Effects Of Soil Water On Estimating Soil Surface CO₂ Efflux By Gradient Method And Gas Diffusivity

In order to study the effects of soil water on estimating soil surface CO₂ efflux by gradient method and its corresponding soil gas diffusivity, selecting Sand soil and Loessial in Liudaogou, Shenmu County, Yulin City, Shanxi Province, Black Loessial soil in Changwu County, Xianyang City, Shanxi Province, and Cumulic cinnamon soil in Yanglin, Xian City,Shanxi Province as soil samples, respectively. Based on the soil column experiments, gradient method combined the LI-8100 chamber studied the effects of different soil textures on the surface CO₂ efflux by gradient method. On this basis, analyzed the differences in selection of gas diffusion model in different soil texture, put forward to a suitable gas diffusion model and parameters used to estimated the soil CO₂ efflux by gradient method in the Loess Plateau.Mainly on repacked soil and undisturbed soil column experiments, analysis of the effects of the soil moisture as a single factor on soil CO₂ efflux; SAPHIR model was selected to predict relative gas diffusion coefficient and compared with measured values; Gradient method also used to estimate the soil CO₂ efflux in the field, and measured the soil volumetric water content in soil profile, soil temperature, soil CO₂ concentration, air temperature, air CO₂ concentration and the soil surface CO₂ efflux. At the same time,application of the gas chamber measured soil surface CO₂ efflux was compared with the gradient method. The results show that:（1） At the soil column experiments, the estimated soil CO₂ efflux using the gradient method has a agreement with the efflux obtained using the soil chamber method in different soil texture. When there is no added water or a small pulse of water, the gradient method can be instead of the chamber method.（2） Laboratory temperature remained relatively stable, the soil profile CO₂ production and transmission is only affected by soil water content, the CO₂ concentration and soil CO₂ effluxes with increasing soil water content increases, Changes in repacked soil and undisturbed soil CO₂ concentrations and CO₂ effluxes has a consistent trend.（3） Soil texture has a great influence on the selection and set the parameters of the gas diffusion model, this paper chosen SAPHIR model to simulate soil gas diffusivity, the model parameters vary depending on soil texture, p in SAPHIR model was varied with silt contents.For Sand soil and Loessial soil, which have low levels of silt content, SAPHIR model parameters p = 0.6, while in the Black Loessial soil and Cumulic cinnamon soil, high content of the silt（>60%）, p = 0.95. However, w is controlled by soil total porosity and w should be calculated through data fitting.（4） Undisturbed soil has a same gas diffusion model with repacked soil, but with different parameters w. Repacked soil with Loessial soil and Cumulic cinnamon soil, w respectively 3.7 and 3.6, However, undisturbed soil w value are 6.5 and 7 respectively.（5） Soil CO₂ concentration increased with soil temperature and soil water content increases in field test. However, after the volumetric soil water content was greater than field capacity, it has a strong inhibition with soil CO₂ concentration. Seasonal variation of soil CO₂ concentration reached the highest value in July, and the diurnal variation of soil CO₂ concentration reached a maximum between 13:00 to 15:00.（6） After the suitable diffusion coefficient was chose, the gradient method can be used successfully to estimate soil CO₂ efflux in field. Seasonal variation and diurnal variation of soil CO₂ efflux has a same changes trend with soil CO₂ concentration.（7） Air temperature and the soil temperature at different depths affected significantly on soil CO₂ efflux. However, changes of soil water content will affect the sensitivity of the soil temperature of CO₂ efflux. In dry conditions, increasing water content will increase the value of Q10, but it will limit the soil respiration in response to temperature changes if the soil water reached a certain limit value, which broken the relationship between soil water content and CO₂ efflux.