A world beneath your feet!

In our mission to understand how Nature-based solutions (NBS) can be used to build more resilient cities, PhD student Nerea Ferrando, from the University of Reading and Earthwatch, has been investigating the world beneath our feet. Researching how the soil fauna, is a vital part of the equation.

Image: Researcher (Nerea Ferrando) from the University of Reading identifying soil mesofauna (Zhang, Y., 2019. [photograph]).

Her work is part of a wider research project, Climate-Proof Cities, led by Earthwatch. It explores how we can manage urban environments more effectively to help mitigate the effects of climate change, Making cities more resilient to flooding, drought and heatwaves.

During the research, HSBC citizen scientists participated in Earthwatch’s Sustainability Training Programme (STP). They collected soil samples and tree data from study sites in urban parks in the UK and France. In the UK, citizen scientists looked at how the management of trees in urban parks affects soil health and the ability of soil to reduce flooding by soaking up water.

So, what happened to these soil samples in the lab?

The soil samples collected were sent to the University of Reading. In a previous blog, Angie Elwin explains how two PhD students, Vicky Struthers and Nerea Ferrando, measured the soil health, using different methods that capture soil chemical and physical properties.

Following on from that, Nerea has shared some of the additional measurements that she made while looking at soil biology as part of her PhD. To do this, she worked with two expert researchers in soil biology from the University of Manchester.

They wanted to understand what happens to the creatures living in the soil at these urban sites when leaf litter, the main food source for most soil life, is cleared from the soil surface; and how this, in turn, affects soil health, the environment and, ultimately, us!

Soil and its Inhabitants

Soil may appear an insignificant material, something on which we build roads and buildings or grow our plants. However, soil is much more than just dirt. Soil is alive! In fact, there can be more creatures in a teaspoonful of soil than there are people in the entire planet (FAO, 2015).

Soil life varies in size: from bacteria and fungi, which require powerful microscopes to see, up to larger burrowing animals, such as moles and rabbits, that spend part of their time in the soil. In most ecosystems, more life and diversity live underground than above ground. But, much of this life is too small to be seen with the naked eye.

Image: A selection of photographs showing the diversity of soil life found in our soil samples from Kew Garden and Cannon Hill. (a) Macro- and mesofauna, including millipede, springtails and mites (b) Collembola (springtail) order Symphypleona (c) Collembola (springtail) order Poduromorpha (d) Myriapod (centipede) (e) Oribatida (beetle mite) (Ferrando, N., 2019. [photograph]).

Why Protect Soil?

Soil inhabitants are responsible for supplying a number of critically important ecosystem services. These range from climate regulation, mitigation of drought and floods, soil erosion prevention, and water filtration.

Soils, and the creatures living in it, are like the stomachs of the earth; consuming, digesting, and cycling nutrients. Just as human health is linked to the health of microbes in our guts, soil organisms are linked to healthy soil. For this reason, it is vital to protect soil and its inhabitants.

However, globally, soil is under threat from increased human population, poor land management and pollution. The rapid deterioration of soil endangers the quality of our lives by posing global challenges to food safety, climate change adaptability, and water quality.

Soil is often a forgotten part of the equation, particularly in our cities. In urban ecosystems, such as parks, litterfall is swept and taken away as a common land management practice. This litter, together with plant root exudates, are the main food sources that drive the soil food web. Its removal can affect the dynamics of the soil communities that are so critical to urban trees. There have been very few scientific investigations that look at the impact of these disturbances on soil biota and how this impacts the chemical and physical properties, like soil carbon and infiltration rate. It is crucial that we understand how litter removal can be used as a form of urban tree management and how it can affect soil’s biological properties in our cities.

How do we find life in the soil?

We used three different tests to measure soil biology:

• First, we measured the soil respiration of the samples while still in the field where collected. This is a crude estimation of biological activity.

• Then in the lab, we used a simple and robust method to extract soil meso-fauna abundance and diversity by encouraging them to crawl out the soil into a collection pot. This is called the Tullgren method and its possible to also do this at home.

• Finally, we used more advanced laboratory methods called PLFA and NLFA profiling to provide a real-time snapshot of the soil bacterial and fungal community.

The soil breathes!

Soil respiration is a good indicator of overall biological activity and health. It is a key process that releases carbon from the soil in the form of carbon dioxide (CO2) into the atmosphere. In fact, although carbon is stored in the soil and litter as organic matter, carbon dioxide diffuses out when plants, bacteria, fungi and soil animals respire.

