Carbon stability in mining-impacted soils through calcium-doped biochars

Biochar application is a promising strategy to enhance soil carbon (C) sequestration, and chemical modification with alkaline cations, like calcium (Ca), may further improve its stability and functionality, particularly in contaminated soils. The mechanisms by which Ca-doped biochars influence C stabilization and metal immobilization in mining-impacted soils remain insufficiently understood, and we evaluate the effects of Ca-modified biochars on C sequestration and stabilization mechanisms in a Zn- and Cd-contaminated soil from a mining area. Biochars produced from urban pruning waste and modified with CaCO₃, CaSO₄, and CaCl₂ were characterized by proximate analysis. CaSO₄- and CaCl₂-modified biochars exhibited higher ash contents and lower fixed carbon compared to the control (p < 0.01), while CaCO₃-modified biochars had intermediate values. Water content also differed among treatments (p < 0.05), whereas volatile matter did not show differences. The observed fixed C levels reflected the use of lignocellulosic urban biomass and the incorporation of Ca-based modifiers, which increased the mineral fraction and likely altered thermal decomposition during pyrolysis. Although these biochars exhibit lower fixed C contents than highly carbonized materials, their increased mineral contents and potential surface reactivities may enhance functional groups´ availabilities, which is relevant for metal immobilization. Ongoing soil incubation experiments will be carried out to evaluate total and oxidizable soil organic C to elucidate the role of Ca doping on biochar properties and soil C dynamics in mining-impacted soils.