Microbe-biochar interaction in improving plant growth and water reuse in a green wall system

Given the increasing scarcity of potable water and the rising generation of domestic and industrial wastewater, the reuse of greywater to meet part of plant water requirements, along with its environmentally friendly approach, has gained increasing attention. The present study was conducted in 2023 to investigate the effects of microorganisms and biochar on the establishment and growth of Crassula capitella Thunb., as well as on greywater treatment. Cyanobacteria (C0, C0.4, and C0.8 g), mycorrhiza (M0, M5, and M10 g), and biochar (B0, B5, and B10 g) were evaluated under two irrigation regimes including municipal water (MW) and greywater (GW) within a green wall system in Mashhad, Iran. The results indicated that the combined treatments and irrigation water type had statistically significant effects on most of the measured traits. The C0.4-M10-B0 treatment increased chlorophyll content by more than 97% (from 4.93 to 9.74 µg g⁻¹ FW), while the C0-M10-B5 treatment increased RFW and RDW by 189 and 298%, respectively (from 2.530 to 7.309 g and from 0.353 to 1.407 g), compared to their respective minimum values. The initial wastewater COD (376 mg/L) was 118% greater than the COD measured after treatment with C0.4-M10-B0 (173 mg/L). Overall, the C0-M10-B5 and C0.4-M10-B0 treatments showed superior performance in improving plant establishment and growth of C. capitella in the green wall system, while the C0.4-M10-B0 and C0.4-M5-B5 treatments were more effective in greywater treatment. Based on these findings, the integrated use of cyanobacteria, mycorrhizae, and biochar may contribute to the establishment of Crassula in green wall systems and to urban water resource management under the conditions tested in Mashhad, Iran. However, further studies are needed to assess the applicability of these results to other cities or environmental conditions.