The samples were wet-sieved through a 63-mm sieve with distilled water, dried at 105°C, and then homogenized with an agate mortar. Triplicate of 1 g of sieved, dried and homogenized samples, with mesh size less than 63 mm (de Groot et al., 1982) was digested using aqua regia consisting of 750 ml/l HCl and 250 ml/l of HNO3 at 115°C for 24 h.