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

Green roofs are a key component in providing nature-based solutions in cities. However, most green roofs installed in the Northern hemisphere are shallow, Sedum planted systems (“extensive” green roofs), which have been shown to support limited biodiversity and could be designed to be more effective at providing ecosystem services. One major issue with extensive green roofs is that rootzones are almost sterile on construction, relying on natural colonisation to provide a soil food web, which is key to regulating nutrient cycles. Vegetative establishment and subsequent plant growth is therefore slow.
Our aim was to determine if a soil food web could be artificially constructed when the green roof is built and to determine if this could speed up green roof development. We applied microbial inoculants in the form of mycorrhizal fungi and bacteria (Bacillus sp..) to a new green roof and monitored plant growth and the soil food web, including bacteria, fungi and microarthropods.
We found that bacteria were effective in aiding mycorrhizal colonisation of plants roots, but that this colonisation was detrimental to plant growth, most likely because the mycorrhizal species within the inoculant mix were parasitic on Sedum spp.. We also found that 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 its long-term resilience. In addition we suggest that if a beneficial mycorrhiza could be found to promote the growth of Sedum spp. on green roofs, bacteria could be effective “helper” species to aid colonisation; application at the construction of a green roof is the optimum time to apply these