Speciation of nickel and enzyme activities in fluvo-aquic soil under organic amendments treatment

To investigate the efficacy of humic acid and cow dung for nickel (Ni) immobilisation and detoxification in fluvo-aquic soil, the Ni-polluted soil was mixed with amendments of humic acid and cow dung. The Ni speciation (modified BCR sequential extraction method) and enzyme activities were scrutinised. As the humic acid concentration increased (0 to 12 g kg -1), the amount of Ni in the acetic acid-soluble fraction (the bioavailable fraction with high environmental risk) first decreased and then increased. In contrast, the amount of Ni in this fraction continued to decrease as the cow dung concentration increased (P < 0.05). The Ni in the acetic acid-soluble and residual fractions transformed to the oxidisable fraction, leading to lower environmental risk. The fluorescein diacetate hydrolysis and alkaline phosphatase activity were negatively correlated with the amount of Ni in the acetic acid-soluble fraction (r = -0.695 and -0.773, respectively, P < 0.01), indicating that Ni in the acetic acid-soluble fraction was toxic and led to enzyme activity inhibition, and the amendments could reduce subsequent damage. When humic acid concentration ranged from 5.01 to 6.47 g kg -1, the acetic acid-soluble Ni level reached the minimum, while the fluorescein diacetate hydrolysis and the alkaline phosphatase activity reached their maxima. In conclusion, humic acid and cow dung are effective amendments for Ni immobilisation and reduce devastation of soil enzymes. The humic acid concentration should be between 5.01 and 6.47 g kg -1 to achieve the optimum efficacy.