Gas transport enhancement under windy condition in poorly tilled seedbed of clayey rotational paddy field

Abstract

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 and , 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.