ABSTRACT

Gas exchange between soil and atmosphere governs the growth of plants and global circulation of the gaseous component of air. Enhancement of gas exchange by wind has been reported in media with large pores. In clayey paddy fields, large aggregates (clods) and inter-aggregate pores are prone to be formed by tillage. However, the effect of wind on gas transport has not been considered in such fields. This study quantified the wind-induced gas movement through the paddy soil layer composed of large clods. The applicability of the analysis using the gas diffusion equation was also verified regarding the mean clod size and wind condition. Undisturbed soil columns were taken from the ‘poorly tilled’ and ‘fairly tilled’ paddy field under the cultivation of soybean. The mean diameters of the clods were 3.4 and 1.9 cm for the ‘poorly’ and ‘fairly’ tilled columns, respectively. A repacked silica sand column was also prepared as a reference. Diffusion tests were conducted for the columns in the field under natural wind and in the laboratory under calm conditions, with CO₂ as a purge gas and atmospheric oxygen as a tracer gas. The gas transfer efficiency was evaluated by $\mathrm{R}{\mathrm{t}}_{50}$ and $\mathrm{R}{\mathrm{t}}_{90}$, which is the ratio of the time until the oxygen concentration reaches 50% and 90% of that in the atmosphere in the field to that in the laboratory, respectively. The results showed that gas transfer through the paddy soil layers, comprised a few centimeters of clods, under natural wind was up to 11 times quicker than under the calm conditions. The wind-induced gas exchange remained at maximum under the wind speed of more than 1.2 m s⁻¹ at 20 cm above the soil surface in the poorly tilled soil. In contrast, in the fairly tilled soil, the effect of wind gradually increased with wind speed and finally became close to that in the poorly tilled soil at the wind speed around 2.0 m s⁻¹. The applicability of the gas diffusion equation deteriorated as the wind speed increased from 1.2 to 2.6 m s⁻¹ or the mean aggregate size increased from 1.9 to 3.4 cm.

摘要

土壤和大气之间的气体交换控制着植物的生长和空气中气体成分的全球循环。据报道，在具有大孔隙的介质中，风能增强气体交换。在粘土质的稻田中，耕作容易形成大的团聚体（土块）和团聚体间的孔隙。然而，在这些领域尚未考虑风对气体输送的影响。这项研究通过由大块土块组成的水稻土层量化了风引起的气体运动。使用气体扩散方程的分析的适用性也得到了关于平均土块大小和风况的验证。未受干扰的土柱取自种植大豆的“耕作不良”和“耕作良好”的稻田。土块的平均直径为 3.4 和 1。“较差”和“较好”耕作的柱子分别为 9 厘米。还准备了重新填充的硅砂柱作为参考。在自然风下的野外和平静条件下的实验室中对柱子进行了扩散测试，CO₂作为吹扫气体，大气氧作为示踪气体。气体传输效率的评价是$\mathrm{电阻}{\mathrm{吨}}_{50}$ 和 $\mathrm{电阻}{\mathrm{吨}}_{90}$, 即现场氧气浓度分别达到实验室大气氧气浓度的 50% 和 90% 所需时间的比值。结果表明，在自然风下，气体通过水稻土层（包括几厘米的土块）的传输速度比平静条件下快 11 倍。在耕作不良的土壤中，在土壤表面上方 20 cm 处风速超过 1.2 ms ^-1时，风引起的气体交换保持最大。相比之下，在充分耕作的土壤中，风的影响随着风速逐渐增强，最终在风速为2.0 ms ^-1左右时接近未耕作的土壤中的作用. 随着风速从 1.2 增加到 2.6 ms ^-1或平均聚集体尺寸从 1.9 增加到 3.4 cm ，气体扩散方程的适用性变差。