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

Abstract. Silicon (Si) is a beneficial plant element that has been shown to mitigate the effects of potentially toxic elements (PTEs) on crops. Biochar is a soil amendment that sequesters soil carbon, and that can immobilize PTEs and enhance crop growth in soils. Considering these beneficial properties, it remains to be investigated how the simultaneous utilization of Si and biochars affects PTEs immobilization in soils. Therefore, the aim of this study was to examine the interaction effects of Si levels and biochars, to alleviate soil Ni bioavailability and its corresponding uptake in corn (Zea Mays) in a calcareous soil. A 90-day factorial greenhouse study with corn was conducted. Si application levels were applied at 0 (S₀), 250 (S₁₎ and 500 (S₂) mg Si kg^-1 soil and biochar treatments (3 % wt.) included rice husk (RH) and sheep manure (SM) biochars produced at 300 °C and 500 °C (SM300, SM500, RH300 and RH500). At harvest, corn shoot Ni-concentrations, soil chemical Ni fractions and DPTA-release kinetics were determined. Simultaneous utilization of Si and SM biochars led to a synergistic reduction (15–36 %) of soluble and exchangeable soil Ni fractions compared to application of Si (5–9 %) and SM (5–7 %) biochars separately. The application of the Si and biochars also decreased DPTA-extractable Ni and corn Ni shoot concentration (by up to 57 %), with the combined application of SM500+S₂ being the most effective. These effects were attributed to the transformation of Ni from more bioavailable fractions to more stable iron oxide bound fractions, related to soil pH increase. The 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 Si and sheep manure biochar at immobilizing Ni in contaminated calcareous soils.