Effects of Wetting-drying Cycles on Bulk Density and Saturated Hydraulic Conductivity of Soils

Abstract

【Objective】 Changes in density and hydraulic conductivity of soil not only modulate subsurface hydrological processes but also affect root growth and root uptake of water and nutrients from soil. Understanding the mechanisms underlying their changes is thus critical to mitigating flooding and improving bioavailability of soil water. In this paper we studied how wetting-drying cycles often seen in the field change bulk density and saturated hydraulic conductivity of the soil. 【Method】 Intact (mimicking zero-tillage) and disturbed (mimicking tillage) soil samples were taken from a cropped field. They were then subject to wetting-drying at different frequencies. During and at the end of the wetting-drying cycle experiment, we measured bulk density and saturated hydraulic conductivity of each soil sample. 【Result】 Bulk density of the intact and disturbed soil samples varied in the range of 1.41~1.89 g/cm3 and 1.47~1.85 g/cm3 respectively. The bulk density of intact soils after the first wetting-drying cycle increased significantly followed by slight fluctuations in other wetting-drying cycles, with an average coefficient of variation 8%. In contrast, bulk density of the disturbed soils increased with the increase in wetting-drying cycles, with an average coefficient of variation 5%. Overall, the wetting-drying cycles affected the bulk density of intact soils more significantly than the disturbed soils (P<0.05). The saturated hydraulic conductivity of the intact and disturbed soils varied in the range of 0.88~3.22 cm/h and 0.21~3.54 cm/h, respectively. The saturated hydraulic conductivity of the intact soils increased with the increase in wetting-drying cycles, with an average coefficient of variation 39%. In contrast, the hydraulic conductivity of the disturbed soil decreased first followed by a decline with the increase in wetting-drying cycles, with an average coefficient of variation 94%. Wetting-drying cycles affected the hydraulic conductivity of the disturbed soils more significantly than the intact soils (P<0.05). 【Conclusion】 Wetting-drying cycles resulted in cracks thereby reducing the bulk density and increasing saturated hydraulic conductivity of the soils. The effects, however, depend on tillage. Overall, wetting-drying cycles affect the bulk density of zero-tillage soil and saturated hydraulic conductivity of tilled soil more significantly.

Keywords

• wetting-drying cycles
• bulk density
• saturated hydraulic conductivity
• soil moisture characteristics