Manipulating soil microbial communities in extensive green roof substrates

There has been very little investigation into the soil microbial community on green roofs, yet this below ground habitat is vital for ecosystem functioning. Green roofs are often harsh environments that would greatly benefit from having a healthy microbial system, allowing efficient nutrient cycling and a degree of drought tolerance in dry summer months. To test if green roof microbial communities could be manipulated,we added mycorrhizal fungi and a microbial mixture (‘compost tea’) to green roof rootzones, composed mainly of crushed brick or crushed concrete. The study revealed that growingmedia type and depth play a vital role in the microbial ecology of green roofs. There are complex relationships between depth and type of substrate and the biomass of different microbial groups, with no clear pattern being observed. Following the addition of inoculants, bacterial groups tended to increase in biomass in shallower substrates, whereas fungal biomass change was dependent on depth and type of substrate. Increased fungal biomass was found in shallow plots containing more crushed concrete and deeper plots containing more crushed brick where compost tea (a live mixture of beneficial bacteria) was added, perhaps due to the presence of helper bacteria for arbuscular mycorrhizal fungi (AMF). Often there was not an additive affect of the microbial inoculations but instead an antagonistic interaction between the added AM fungi and the compost tea. This suggests that some species of microbes may not be compatible with others, as competition for limited resources occurs within the various substrates. The overall results suggest that microbial inoculations of green roof habitats are sustainable. They need only be done once for increased biomass to be found in subsequent years, indicating that this is a novel and viable method of enhancing roof community composition

Soil microarthropod community dynamics in extensive green roofs

Green roofs are of increasing interest to ecologists, engineers and architects, as cities grow and aim to become more sustainable. They could be exploited to improve urban biodiversity and ecosystem services, yet almost nothing is known about them from a soil community ecology perspective, despite how critical soil food webs are to ecosystem functioning. This paper provides the first comprehensive study incorporating the annual cycle of green roof soil microarthropods.Microarthropod communities were monitored over 14 months on two extensive green roofs. Abiotic factors, including substrate moisture, were recorded, as were biotic factors such as plant and mycorrhizal colonisation. Microarthropod interactions with these variables were then examined.Microarthropod diversity was low overall, with a few dominant species peaking seasonally. On occasion, total abundance was comparable to other early successional soils. The majority of species present were drought tolerant collembola and xerophillic mites, suggesting that moisture levels on green roofs are a major limiting factor for soil microarthropods.Our results suggest that the microarthropod community present in extensive green roof soils is impoverished, limiting the success of above-ground flora and fauna and ultimately the success of the roof as an urban habitat. We conclude that green roof building guidelines should incorporate soil communities in their design and should aim to be heterogeneous at the roof and landscape level, for the purpose of supporting soil biodiversity and creating sustainable habitats

Using soil microbial inoculations to enhance substrate performance on extensive green roofs

Green roofs are increasing in popularity in the urban environment for their contribution to green infrastructure; but their role for biodiversity is not often a design priority. Maximising biodiversity will impact positively on ecosystem services and is therefore fundamental for achieving the greatest benefits from green roofs. Extensive green roofs are lightweight systems generally constructed with a specialised growing medium that tends to be biologically limited and as such can be a harsh habitat for plants to thrive in. Thus, this investigation aimed to enhance the soil functioning with inoculations of soil microbes to increase plant diversity, improve vegetation health/performance and maximise access to soil nutrients. Manipulations included the addition of mycorrhizal fungi and a microbial mixture (‘compost tea’) to green roof rootzones, composed mainly of crushed brick or crushed concrete. The study revealed that growing media type and depth play a vital role in the microbial ecology of green roofs, with complex relationships between depth and type of substrate and the type of microbial inoculant applied, with no clear pattern being observed. For bait plant measurements (heights, leaf numbers, root/shoot biomass, leaf nutrients), a compost tea may have positive effects on plant performance when grown in substrates of shallower depths (5.5 cm), even one year after inoculums are applied. Results from the species richness surveys show that diversity was significantly increased with the application of an AM fungal treatment and that overall, results suggest that brick-based substrate blends are most effective for vegetation performance as are deeper depths (although this varied with time). Microbial inoculations of green roof habitats appeared to be sustainable; they need only be done once for benefits to still been seen in subsequent years where treatments are added independently (not in combination). They seem to be a novel and viable method of enhancing rooftop conditions

Multitrophic links between arbuscular mycorrhizal fungi and insect parasitoids.

