For total organic carbon (TOC) analysis, the soil samples (ca.
1.0 g) were weighted into ceramic crucibles, and then added with
10 mL 5% (v/v) HCl. The crucibles were then heated in a 60 C water
bath for 1 h to remove the carbonate minerals (He et al., 2009). The
residue was washed by triple-distilled water until the pH of the
solution reached neutral. The TOC contents of the residues were
analyzed with a Leco C230 elemental analyzer after being oven
dried at 60 C for 24 h.
peaks based on calibration curves established using
standard solutions.