Briefly, to a sample of wet soil equivalent to 1 g of oven-dried (105°C) soil was added 1.5 mL of 1 M tris–HCl buffer of pH 7.5, followed by 2 ml of 0.5% INT solution (Sigma product No. I 8377), and the suspension was kept at 40°C for 1 h. Then 10 ml of extractant (methanol or DMF-ethanol) was added and the sample was mixed using a vortex mixer and then left in the dark for 10 min. Finally, the solids were filtered out (paper Whatman No. 40) and the absorbance of the filtrate was determined at 490 nm.