Green roof soil organisms: anthropogenic assemblages or natural communities?

Green roofs provide a range of ecosystem services, from stormwater retention to thermal insulation. They can also provide habitat for biodiversity, remediating land lost in development. However, few extensive green roofs are designed with this benefit in mind and, as such, biodiversity often does not reach its full potential. In particular, the soil ecology of green roofs is poorly understood, despite soil microorganisms having a large impact on nutrient cycling and thus plant diversity. In particular, whilst there are studies describing the soil microarthropods and microbial communities present on green roofs, little is known about how these species arrive there. This paper aims to determine how soil microarthropods and microbes colonise green roofs and which species survive post-construction, to inform green roof technosol design and to understand if remediation of impoverished green roof soils is possible. To do this, we conducted a preliminary study by analysing green roof construction materials (substrates and Sedum plugs) for microarthropods, bacteria and fungi before constructing a new green roof. We then monitored survival and independent colonisation over eleven months. Whilst green roof substrates were a poor source of colonisation, Sedum plugs showed potential as a vehicle for colonisation by microbes and, especially, by soil microarthropods. However, the majority of the species present within Sedum plugs were not adapted to the harsh conditions of the green roof, resulting in high mortality. Two ubiquitist species, the Collembola species complex Parisotoma notabilis and a mite of the family Scutoverticidae survived in high abundance after the eleven month sample period, and the functional role of these species on a green roof should be investigated. Some species colonised independently during the study, highlighting that microarthropods and microbes in green roofs consist of a mix of anthropogenic assemblages and natural communities. Mycorrhizal fungi were extremely successful, independently colonising almost all Sedum plants by the end of the study. However, the absence of arbuscules suggests that this colonisation may not have a benefit to plant growth in this instance. Demonstrating that the succession of soil organisms is influenced by the communities present in construction materials has implications for substrate design, demonstrating that soil organisms may be inoculated onto green roofs to provide functioning technosols. In addition, the independent colonisation of mycorrhiza in this study stimulates discussion about the role of commercially applied mycorrhizal fungi in green roof construction.