<p>Globally speaking, the addition of biochar to the soil has, among other objectives, the aim of reducing the toxicity of metals by decreasing their mobility and bioavailability, which will ultimately result in slowing metal uptake by plants. With this idea, our study focuses on the effect of biochar promoting microbial growth rendering the heavy metals (HMs) unavailable to plants or microorganisms in contaminated site. In our pot experiment, we collected HMs contaminated soil from farmers’ field, usually irrigated with treated tannery effluent located in Kanpur city, India’s tannery hub. Soil was collected from the rhizosphere zone of mustard plant. Mustard was grown after rice cultivation to observe the residual impact of biochar application. The primary objective of the study was to compare the residual effect of unmodified parthenium biochar (UM-B) along with two types of modification of it, H₃PO₄-modified biochar (H₃PO₄-B) and FeCl₃-modified biochar (FeCl₃-B), in reducing HMs bioavailability as well as promoting soil biological properties at the graded application dose of 5, 7.5 and 10 t ha^− 1, with one control treatment consisting a total of 10 treatments in completely randomized design. Results revealed the residual impact of H₃PO₄-B at 10 t ha^− 1 increased the bacteria, actinomycetes and fungi population by ~ 2 times over control. Besides this, soil enzymatic activities were also enhanced. The dehydrogenase and urease activity were increased by 108% and 63% respectively, in H₃PO₄-B at 10 t ha^− 1 treatment over the control. Besides this, we also noted a revamp in microbial biomass carbon, phosphorus and nitrogen. The performance of UM-B at higher application dose was comparable to the modified biochars of lower dose. Thus, our study confirms a positive residual impact of biochar application on soil biological properties and nutrient availability of soil besides reducing DTPA-extractable HM content.</p>