| Soil surface CO2 flux is an important part of the terrestrial carbon cycle and has a significant impact on climate change. Numerous researchers focused on the study of soil surface CO2 flux. The application of concentration gradient method is to estimate the soil surface CO2 flux by measuring the CO2 concentration gradient and gas diffusion coefficient. "Surface flux approximation method" and "Linear flux extrapolation method" were widely used to estimate soil surface CO2 flux. Soil CO2 production rates were assumed to be zero and constant value in "Surface flux approximation method" and "Linear flux extrapolation method" respectively. When these assumptions were not established, the calculation of soil surface CO2 flux will not be accurate. In this study, we showed that the gas diffusion coefficient model can lead to inaccurate CO2 flux. We estimated the soil surface CO2 fluxes in cropland (located in Shangzhuang experimental station of China Agricultural University) and grassland (located in Guyuan County, Hebei Province) respectively. The main conclusions are as follows.First, we designed a gas diffusion device to measure soil gas diffusion coefficient. The results showed that the value of leakage error was 6×10-5 (Dleak/D0) and this value was far less than measured values. So, the data measured by the gas diffusion device was reliable. The relative diffusion coefficient of quartz sand (2-3 mm) was 0.25, when air filled porosity was 0.42 cm3 cm’3. The relative diffusion coefficient of quartz sand (0.15-0.5 mm) was 0.38 and 0.40 cm3 cm-3, when air filled porosity was 0.20 and 0.21 cm3 cm-3 respectively. These results were close to the results of Currie and Hamamoto et al. The above indicated that this gas diffusion device work reliably.Second, we evaluated different gas diffusion coefficient models in three kind of soil. The results showed that, there was no universal model. Marshall (1959) model predicted dry quartz sand accurately. The results of WLR-Marshall (1959) model had good performance both on quartz sand and sandy soil. However, the best prediction results of sandy loam soil were observed on Buckingham (1904) model. WLR-Marshall (1959) model overestimated cumulative CO2 flux in 2.5 and 7.5 cm by 6.6 and 4.2 mol m-2 and underestimated cumulative CO2 flux in 12.5 cm by 1.7 mol m-2. All above reflected the necessity using measured gas diffusion coefficient to estimate CO2 flux.Third, "Surface flux approximation method" obtained the most accurate result in cropland. Diurnal amplitudes of soil surface CO2 flux estimated by gradient method were small than chamber method measurement. Moreover, gradient method delayed 1-2 hours than chamber method. Production rates of 2.5-7.5 cm soil layer had obvious seasonal variation, and production rates of 7.5-12.5 cm soil layer remained stable. The mean values of production rate in 2.5-7.5 cm and 7.5-12.5 cm soil layer was 1.5 and 0.7 μmol m-2 s-1 respectively (DOY 94-181). The cumulated CO2 emission of 2.5-7.5 cm,7.5-12.5 cm and>12.5 cm soil layer was 0.45、0.40 ' 0.21 gCO2 m-2 respectively and accounted for 42%、38% and 20% to the total surface CO2 emission.Fourth, we proposed "Exponential flux extrapolation method" to improve the accuracy of estimating soil surface CO2 flux. The CO2 production rate cannot be used as a constant value for situation when the depth of CO2 measurements was far away from each other. Exponential flux extrapolation method assumed that the production rate increases with the decrease of soil depth. The results in grassland showed that the "Exponential flux extrapolation method" yield soil surface CO2 flux closest to the result of chamber method. "Surface flux approximation method" and "Linear flux extrapolation method" underestimated 40% and 30% in grassland. |
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