We used a Li-8100 Automated Soil CO2 Flux System to calculate how much carbon dioxide accumulates in a closed chamber during a specific amount of time. This way, we could measure soil microbial activity and contrast it between sites to evaluate the impact of litter removal.

Image: Soil respiration measurement at Kew Garden.

Identifying Soil Critters

We collected soil samples from our study sites and extracted the invertebrates living in the soil, or on the leaf layer on the surface, using the Tullgren funnel method. This method works by creating a temperature gradient over the samples so that mobile soil organisms move away from the higher temperatures and fall into a container filled with ethanol where they perish and are preserved for examination.

Image: Soil samples set-up for dry extractions of invertebrates (Ferrando, N., 2019. [photograph]).

The soil mesofauna (invertebrates between 0.1 – 2 mm in size and can be mites, springtails, spiders, insect larva, and other tiny critters) are later identified and categorized. We do this by investigating the head and mouth structures using a powerful microscope. This soil community is a valuable indicator of soil quality.

Image: A selection of photographs from the soil fauna identification process in the laboratory. Researcher Nerea Ferrando identifying soil mesofauna (left) Leica microscope (right) (Zhang, Y., 2019. [photograph]).

Revealing what you can’t see!

We did further chemical analysis of the soil to see how soil microbes (bacteria and fungi) respond to changes in litter management.

Phospholipid Fatty acids (PLFAs) and Neutral Fatty Acids (NLFAs) are key components of microbe’s skin. They can serve as unique indicators for different microbial classes, such as soil bacteria and fungi. With this analysis we can see the microscopic soil community.

After a complex removal method, with different organic solvents, we used a Gas Chromatograph (GC) to detect the different lipids in our soil samples.

Image: Gas chromatograph with soil samples (Ferrando, N., 2019. [photograph]).

What Next?

After a temporary delay due to the Coronavirus Pandemic restrictions, the lab work is almost finished.

We are continuing to share our work with other researchers and the wider community, with Nerea due to present at the Engaging Sustainability Joint DTP Conference 2020 and Treescapes 2021 conference.

Once all the results are in and the data has been analysed, we plan to publish them in peer-reviewed papers within the scientific community. Published papers will underpin key insights and recommendations for policymakers and urban planners. We hope that this will help improve the management of soil and trees, making them a more viable way to help mitigate the effects of climate change and flood risk within our cities.

As well as this, we are also interested in engaging the general public in our scientific research, sharing our work on social media to reach a broader audience – you can follow Nerea on Twitter, @FerrandoNerea, for latest updates.

Further information about our project

Our research project is investigating the impact of removing litter, compared to leaving litter in situ, on soil’s physical, chemical and biological properties in three urban parks in the UK and France. This is part of the Climate-Proof Cities programme, an initiative created by Earthwatch. It is bringing together leading researchers and citizen scientist to address the knowledge gap in soil and tree management practices, as well as other nature-based solutions.

Experts from University of Reading, Imperial College London, INRA France and CNRS are investigating interlinkages between tree health and productivity of Tilia Europaea species (common lime) and soil health.

Nerea Ferrando has been collaborating with two researchers from the Soil and Ecosystem Ecology Group at the University of Manchester to support her PhD research. They are assessing abundance, activity and diversity of soil biota to capture their response to soil litter removal at our UK sites: Cannon Hill (Birmingham) and Kew Garden (London). This collaboration will be key for understanding complex interlinkages between soil properties to support the development of effective management strategies for urban parks and the delivery of key regulating ecosystem services.

Further information about our project

Nerea Ferrando is a PhD student at the University of Reading, she is collaborating with Earthwatch as part of the Climate-Proof Cities project, exploring the role of Citizen Science for advances in urban soil management and inspiring sustainable action.

Dr Irene Cordero is a research fellow at the University of Manchester. She mainly focuses her research on the soil microbial communities and how different perturbations (particularly drought and land management) affect them.

Dr Mathilde Chomel is a research associate in the Department of Earth and Environmental Sciences, University of Manchester, who specialises in plant-fauna interactions. Currently, she is working in a NERC funded project: Disentangling mechanisms of co-adaptation between trees and soil food webs in response to environmental perturbations.

Prof Joanna Clark is a professor on environmental science at the University of Reading, working on carbon and water cycles on land and how they are affected by management, climate change and pollution. She is the lead researcher at the University of Reading for the Earthwatch Climate Proof Cities project.