Soils from city parks are important hotspots of microbial diversity: study

City parks and residential gardens have a unique functional microbiome with more potential human and plant pathogens, and fewer plant-symbionts than neighbouring natural ecosystems, according to a study that unpacks the soil microbiome of global urban greenspaces.

The findings indicate that warmer cities support larger proportions of plant pathogens, which could make fighting against city park pests, more difficult and economically challenging in the future, said study lead author Manuel Delgado-Baquerizo. Densely populated cities host a higher proportion of key antibiotic resistance genes, demonstrating that climate and socio-economic conditions can influence the microbes inhabiting city parks and gardens.

The study also reports that soils from urban greenspaces harbour a higher proportion of microbes tied to methane production and nitrogen dioxide emissions compared with natural ecosystems, suggesting that urban greenspaces could potentially be important sources of greenhouse gas emissions (methane and nitrous oxide) to the atmosphere.

“These results, together with the fact that many urban ecosystems are overly fertilised and watered, suggest that city parks could play a role as regulators of greenhouse emissions. However, further investigations are needed to better understand the role of urban greenspaces in providing nature-based solutions and control emissions,” Delgado-Baquerizo from Spain’s Universidad Pablo de Olavide, told Mongabay-India.

City parks and gardens are “living ecosystems” and not ecological deserts, said researchers on the media release on the study, stressing that every green spot accounts for conserving soil biodiversity. “In the past, it was shown that the human gut has a particular microbiome; likewise, city parks and gardens have a very distinctive soil microbiome that we are just starting to understand,” Delgado-Baquerizo said.

Researchers examined soil samples from different types of urban green spaces and comparable neighbouring natural ecosystems in 56 cities from 17 countries across six continents. In India, researchers from Banaras Hindi University (BHU) looked at Durgapur, Agra and Mirzapur as part of the global work. Over five hundred species of archaea, bacteria, fungi, and protists were characterised in the soils of urban parks across the globe.

Sixty-eight percent of the global population is set to live in cities by 2050, increasing the environmental and social stresses for the billions of humans living in urban areas. India is projected to double its urban dwellers from 400 million in 2011 to 800 million by 2050 which means urban green spaces for physical and mental wellbeing of residents will be critical.

The realities of climate change for urban areas are underscored by the latest report of the Intergovernmental Panel on Climate Change that warns on future urbanisation amplifying projected air temperature change in cities regardless of the characteristics of the background climate, increasing minimum temperatures likely to be large as the global warming. Additionally, it emphasises that impact assessments and adaptation plans in cities require high-spatial-resolution climate projections along with models that represent urban processes, ensemble dynamical and statistical downscaling, and local-impact models.

In India, urban green spaces have garnered support in policy (including the Green India Mission, Nagar Van Scheme) and political will in recent years but there is a need to plan for these restoration projects considering the larger landscape and in discussion with key stakeholders to not infringe on local rights and plan interventions and species that are ecologically appropriate, as well as develop better monitoring systems, shared Ruchika Singh, Director – Sustainable Landscapes & Restoration, at World Resources Institute India (WRI India).

For example, the National Mission for a Green India (Green India Mission or GIM), launched in 2014, is one of the eight Missions under the National Action Plan on Climate Change (NAPCC) GIM focusses on multiple ecosystem services such as biodiversity, water, biomass, preserving mangroves, wetlands, critical habitats etc. along with carbon sequestration.

One of its aims is to improve forest and tree cover across 0.2 Mha in urban and peri-urban areas. The policy emphasises the setting up of local citizen groups to maintain and monitor urban green spaces. It seeks the support of institutions, businesses, and corporates to achieving the target. “However, implementation of GIM has been slow, with the scheme achieving only 2.8 percent of its overall target between 2015 and 2020. In addition to being underfunded, the scheme has faced criticism for lacking consideration around local soil and other ecological conditions,” Singh told Mongabay-India.

Soil microbes in urban parks globally are very similar

Jay Prakash Verma at BHU’s Institute of Environment and Sustainable Development, one of the co-authors of the global study, said soils of urban parks supported a significantly greater proportion of Gammaproteobacteria, Deltaproteobacteria, Bacteroidetes, Gemmatimonadetes, Ascomycota, Mucoromycota, Chlorophyta and Amoebozoa than natural ecosystems. 

