水分对梯度法估算土壤表面CO_2扩散通量的影响

Effects of Soil Water on the Estimation of Soil Surface CO_2 Diffusion Efflux by the Gradient Method

土壤水分是土壤CO2产生和传输的主要控制因子,对其进行研究可以更好的理解土壤水分对土壤碳通量的影响。梯度法是估算土壤CO2扩散通量的一种常见方法,该方法利用菲克第一定律计算土壤CO2扩散通量,其关键参数是气体扩散系数(DP)和CO2浓度梯度。本实验用放置在恒温室内的土柱研究土壤水分变化对梯度方法估算土壤表面CO2扩散通量的影响,利用Millington-Quirk模型对气体扩散率进行模拟。结果表明,土壤CO2浓度随着土壤深度的增加而增加,15 cm处的土壤CO2浓度平均值(1991.8μmol mol-1)明显大于5 cm处土壤CO2浓度平均值(1495.1μmol mol-1),土壤水分与两层土壤CO2浓度变化趋势具有一致性。在土壤温度保持相对稳定的情况下,CO2通量随体积含水量的变化而变化,在干燥或含水量很高的条件下土壤CO2通量较低,接近田间持水量时CO2通量最大;在大多数情况下,梯度法和动态气室法测得的土壤碳通量具有较好的一致性。

Soil moisture is the main factor controlling the production and transmission of soil CO2. This study aimed to investigate the effect of soil water content on CO2 efflux in soil using the gradient method. Both gas diffusion coefficient(DP) and CO2 concentration in the soil depth profile are key parameters in Fick' Law to calculate soil CO2 efflux. A soil column study was used to investigate the effect of vertical water distribution in soil on estimating soil CO2 efflux by the gradient method. Millington-Quirk model was used to estimate the relative gas diffusion coefficient(Dp). The estimated soil CO2 efflux was then compared to that measured by the chamber method. The results showed that the soil CO2 concentration increased with the increase of soil depth and moisture. The average value of soil CO2 concentration at 15cm(1991.8 mol mol-1) was significantly higher than in 5 cm(1495.1 mol mol-1), but soil moisture and the soil CO2 concentration variation trend is consistent. When the temperature was relatively stable, CO2 efflux changed with the volumetric water content as follows: the soil CO2 flux is low under dry or high soil water content. Soil CO2 flux reached maximum when the soil water content was close to field capacity. In most cases, the estimated soil CO2 efflux using the gradient method reasonably approximates the efflux obtained using the chamber method.