Effects of sewage sludge amendment and wetting-drying cycles of wastewater irrigation on structural improvement of clay soil

Improving the structure of clay soils with sewage sludge to achieve better aeration and infiltration is one of the practical ways to maintain sustainable use of agricultural land. Furthermore, wetting-drying processes on the surface caused by changing irrigation intervals can supplement benefits to structural improvement under wastewater irrigation conditions. Therefore, a laboratory model study was carried out using three sewage sludge doses (50, 100, and 150 Mg ha(-1)), two types of water (freshwater, recycled wastewater) and three irrigation intervals (4, 8, and 12 days; as wetting-drying cycles) in clay soil containers. The highest dose caused a lower pH (6.92) and particle density (2.612), while organic matter (4.33%), CaCO3 (2.21%), electrical conductivity (2.476 dS m(-1)), wet aggregate stability (74.3%), field capacity (0.395 m(3) m(-3)), stable and mean infiltration rates (10.4 and 13.9 mm h(-1)) increased. Wastewater increased electrical conductivity, CaCO3 content and bulk density, and decreased porosity and macro-porosity. The highest irrigation interval decreased particle density, and increased electrical conductivity and organic matter content. The moderate irrigation interval was the most effective option in delivering higher mean infiltration rates. It can be concluded that adding sewage sludge to clay soil, especially at a dose of 150 Mg ha(-1) is an appropriate option to improve infiltration with better wet aggregate stability. Wastewater irrigation in clay soil can be a practical way both to dispose waste and to save freshwater. Long irrigation intervals can maintain organic matter for a longer time.