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Development of suitable growing media for effective green roofs
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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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... The production of green roof substrates that are physically, chemically and biologically comparable to those of brownfield areas (Molineux et al. 2009) are desirable, as this should provide green roofs with the most favourable conditions for a diversity of wildflowers that are associated with urban wastelands. If the growing media is designed and engineered correctly, then these types of green roofs should be more attractive than they are at present because the plants should perform better in terms of growth, diversity and ecosystem services such as pollination provision (Molineux 2010). ...
... There are very few studies on the microbial populations of green roofs, (Molineux 2010;McGuire et al. 2013;Molineux et al. 2014) yet these communities are fundamental to the health of the green roof ecosystem (Rumble and Gange 2013). It has been shown that wellestablished underground communities have huge benefits on the aboveground forbs and grasses, in early successional communities (Gange and Brown 2002;Wardle et al. 2004). ...
... If there is a viable microbial community then plant growth-and to some extent plant diversity (Gange et al. 1990;Van Der Heijden et al. 2006;Vogelsang et al. 2006)-should be increased. Molineux (2010) found that the inoculation of green roof substrates with either arbuscular mycorrhizal fungi (AMF) spores or with an addition of 'live' microbes in a compost tea could enhance the microbial biomass within substrate and subsequently produced plants that were larger with more foliage cover. However, the overall success of the inoculation experiments was that they needed only to be applied three times in 1 year for the effects to be sustained the following year (Molineux et al. 2014). ...
Green roofs have been implemented on new buildings as a tool to mitigate
the loss of post-industrial or brownfield land. For this to be successful, the roofs must
be designed appropriately; that is with the right growing media, suitable substrate
depth, similar vegetation and with a comparable soil microbial community for a
healthy rhizosphere. This study compared soil microbial communities (determined
using phospholipid fatty acid or PLFA analysis) of two extensive green roofs and
two post-industrial sites in Greater London. It was found that green roof rootzones
constructed using engineered growing media are not depauperate, but can have an
abundant soil microbial community that in some cases may be more diverse and
numerous than communities found in brownfield areas. In this preliminary study, one
green roof supported abundant soil microbial communities that were dominated by
gram negative and aerobic bacteria, whilst fungal abundance was similar across all
sites analysed. Furthermore, ratios of fungal: bacterial PLFA’s were larger from postindustrial
sites but overall were consistent with bacterial dominated soils typical of
early successional habitats.
... Arbuscular mycorrhizal (AM) fungi are an important part of this community; they comprise of about 150 known fungal species and are said to be associated with around 80% of all plant species root systems (Hodge, 2000). Preliminary studies of microbial communities on green roofs have shown that there are relatively few microorganisms present within the substrates (Molineux, 2010). This may mean that nutrient recycling is not as efficient as it could be, resulting in reduced plant growth and diversity. ...
... This may mean that nutrient recycling is not as efficient as it could be, resulting in reduced plant growth and diversity. There is a huge gap in the literature about the effects of soil microbes on plant diversity on green roofs (Molineux, 2010) and very little data on how these microbial communities could be enhanced for improved greening. If a healthy rhizosphere is achieved then green roofs may be more resilient to harsh conditions in hot, dry months. ...
... This may indicate that other factors were involved such as nutrient availability (Blume et al., 2002;Fierer et al., 2003), pH (Smith and Bowen, 1979;Graham, 1992;Bayoumi et al., 1995), substrate aeration (Fomsgaard, 1995) or microbial succession (Carpenter-Boggs et al., 1998). However, there was no correlation with soil microbial biomass and SOM (soil organic matter) in 2007 (Molineux, 2010) nor was there a significant change in substrate pH over the 3 years (Molineux, 2010). Microbial succession may have caused microorganism abundance to increase with time, but for this to occur more nutrients (organic material in the root zone) must have become available to support the larger biomass. ...
... Little such testing has been carried out on green roofs, but the few studies that exist have also reported unpredictable findings. Molineux (2010) found that the addition of mycorrhizas and compost tea (liquid obtained via aerobic digestion of composts) to green roofs planted with Plantago lanceolata improved plant growth for the first year alone and some competitive effects between inoculants were noted. She also found that fungal and bacterial biomass on green roofs could be enhanced with the addition of microbial inoculants (Molineux et al., 2014). ...
... In addition, commercial inoculants typically contain mixes of species, in order to increase the probability that a species specific relationship can develop (Koomen et al., 1987;Gadhave et al., 2016) . There is evidence to suggest that in some cases, however, an antagonistic relationship may develop between inocula species (Molineux, 2010), negating their desired effect. Here we describe a study in which three commercial inocula mixes, encompassing mycorrhizas, bacteria and Trichoderma were added to a mature green roof to determine if commercial inocula applied singly, or in combinations, affects the soil microarthropod community, and if this has resultant (or independent) effects on plant growth. ...
