Microbes help urban greenery plants to thrive

Urban greening provides additional green spaces in cities, but plants need special attention in order to grow in harsh conditions. The recent doctoral thesis from the University of Helsinki shows how adding plant growth-promoting microbes to the soil helps plants to tolerate stress and thrive in the city.

Urban greening has gained increasing popularity in cities, helping to create a more liveable environment. Vegetated building envelopes include vegetated roofs, indoor and outdoor living walls and balcony gardens. They can provide additional green spaces and deliver ecosystem services, such as storm water management, air pollution mitigation, energy conservation and urban heat island effect reduction.

The growing conditions on vegetated building envelopes are generally harsh, so it is crucial to investigate how to maintain plants on them in optimum conditions. Long Xie’s doctoral dissertation from the Faculty of Agriculture and Forestry focuses on maintaining plants in vegetated building envelope systems by introducing two beneficial plant growth-promoting microbes into the substrate, that is, the growth medium for plants, such as soil.

Rhizophagus irregularis is an arbuscular mycorrhizal fungus (AMF) residing in plant roots. AMF features tree-like fungal structures called arbuscules growing inside root cells. They are important for nutrient exchange between the fungus and host plants. Bacillus amyloliquefaciens, on the other hand, is a spore-forming bacterium residing outside the root surface.

Both of these microbes can protect host plants against nutrient deficiency, pathogen infection, drought and high salinity. In return, the host plants support the microbes with chemicals and nutrients, rich in photosynthetic compounds, from the plant root system.

The dissertation showed that both microbes are able to survive and inhabit the roots of greenery plants. In addition, the R. irregularis fungus was thought to promote the growth of B. amyloliquefaciens. The results also suggested that the colonisation of R. irregularis was affected by the pH of the growth medium. The effect of soil pH on R. irregularis colonisation was dependent on the plant species: some plants had higher colonisation levels in acidic soil than neutral soil, and other showed the opposite. Biochar supplementation showed a trend to reduced R. irregularis colonisation. The growth of B. amyloliquefaciens was affected by weather conditions – heat and drought reduced B. amyloliquefaciens density.

Under laboratory conditions, both microbes were able to simultaneously inhabit most of the test plants, and B. amyloliquefaciens promoted the growth of R. irregularis. Above all, co-inoculation, which means injecting the soil with both microbes, resulted in higher shoot biomass and photosynthetic efficiency than inoculation of a single microbe.

The increase in plant growth observed in co-inoculation occurred on vegetative roofs. In addition, plant species, planting methods (different ways of introducing plants into vegetated roofs), and their interactions affected the colonisation of R. irregularis, whereas the survival of B. amyloliquefaciens was influenced only by the plant species.

Recipe for success – urban greening with beneficial microbes

“I would recommend using substrates and plant species that support plant growth-promoting microbes when vegetated building envelopes are constructed. It is also advisable to use both R. irregularis and B. amyloliquefaciens to achieve synergistic effects on plant growth,” says Long Xie.

There are several different planting methods to introduce plants into vegetated building envelope systems, e.g., pre-grown vegetation mats, pre-grown plug plants, and seed-grown plants. Using a mixture of planting methods can achieve instant greening and maintain the building envelope’s ability to host arbuscular mycorrhizal fungus. It also necessary to irrigate the plants moderately during prolonged hot and dry periods in the first few years and, finally, to avoid invasive microbe and plant species.

Xie's research was funded by China Scholarship Council, Maiju ja Yrjö Rikalan Puutarhasäätiö, Finnish Cultural Foundation and August Johannes ja Aino Tiuran maatalouden tutkimussäätiö.

For more information

Long Xie, MSc will defend the doctoral dissertation entitled “Application of Plant Growth-Promoting Microbes on Urban Building Vegetated Envelopes, from Lab to Field” at the Faculty of Agriculture and Forestry, University of Helsinki, on 6 May 2020 at 12.00. You can follow the dissertation defending online and post your questions via Presemo.

Professor Alan Gange, Royal Holloway, University of London, will serve as the opponent, and Professor Paula Elomaa as the custos.

The dissertation will be published in the series Dissertationes Schola Doctoralis Scientiae Circumiectalis, Alimentariae, Biologicae.

The dissertation is also available in electronic form through the E-thesis service.