[AGE28-P06] Measurement of apparent gas diffusion coefficient in poorly tilled seedbed under windy condition.
Keywords:clayey rotational paddy field, tillage, clod, movement of gas, pressure-pumping
In the cultivation of soybean in clayey paddy fields, huge clods with diameters of some centimeters are formed by tillage. In soil, movement of gas is usually assumed to be governed by molecular diffusion. On the other hand, in soil layer with huge clods, movement of gas can be enhanced by wind-induced pressure-pumping through large inter-clod pore space under windy condition. Although pressure-pumping is not a diffusive process, this phenomenon can be represented as a diffusive process by the Fick’s law with the apparent gas diffusion coefficient under windy condition (Da). However, little work has been done on Da in poorly tilled soil, especially in situ. This study aimed to compare Da and molecular gas diffusion coefficient (Dm) in poorly tilled seedbed and quantify how many times Da is larger than Dm.
The undisturbed poorly tilled soil was sampled on June 2018 just after tillage from the surface of the clayey field where conversion from flooded rice to soybean was conducted this year using the soil column with a diameter of 15.3 cm and a height of 9.7 cm. The porosity and mean clod diameter of the soil were 0.69 and 2.8 cm, respectively. As a control, Mikawa silica sand was packed into the column with a porosity of 0.49. The empty cap with a height of 2.9 cm were attached to the lower end of the column to conduct gas diffusion experiments. The oxygen concentration sensors were installed to the column and cap at 4.7 and 11.7 cm depth from the top of the column, respectively. The top of the column was closed, and then CO2 gas was injected from the cap to saturate the column with CO2. After opening the top of the column, O2 concentration was continuously measured. The gas diffusion coefficient (Da or Dm) was estimated by fitting the measured O2 concentration at 4.7 cm and 11.7 cm depth to the numerical solution of the diffusion equation. The diffusion tests under the wind for evaluating Da was conducted on November 2018 at the field where the soil specimen were taken. Three-dimensional wind velocity at 2 m above the column (U2) was measured by the ultrasonic anemometer. The diffusion tests without the effect of wind for evaluating Dm was conducted in the laboratory in the same manner.
Under the wind speed U2 of 1.0 and 1.4 m s-1, the apparent diffusion coefficients Da for the sand column were both 0.020 cm2 s-1, which was close to the measured Dm for the column: 0.019 cm2 s-1. Thus, the gas diffusion in the sand column was governed by the molecular diffusion alone. On the other hand, the Das for the poorly tilled soil column under U2 of 1.5, 2.3, and 4.1 m s-1 were 0.065, 0.075, and 0.097 cm2 s-1, respectively, while the Dm for the column was 0.036 cm2 s-1. This result shows the wind induced two to three-fold increase of apparent gas diffusion coefficient through the inter-clod pores of the poorly tilled soil, while the effect does not work on the sand layer.
The undisturbed poorly tilled soil was sampled on June 2018 just after tillage from the surface of the clayey field where conversion from flooded rice to soybean was conducted this year using the soil column with a diameter of 15.3 cm and a height of 9.7 cm. The porosity and mean clod diameter of the soil were 0.69 and 2.8 cm, respectively. As a control, Mikawa silica sand was packed into the column with a porosity of 0.49. The empty cap with a height of 2.9 cm were attached to the lower end of the column to conduct gas diffusion experiments. The oxygen concentration sensors were installed to the column and cap at 4.7 and 11.7 cm depth from the top of the column, respectively. The top of the column was closed, and then CO2 gas was injected from the cap to saturate the column with CO2. After opening the top of the column, O2 concentration was continuously measured. The gas diffusion coefficient (Da or Dm) was estimated by fitting the measured O2 concentration at 4.7 cm and 11.7 cm depth to the numerical solution of the diffusion equation. The diffusion tests under the wind for evaluating Da was conducted on November 2018 at the field where the soil specimen were taken. Three-dimensional wind velocity at 2 m above the column (U2) was measured by the ultrasonic anemometer. The diffusion tests without the effect of wind for evaluating Dm was conducted in the laboratory in the same manner.
Under the wind speed U2 of 1.0 and 1.4 m s-1, the apparent diffusion coefficients Da for the sand column were both 0.020 cm2 s-1, which was close to the measured Dm for the column: 0.019 cm2 s-1. Thus, the gas diffusion in the sand column was governed by the molecular diffusion alone. On the other hand, the Das for the poorly tilled soil column under U2 of 1.5, 2.3, and 4.1 m s-1 were 0.065, 0.075, and 0.097 cm2 s-1, respectively, while the Dm for the column was 0.036 cm2 s-1. This result shows the wind induced two to three-fold increase of apparent gas diffusion coefficient through the inter-clod pores of the poorly tilled soil, while the effect does not work on the sand layer.