The study, published in Science Advances, found that even humble roadside verges contribute a range of important microbial communities that are critical for sustaining productive ecosystem services, such as filtering pollutants and sequestering carbon dioxide.
The researchers, including Professor David Eldridge from the Centre for Ecosystem Science in the University of New South Wales (UNSW), noted that parks are not the homogenised ecological deserts that people think they are - even road verges are full of important microbes.
"Urban greenspaces harbour important microbes, so if you want to sustain a bunch of ecosystem services, you need to have plenty of parks and green spaces," Eldridge said.
The study took soil samples from different types of urban green spaces and comparable neighbouring natural ecosystems in 56 cities from 17 countries across six continents.
Researchers found green spaces support many fast-growing microbes that use fertilisers and irrigation water, and that can colonise bare soils. These included important fungal root pathogens such as fusarium, microorganisms capable of removing nitrogen from sewerage, and many bacteria-feeding amoebae.
Eldridge said the results mirror a study in Central Park in New York, which found there was as much microbial diversity in the city park as there is globally.
"We think of roadsides as being barren, but we found a great variety of different microbes in some roadside verges; they are not barren wastelands at all," Eldridge said.
With 68 percent of the global population set to live in cities by 2050, the study also suggested that urban green spaces are critically important for promoting mental and physical wellbeing, because human exposure to soil microbes has been shown to be beneficial to people's health by promoting effective immunoregulation functions and reducing allergies.
But scientists also warned that the study found in some countries, green spaces also harboured a greater proportion of fungal parasites and plant pathogens that are often economically important pests. This may link to human pathogens, such as listeria and diphtheria.
"The really interesting thing is that there was a strong link between a country's Gross Domestic Product and the abundance of the microbes that caused human diseases," Eldridge said.
He added that the reasons could be a greater use of antibiotics in developing nations, and therefore it caused greater microbe controlled antibiotic resistance.
Eldridge explained that sewerage water containing antibiotics is then used to water greenspaces, so while parks are good, there is a warning that some of the soil in these urban green spaces do harbour some of these toxic microbes, particularly in poorer cities.
The international study is part of a series of research looking at the importance of green spaces for ecosystem health. The next study will examine the importance of mosses in urban green spaces for supporting healthy soils and important habitat for microbes.
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