The Effects of Wetting-drying Cycles on Degradation of Sands and Gravels Used for Soil Mulching in Ningxia Province

Abstract

【Objective】 Sand and gravel mulching is a technology widely used for crop production in the arid regions in northwestern China. Continuous weathering could result in their degradation; the purpose of this paper is to study the effects of wetting-drying cycles on degradation of gravels and sand used for soil mulching in the arid belt in central Ningxia province. 【Method】 We took lime-green slate - a commonly used gravel and sand for soil mulching - as an example, mimicking wetting-drying cycles in laboratory by spraying. Degradation of the gravels were measured and calculated using SEM and PCAS technologies. 【Result】 ①Large gravels degraded faster than small ones at the early stage, and wetting-drying increased pore connectivity of the gravel mulch. A great number of debris and flocculent minerals were found. ②At the end of wetting-drying cycles, gravel size was proportional to the number of pores and was inversely proportional to the average pore area of the gravels. Wetting-drying increased the uniformity coefficient of the pores by up to 129.4%. The ability of the gravels to adsorb water decreased as the gravel size increased. The crushed sand and gravel in the size range of 0.5~2 mm had the highest water adsorption under water application of 250 and 125 mL, increasing water adsorption by 75.05% and 56.60% respectively, compared to the initial water content. ③At the end of the wetting-drying cycles, the mass loss rate and cumulative mass loss in the sand and gravel were inversely proportional to particle size. Among them, sand and gravel with size in the range of 0.5~2 mm were more sensitive to temperature. Their cumulative mass loss under water application of 250 mL was 27.66% higher than that under water application of 125 mL. 【Conclusion】 Wetting-drying cycles affect degradation of small and compacted sand and gravel more than the large ones. Our results have important implications for improving management of soils mulched by sand and gravel in the arid regions in northwestern China.

Keywords

• drying-wetting cycle
• sem
• porosity
• water absorption
• mass loss rate
• pcas