Biochar (BC) is a material that finds many applications in agriculture and environmental activities. The aim of the study was to define the influence of biochar produced from various organic materials: mellow compost (MC), stabilized municipal sewage sludge (MSS), pine sawdust (PS), sycamore sawdust (SS) and oak leaves (OL) on soil enzyme activity, as well as its relations with carbon and nitrogen content. After a 60-day incubation of soil and BC, the activity of dehydrogenases (DEH), catalase (CAT), alkaline (AlP) and acid (AcP) phosphatases was investigated. The basic parameters of soil were also determined: TOC, TN, DOM, pH in H₂O, available phosphorus (AP). The highest AP content was obtained in the S + MSS, S + OL and S + MC variants. Enzyme activity was highest in soil with MSS BC, regardless of incubation time. After 60 days, the activity of soil enzymes was inhibited. The obtained results indicate that the response of enzymatic activity to biochar depends on the feedstock material and the incubation time. When using BC as an exogenous matter, it is necessary to determine the TOC/TN ratio. For the very wide range of this parameter, supplemental nitrogen fertilization or mixtures of different biochars should be applied.

Synchronous effects of biochar on heavy metals stress, microbial activity and nodulation process in the soil are rarely addressed. This work studied the effects, under greenhouse conditions, of selected heavy metals Cd²⁺, Pb²⁺ and Ni²⁺ on soybean plants grown in two different soils amended with biochar, and studied their effect on the microbial and enzymatic activity. As a result of the interference between heavy metals and biochar, biochar overcame heavy metal problems and maintained a microbial population of major groups (bacteria–fungi). There was an increase in the degree of resistance (RS) of the major microbial groups to heavy metals when biochar was added to the soil under study. Numbers of bacterial nodules significantly increased, particularly by using the higher rate of biochar compared to the control, either by adding biochar alone or by mixing it with the selected heavy metals. The arginase activity was increased by 25.5% and 37.1% in clay and sandy soil, respectively, compared to the control. For urease (UR), the activity was increased by 105% and 83.8% in clay and sandy soil, respectively, compared to the control. As a result, considerations of using biochar as a soil amendment should be first priority.