The soil suspensions were shaken for 1 hr at (25 _ 2)°C in the constant-temperature shaker and then the solutions were separated by centrifugation at 5000 r/min for 10 min (Xu et al., 2005).
The soil suspensionswere shaken for 1 hr at (25 _ 2)°C in the constant-temperatureshaker and then the solutions were separatedby centrifugation at 5000 r/min for 10 min (Xu et al.,2005).