These phyla include multiple fast-growing organisms that could take advantage of the often-fertilised and irrigated conditions in urban ecosystems. They aid the speedy decomposition of organic matter to release more greenhouse gases as part of their metabolism. “Soil microorganisms are largely responsible for the cycling of soil organic carbon (SOC) and other nutrients. So they have a key role in climate feedback, including the production or consumption of greenhouse gases such as carbon dioxide, methane and nitrous oxide,” Verma shared.

Once this cycling of SOC and other nutrients is disrupted it may lead to the emission of greenhouse gases like CO2, methane and nitrous oxide. “If we keep adding synthetic nutrients and that too in higher proportions, this may lead to change in the microbiome and consequently disturb the recycling of elements like carbon and nitrogen,” said microbial ecologist Praveen Rahi, who was not associated with the research.

“With increasing urban population, most city planners are left with very few choices to maintain natural parks, as they require lots of land. Even the existing natural parks also witness a tremendous inflow of people, so there is little or no time for the park to restore organically. As an alternative, most urban greenspaces are now fertilised using chemical fertilisers and the plant residue is either burnt or moved to some other place to keep these spaces presentable all the time,” Rahi at National Centre for Cell Science, Pune told Mongabay-India.

The research, says Rahi, is “an eye-opener” for governments and policymakers, to treat the local biodiversity (both above and below-ground) with respect and adopt technologies to maintain it, especially since the findings spotlight that urban greenspaces, globally, are very homogeneous − they are very similar, support similar microbial species and often have large portions of land covered by intensively managed lawns. 

“The same concept can be applied to pigeons or squirrels, which can be found everywhere. Creating more heterogeneous urban greenspaces, including native vegetation, is probably one of the keys to supporting a larger number of soil species while reducing pest incidence in the future,” Delgado-Baquerizo adds.

The urban greenspace taxa included fungal and oomycete plant pathogens such as Fusarium intricans, Pythium rostratifingens, Pythium uncinulatum, and fungal decomposers such as Mortierella elongata, archaeal nitrifiers such as Nitrososphaera sp., bacteria such as Streptomyces and Pseudomonas spp., and multiple species of bacteria-feeding amoebae.

For a healthy soil microbiome, Verma recommends using more bio-fertilisers, green manure, farmyard manure, compost, and crop residues but advises against using contaminated or sewage water for irrigation of city parks gardens because such water sources have many disease-causing microbes. An organic framework is more sustainable.

“Using natural grass rather than synthetic grass transplantation will have a more healthy microbiome and be in synergy with the local environment. Natural grass fortified with biofertilisers, vermicomposting and compost from grass residues and leaf litter in park spaces will also reduce investment costs and chemical fertiliser application,” Verma elaborated.

More research on urban green spaces required

More research on urban green spaces, their management and links to human health is required. How we think about microbes beneath our feet in city parks and gardens matters because human exposure to soil microbes is beneficial to human health by promoting effective immunoregulation functions and reducing allergies. 

Some soil microbes (e.g., Mycobacterium, Listeria, and Fusarium spp.) can also endanger the sustainability of urban greenspaces and animal, human, and plant health. In 2017, scientists put forth the Microbiome Rewilding Hypothesis which proposed that “restoring biodiverse habitats in urban green spaces can rewild the environmental microbiome to a state that helps prevent human disease as an ecosystem service.”

Recent research spanning urban greenspaces in Australia, the United Kingdom and India showed that after exposure to city parks, the diversity of microbes on the skin and nose of sampled adults increased and the microbial composition became more similar to environmental sources which could have implications in reducing the development of non-communicable diseases.

“There is a need to emphasise and plan how urban land use planning is undertaken and how areas for different restoration interventions are identified. In Kochi, for instance, as part of the Cities4Forest Initiative, we are working with Kochi Municipal Corporation and local stakeholders to systematically identify the potential for restoration and interventions that can mitigate urban heat island risks,” said Marie Duraisami, Manager – Sustainable Landscapes & Restoration at WRI India.

“Our focus is on a participatory consultative approach to identify key interventions that mitigate the risks of heat islands and build resilience. Climate-friendly practices for nurturing green spaces include a focus on natural farming practices, use of natural inputs using a holistic systems approach has potential to improve soil health,” Duraisami added.

Praveen Rahi adds that there are tools to measure even the smallest shift in microbial communities, which can lead to developing methods to predict future climatic impacts. “Microbes can be the biomarkers to predict future climate impacts.”

Although the importance of soil microbial ecology for the prediction of future climate impacts has been recognised, Verma adds that it remains a challenge to integrate with landscape-scale climate models.

Banner image: Girls at Science Park in Kolkata. Photo by Biswarup Ganguly/Wikimedia Commons.