Effect of Sewage Sludge and Its Biochar Application on Water Repellency and Structural Stability of Two Calcareous Soils under Corn Cultivation

Sewage sludge has positive effects on chemical and physical properties of soil. Biochar production from sewage sludge, while having positive effects on the soil, can lead to its effective management, especially in relation to the environment. The objective of this study was to investigate the effect of sewage sludge and its biochar on water repellency and structural stability of two calcareous soils with clay loam and loam texture under corn cultivation. Sewage sludge was mixed with soils in the rates of 10, 20 and 40 t ha–1 (S1, S2 and S3). The equivalent application rates of biochar were 7.3, 14.5 and 29 t ha–1 (B1, B2 and B3). At the end of corn growing period, soil water repellency was measured by the intrinsic sorptivity method. Water-dispersible clay (WDC) was also measured as an indicator of soil structural instability. Results showed that the water repellency index (RI), ethanol sorptivity (SE) and soil-water contact angle (β) were significantly higher in sewage sludge and biochar treatments than those of control. However, the WDC in the sewage sludge and biochar treatments was significantly lower than the control. The values of RI and β in the clay loam soil were significantly higher than the loam soil. In both sewage sludge and biochar treatments, an increment in the application rate increased the RI. In the clay loam soil, the highest RI and SE were observed in the sewage sludge and then in the biochar treatment; while in the loam soil, the RI had the trend of biochar > sewage sludge > control. Overall, the results indicated that the use of organic treatments such as sewage sludge and its biochar would increase sub-critical soil water repellency and structural stability.