slopes. Nighttime əc/ət was significantly negatively correlated with soil surface CO₂ concentration, suggesting that the decreased CO₂ diffusion gradient across the soil surface strongly influences the əc/ət. For the closed-chamber method, linear curve fitting significantly underestimated the əc/ətrate during the nighttime. These results demonstrated the importance of estimating the əc/ət at ambient soil surface CO₂ concentration. The response of the əc/ət 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.