Nowadays, antibiotic-polluted soils have raised concerns regarding the health of soil ecosystems. It has been suggested that biochar can be used as an amendment to improve soil health conditions in polluted sites. The present research aimed to investigate the influence of walnut shell biochar and methanol-activated walnut shell biochar on tetracycline concentration in the presence of copper and the assessment of soil biological parameters (microbial biomass, urease and dehydrogenase enzymes). Therefore, a factorial experiment was conducted as a completely randomized design. The factors included two biochar levels without or with copper (0% biochar (control soil), soil amended with 5% walnut shell biochar and soil amended with 5% methanol-activated walnut shell biochar) and two tetracycline levels (0 and 400 mg/kg soil), which were measured at 10-, 30- and 50-day intervals. The results revealed that biochar and methanol-activated biochar efficiently decreased tetracycline concentration up to 46.5% and 80.4%, respectively. The application of biochar decreased tetracycline concentration through the formation of a stable complex with tetracycline and the reduction of its extractability as well as biodegradation. The decrease in tetracycline extractability was also facilitated by the presence of copper. Neither biochar had significant effects (P > 0.05) on microbial biomass. They also reduced enzymatic activities, probably due to a high application rate. However, they weakened the negative effects of tetracycline on microbial activities. It was concluded that the application of biochar in tetracycline-polluted soils could help expedite the decrease of residual tetracycline concentration, which, in turn, could improve soil biological parameters and soil health.

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