Abstract The effects of arbuscular mycorrhizal colonisation of Leucanthemum vulgare on parasitism of a leaf-mining insect was studied in a field and a laboratory experiment. In the field, parasitism of Chromatomyia syngenesiae by Diglyphus isaea was lower on mycorrhizal plants, compared with plants where the association was reduced. A laboratory experiment, in which L. vulgare was inoculated with three species of AM fungi, showed that the effects on parasitism rates were mycorrhizal species dependent. Some fungal combinations increased parasitism, some decreased it, while others had no effect. It is concluded that the most likely cause of these differences is plant size, with parasitoid searching efficiency being reduced on the larger plants, resulting from certain mycorrhizal species combinations. However, a mycorrhizal effect on herbivore-produced plant volatiles cannot be ruled out

Using recycled aggregates in green roof substrates for plant diversity

Extensive green roofs are becoming a popular tool for restoring green infrastructure in urban areas, particularly biodiverse habitats such as post-industrial/brownfield sites. This study investigated the use of six recycled lightweight aggregates and combinations of them in green roof growing substrate, to determine their effectiveness for enhancing plant abundance and species diversity. In two separate experiments, we examined the roles of substrate type and depth on the establishment of a perennial wildflower mix over a 15-month period. We found that some of the alternative substrates are comparable to the widely used crushed red brick aggregate (predominantly found in commercial green roof growing substrate) for supporting plant establishment. For some materials such as clay pellets, there was increased plant coverage and a higher number of plant species than in any other substrate. Substrates that were produced from a blend of two or three aggregate types also supported higher plant abundance and diversity. Generally, increasing substrate depth improved plant establishment, however this effect was not consistent across substrates. We conclude that recycled materials may be viable constituents of growing substrate for green roofs and they may improve green roof resilience, through increased plant cover and diversity. The results could provide evidence to support the construction of mosaic habitat types on single roofs using various substrate blends

Three-way fungal interactions affect the potential biological control of Himalayan balsam, Impatiens glandulifera

Himalayan balsam (Impatiens glandulifera) is one of the most invasive weeds across Europe. The rust fungus, Puccinia komarovii var. glanduliferae has been introduced as a biological control agent, but success has been patchy. Here, we investigated whether mycorrhizal and endophytic fungi can affect rust efficacy and plant growth. Over three experiments we found that AM fungi and the rust alone or together consistently reduced plant growth, but this depended on the identity of species in the AM inoculum. Meanwhile, AM fungi increased infection frequency of the endophyte Colletotrichum acutatum. Rust inoculation had no detrimental effects on mycorrhizal colonisation or C. acutatum infection, but the latter two fungi reduced rust sporulation. However, plant size was reduced when all three fungal types were present, suggesting that a combined fungal inoculum offers a promising approach for the control of this weed

Can microbial inoculants boost soil food webs and vegetation development on newly constructed extensive green roofs?

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 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

Infection by a foliar endophyte elicits novel arabidopside-based plant defence reactions in its host, Cirsium arvense

Endophytic fungi live asymptomatically within plants. They are usually regarded as non-pathogenic or even mutualistic, but whether plants respond antagonistically to their presence remains unclear, particularly in the little-studied associations between endophytes and nong-raminoid herbaceous plants. We investigated the effects of the endophyte Chaetomium cochlioides on leaf chemistry in Cirsium arvense. Plants were sprayed with spores; leaf material from both subsequent new growth and the sprayed leaves was analysed 2 wk later. Infection frequency was 91% and63% for sprayed and new growth, respectively, indicating that C. cochlioides rapidly infects new foliage. Metabolomic analyses revealed marked changes in leaf chemistry with infection, especially in new growth. Changes in several novel oxylipin metabolites were detected, including arabi-dopsides reported here for the first time in a plant species other than Arabidopsis thaliana,and a jasmonate-containing galactolipid. The production of these metabolites in response to endophyte presence, particularly in newly infected foliage, suggests that endophytes elicit similar chemical responses in plants to those usually produced following wounding, herbivory and pathogen invasion. Whether en-dophytes benefit their hosts may depend on a complex series of chemically mediated interactions between the plant, the endophyte, other microbial colonists and natural enemies