Investigating the synergistic potential of Si and biochar to immobilize Ni in a Ni-contaminated calcareous soil after Zea mays L. cultivation

Abstract. In Iran, a significant percentage of agricultural soils are contaminated with a range of potentially toxic elements (PTEs), including Ni, which need to be remediated to prevent their entry into the food chain. Silicon (Si) is a beneficial plant element that has been shown to mitigate the effects of PTEs on crops. Biochar is a soil amendment that sequesters soil carbon and that can immobilize PTEs and enhance crop growth in soils. No previous studies have examined the potentially synergistic effect of Si and biochar on the Ni concentration in soil chemical fractions and the immobilization thereof. Therefore, the aim of this study was to examine the interactive effects of Si and biochar with respect to reducing Ni bioavailability and its corresponding uptake in corn (Zea Mays) in a calcareous soil. A 90 d factorial greenhouse study with corn was conducted. Si application levels were 0 (S0), 250 (S1), and 500 (S2) mg Si kg−1 soil, and biochar treatments (3wt %) including rice husk (RH) and sheep manure (SM) biochars produced at 300 and 500 °C (SM300, SM500, RH300, and RH500) were utilized. At harvest, the Ni concentration in corn shoots, the Ni content in soil chemical fractions, and the release kinetics of DPTA (diethylenetriaminepentaacetic acid)-extractable Ni were determined. Simultaneous utilization of Si and SM biochars led to a synergistic reduction (15 %–36 %) in the Ni content in the soluble and exchangeable fractions compared with the application of Si (5 %–9 %) and SM (5 %–7 %) biochars separately. The application of Si and biochars also decreased the DPTA-extractable Ni and Ni content in corn shoots (by up to 57 %), with the combined application of SM500 + S2 being the most effective. These effects were attributed to the transfer of Ni in soil from more bioavailable fractions to more stable iron-oxide-bound fractions, related to soil pH increase. SM500 was likely the most effective biochar due to its higher alkalinity and lower acidic functional group content which enhanced Ni sorption reactions with Si. The study demonstrates the synergistic potential of Si and SM biochar for immobilizing Ni in contaminated calcareous soils.