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Development of suitable growing media for effective green roofs
Abstract and Figures
Many cities in the urban environment are composed of tall office buildings with flat, unused rooftops. These rooftops are now of interest to architects and ecologists as sites for vegetated or green roofs for both aesthetic and conservation appeal. Extensive green roofs are usually constructed with a very thin layer of substrate, like crushed brick, with a small amount of organic component mixed in, and then vegetated with different plant species. These roofs are generally no thicker than 10cm in depth and therefore require no structural modification of the building that they are to be placed upon. The habitats that these extensive roofs try to mimic are that of natural brownfield land (or ‘wasteland’) found in urban environments. These ‘wastelands’ are considered as some of the most species rich habitats and can be a refuge for some of the country’s rarest birds and invertebrates. However brownfield sites are in danger of being developed in cities due to the high demand for housing and industry, so an alternative (such as the green roof) is highly desirable. The aim of this PhD research was firstly to determine if green roof substrates could be engineered with recycled materials in order to provide an effective growing media for native wild flowers and grasses, and secondly, to introduce or modify microbial communities on an existing green roof for enhanced plant growth and plant species diversity. This produced two main themes for the thesis: physical improvements and biological improvements to new and existing extensive green roofs respectively. Results showed that recycled waste materials could provide environmentally safe and effective green roof substrates. Experiments on Bourne roof (the new green roof site) showed that one recycled aggregate (clay and sewage sludge pellets) performed better than the industry standard crushed red brick, in terms of plant coverage and species diversity. Furthermore, manipulation experiments on an existing green roof at London Zoo showed that these artificial habitats are able to support an abundant microbial community. The applications of microbial treatments increased root colonisation levels of arbuscular mycorrhizal fungi, but benefits usually associated with AMF were not fully exploited. The project confirmed above all else that the substrate type and depth on a green roof has the most influence on plant performance, species diversity and soil microbial communities.
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