Crushed concrete and biochar: A sustainable solution for vegetated roofs

Abstract

Besides providing several important ecosystem services, vegetated roofs can provide also ecosystem disservices, such as leaching nutrients that cause eutrophication in adjacent water bodies. Such downsides and limited knowledge on hydrologic performance of vegetated roofs in a cold climate may prevent their wider implementation. Another issue concerns sustainability of materials used in vegetated roofs. A replicated field experiment was run for four years in southern Finland to study the performance of vegetated roof ecosystems with substrates containing crushed lightweight concrete, biochar or crushed brick. All materials were locally sourced and recycled. Nutrient dynamics, soil fauna, vegetation and stormwater management were monitored. Alkaline concrete efficiently reduced phosphorus leaching due to high pH compared with that of a neutral brick substrate. Amending concrete substrate with 20% (v/v) biochar improved retention of nitrogen and in < 20 mm events also rainfall. Vegetation grew equally well in all treatments. The abundance of soil animals, nematodes and enchytraeids, varied markedly over time, driven by changes in substrate moisture. Rainfall was efficiently retained and runoff detained, especially during summer, which is the rainiest season in Finland. We conclude that concrete and biochar as materials used in substrate can provide an effective, innovative and sustainable solution for vegetated roofs.