The CaCl2 procedure
was performed on S2 þ S1 and S3 þ S1for all concentrations. 30 mL of 0.01 M CaCl2
solutionwas mixed with 3 g of soil (1:10 solid solution ratio) in 50 mL polypropylene
centrifugation tubes placed on a shaker table (Heidolph promax 1020) at
50 oscillations min1 for 2 h, 20 C. After extraction, the tubes were centrifuged at
10,000 g for 30 min. The supernatant liquid was then filtered (0.22 mm), acidified to
2% with distilled HNO3 (15 M, suprapur 99.9%) and stored at 4 C before analysis.