Organ specific chrysanthemum biochar for circular waste reuse and sustainable plant production

Abstract Biochar (BC), a low-cost, carbon-rich material produced from a wide variety of biomass sources, is gaining increasing attention in horticultural practice. There is little research on the use of chrysanthemum residues in BC production, especially in the circular production of subsequent plant generations. This study aimed to compare the usefulness of BC derived from various organs of chrysanthemum waste during the acclimatization stage of new chrysanthemum plantlets. Roots, shoots, and flowers of glasshouse-grown chrysanthemums were pyrolyzed at 400 °C for 60 min. The obtained BCs were mixed with peat and perlite (2:1) at concentrations of 4%, 8%, and 12%. In vitro-derived plantlets were grown in the so-prepared substrate for 30 days and then subjected to biometric, biochemical, and physiological analyses. The composition of microorganisms in the growing substrates was compared, and the impact of the tested BCs on substrate utilization patterns of rhizosphere microbiome was evaluated. Both the concentration and source of biochar were found to influence its properties. The longest shoots, the highest number of leaves, and the highest chlorophyll content were obtained in the control, while high BC concentrations (12%, regardless of type) inhibited plant elongation. The application of BC, particularly from shoots or roots, at 4 or 8%, increased the fresh weight of shoots and roots, as well as leaf surface area. Biochar generally did not affect root architecture, but 12% flower‑derived BC reduced root development. Biochar stimulated the synthesis of bioactive compounds and the activity of antioxidant enzymes, with the best results achieved when BC from flowers or shoots was applied at lower concentrations. Most BC combinations did not reduce plant photosynthetic parameters compared with the control, and some (e.g., 8% root BC) improved photosynthesis efficiency. Increasing carbon supplementation to plants reduced the functional diversity of root-associated bacterial isolates; however, the highest BC dose (12%) increased the numbers of bacteria and Actinobacteria in the growing substrate, without affecting filamentous fungi. Consequently, the use of shoot and flower BC (especially at 12%) increased the activity of most of the tested soil enzymes, while BC from roots showed a weaker or inhibitory effect. The obtained results highlight the utility of BC in chrysanthemum production.