Chemical fractions and bioavailability of nickel in a Ni-treated calcareous soil amended with plant residue biochars

Recently, the use of biochars for stabilization of soil heavy metals has been expanded due to their adsorption characteristics, low cost and carbon storage potential. A factorial experiment was performed to investigate the effects of two plant residue biochars (licorice root pulp and rice husk biochar each applied at 2.5% (w/w)) produced at two temperatures (350 and 550 degrees C), and three Ni application rates (0, 150 and 300 mg Ni kg(-1)) on bioavailability and chemical fractions of Ni in a calcareous soil after spinach cultivation. Application of all the biochars significantly reduced Ni bioavailability factor (5-15%) and spinach Ni concentration (54-77%) in Ni-treated soil. The biochars produced at 550 degrees C were more effective at reducing Ni mobility and Ni uptake by spinach than those produced at 350 degrees C, attributed to higher CaCO3 and lower acidic functional group content, which resulted in greater enhancement of soil pH. When comparing the biochars produced at the same temperature, the rice husk biochars were the most effective in reducing Ni bioavailability, likely due to their lower acidic functional group content and higher nano-silica content which resulted in higher soil pH values and potentially promoted the formation of Ni-silicates and hydroxides.