Green roofs are increasingly used in the urban environment to insulate buildings, reduce stormwater runoff and remediate biodiversity lost in construction. Most common in the Northern Hemisphere are extensive green roofs, due to their low-cost and low-maintenance requirements. However, plant growth on these roofs is often limited and this could have implications for ecosystem service provision as well as reduce the economic feasibility of green roofs as an aesthetically successful product. In addition, the increasing popularity of green roofs as an eco-product means that a high number of these roofs, that do not reach their maximum potential in terms of plant growth, already exist, highlighting a need for a successful remediation tool post-build.
... Extensive green roofs are generally designed with a substrate layer (up to 150 mm deep) that contains a high (up to 90%) percentage of aggregate and a small amount of organic material. This not only provides a low nutrient growing substrate ideal for green roof vegetation (Molineux et al., 2009;Molineux, 2010;Nagase and Dunnett, 2011) but also reduces extra roof weight. Problems can occur with either the addition of 'soil' and its attending clay fraction causing reduced water transmissivity or excessive compost/organic matter risking substrate shrinkage (Snodgrass and Snodgrass, 2006). ...
... The purpose of this test plot was to determine if substrate depth altered plant species richness and abundance within the same substrate type. Due to weight restrictions on the roof, only three aggregates could be tested, therefore substrates that had not performed as well in preliminary greenhouse trials (Molineux, 2010) were selected, to see if increasing depth could improve their performance. ...
... Total microbial mass over the study period was within the range reported on other young green roofs by Molineux (2010), characterised by a bacterially dominated microbial community, with all PLFAs low in abundance at the start of the experiment. Fungal PLFAs increased over time. ...
Green roofs are a key to providing nature-based solutions in cities. However, most green roofs installed in the Northern hemisphere are shallow, stonecrop planted systems (“extensive” green roofs), which have been shown to support limited biodiversity and could be more effective at providing ecosystem services. One issue with this type of extensive green roof is that rootzones are almost sterile on construction, relying on natural colonisation to provide a soil food web. This is a slow process, meaning plant growth can also be slow.
Our aim was to determine if a soil food web could be introduced when the green roof is built. We applied microbial inoculants (mycorrhizal fungi and bacteria (Bacillus spp.)) to a new green roof and monitored plant growth and the soil food web (bacteria, mycorrhizal fungi and microarthropods).
Different inoculants altered the composition of microarthropod communities, potentially impacting later succession. In particular, bacterial inoculants increased microarthropod populations. This is one of the first studies to demonstrate that the addition of microbial inoculants impacts not only plant growth, but also faunal components of the soil food web, which could have implications for long-term resilience.
Bacteria were effective at aiding mycorrhizal colonisation of plants roots, but this colonisation had no impact on the growth of our selected selected stonecrops, Sedum album, Petrosedum reflexum and Phedimus spurius. We suggest that if a beneficial mycorrhiza could be found to promote the growth of these specific species on green roofs, bacteria could be effective “helper” species to aid colonisation.
This study enables green roof researchers and the industry to justify further exploration of the impact of microbial inoculants on green roofs.
... The substrates included mineral components like lava, pumice, expanded clay, recycled bricks and organic matter (compost) as a plant nutrient source. A low organic matter content and therefore low nutrient supply are favourable for drought tolerance of plants (Molineux 2010;Nagase and Dunnett 2011). Depending on the speci c quality, an organic matter content of about 10% (related to volume) seems appropriate for sustainable plant growth (Nagase and Dunnett 2011;Schröder and Kiehl 2020a). ...
Green roofs can mitigate negative environmental effects of urban densification to some extent, but they are often covered by species-poor Sedum mixtures with a low value for biodiversity. By combining a habitat template and a seed-provenance approach, we review the suitability of plant species from regionally occurring dry sandy grasslands (Koelerio-Corynophoretea) for extensive roof greening in northwestern Germany. Since 2015, we have studied the effects of species introduction on vegetation dynamics on experimental mini-roofs. Treatments included sowing seeds of regional native origin in two densities (1 g and 2 g/m²) and the transfer of raked material from an ancient dry grassland area classified as Natura 2000 site. The applied raked material contained diaspores of 27 vascular plant species (including seven threatened species) and vegetative fragments of grassland-specific mosses and lichens. Since 2018, we have tested more species-rich seed mixtures in a large-scale experiment on a roof of 500 m² with different engineered green-roof substrates and layering. In 2019, a green roof of 10,200 m² was established in cooperation with a local enterprise to support regional native biodiversity. In this chapter, we summarise the most important results of our studies and discuss how to support regional native biodiversity on green roofs.
