Effects of sewage sludge and biochar amendments on green roof substrate properties and Sedum growth.

The composition of the growing medium is a key factor in the long-term functionality of green roofs, and in recent years, increasing attention has been paid to the use of biochar and other recycled organic materials. The use of biochar in green roof substrates represents an innovative way to improve their hydrological and nutritional properties in accordance with the principles of the circular economy. As part of this study, we incorporated 8.2% organic additives into the experimental substrates, including dried sewage sludge, biochar derived from sewage sludge, food waste biochar, wood biochar, and sewage sludge compost. The main hypothesis was that these additives would induce significant changes in the physicochemical properties of the substrate, nutrient content, and heavy metal concentrations, affecting plant growth. The study involved a three-year monitoring of six modules planted with a mixture of twelve Sedum species. The results showed that dried sewage sludge, food waste biochar, and sludge compost had a highly positive effect on vegetation growth, with coverage significantly higher compared to the control sample. The highest average coverage of plants (89%) was achieved by the substrate with dried sewage sludge due to its high content of available nitrogen and phosphorus. Wood biochar exhibited higher soil moisture content (mean 21.28%) and its stability throughout the year, but without a significant positive effect on plant growth. Dried sewage sludge applications improved soil quality the best with increase in the availability of essential nutrients NPK (an 87% increase compared to the control module). Several heavy metals (notably Cd, Cr, Zn, and Hg) exceeded regulatory thresholds, indicating a potential environmental risk that requires further investigation, particularly with respect to long-term accumulation and leaching.