... Received 4 June 2019; Received in revised form 21 August 2019; Accepted 24 August 2019 (promoting plant performance such as flowering, yield, and growth). Considering that standard EGR substrates often lack a significant amount of beneficial microorganisms such as AMF (Molineux, 2010), the mutualistic symbiosis with AMF could provide the improved drought tolerance and nutrient uptake (Young et al., 2015) necessary for establishing self-sustaining plant populations on EGRs. ...
Standard extensive green roofs (EGRs) with their shallow substrate layers represent extreme sites for plant growth and therefore are planted mostly with drought-resistant species, including non-native plant species. As standard EGR substrates often lack potentially mutualistic soil microorganisms, it has been stated that inoculation with arbuscular mycorrhizal fungi (AMF) might increase plant performance and drought resistance. Aiming to support native biodiversity on EGRs, we tested whether AMF inoculation into standard green roof substrate can enhance plant performance and drought resistance of regionally occurring native dry grassland species.
The results of a pot experiment with 11 native plant species growing with and without AMF inoculation showed considerable differences in fitness-relevant plant traits. Over 88 days of moderate drought conditions, inoculated plants produced 2.5 times more above-ground biomass than control plants. In addition, the number of inflorescences on inoculated plants was significantly higher in 5 out of 7 flowering species. Under severe drought stress created by stopping the water supply, however, inoculated plants wilted on average 2.38 days earlier than control plants.
Although the underlying mechanisms of the observed results remain unresolved, AMF inoculation might help to enhance an earlier and higher seed set, facilitating the establishment of a soil seed bank, which is necessary for a self-sustaining plant population in drought-governed habitats such as EGRs.
... Mallia olisi mahdollista ottaa esimerkiksi Sveitsin Zürichistä, missä katoille on määritelty "hyvä tavoitetaso" ja "erityistaso" ja Baselista, jossa on määritelty viherkaton ominaisuuksia (63). vedenpidätyskapasiteetti sekä orgaanisen aineksen, ravinteiden ja mykoritsan eli sienijuuren määrä (67). Kosteutta pidättävien teknisten kerrosten avulla voidaan lisätä veden saatavuutta ja elinympäristön monipuolisuutta (48,68). ...
... They are designed to be cost effective and low maintenance , but are a challenging environment for non-drought adapted plants (Dunnett and Kingsbury, 2004). Despite their harsh conditions , green roofs support rare insect communities (Kadas, 2006), birds (Fernandez-Canero and Gonzalez-Redondo, 2010) and local plant taxa (Molineux, 2010; Monterusso et al., 2005) and associated pollinators (Kadas, 2006). To date, little work has been done on below-ground communities, despite abundant evidence to suggest that these are inextricably linked to above-ground processes (Wardle et al., 2004). ...
For evaluating the effect of various organic fertilizer ratios on the Spiraeabumalda 'Gold Mound' growth, a container green wall system experiment was conducted in a greenhouse at Konkuk university. The experimental planting grounds were prepared with different organic fertilizer ratios (, , and ) and with drought tolerance and an ornamental value Spiraeabumalda 'Gold Mound' was planted. The change in soil moisture contents, plant height, number of branches, number of dead leafs, number of leaf, number of shoots, length of node, length of leaf, width of leaf, root-collar caliper, chlorophyll contents and survival rate were investigated from April to Jun 2010. 1. The result of soil moisture contents was analyzed with weight unit in the container green wall system during the dry summer season. The soil moisture contents were significantly enhanced in the container green wall system in increasing order as the amount of fertilizer level increased > > > > . 2. Compared to the control treatment (amended soil with 100% + organic fertilizer 0%) application, the highest plant growth was observed in the treatment of (amended soil with 67% + organic fertilizer 13%) application. However, the differences between the organic fertilizer ratio treatments of , , , and the organic fertilizer application were mostly not significant. 3. The survival rate increased with the increasing application of organic fertilizer, but in the control treatment (amended soil with 100% + organic fertilizer 0%) application all the plants died. Experimental results from the presented study clearly demonstrated that the organic fertilizer improved the survival rate more than the Spiraeabumalda 'Gold Mound' growth at different levels of organic fertilizers. This strain can be utilized as a plant growth application in living wall systems during the dry summer season. Therefore, Spiraeabumalda 'Gold Mound' is expected to be a highly valuable shrub for the green wall system if it should be considered in integration with stormwater retention or as a soil conditioner for increasing soil water contents in planting ground.
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