Soil scientist Associate Professor Matt Tighe and PhD student Nicola Forster, from the University of New England (UNE), are venturing where no-one has gone before - investigating the impact of shoe abrasion and microplastics on wilderness trails. They want to reveal if the growing popularity of recreation in nature reserves is slowly putting these vulnerable environments, their soils, plants and animals at risk.
"Much of the research on microplastics concerns marine and water systems or, when it comes to soil, agricultural systems," says Matt. "But over the past 10 years there's been more than a 1600% increase globally in the number of people participating in organised events, mostly on wilderness trails, which amounts to millions of people. We are investigating whether the microplastic signature that shoes leave in these environments is significant enough to contaminate them."
The idea for the research arose during one of Matt's regular runs, when he pondered the wear of his own running shoes. "I was wondering where these micron-sized particles go, how many are out there and whether they are influencing or interfering with soil processes," he says.
Everything from water infiltration to erosion and root growth depends on tiny holes in the soil. Plastic particles can clog the holes, limit aeration and nutrient cycling and, once they begin to degrade, leach chemicals. Only by understanding the abundance, types and distribution of microplastics can we evaluate their effect on soil systems, microbial communities and organisms, and vegetation.
"The soles of shoes are designed to be tough and to last," Matt says. "Once in the soil these microplastics are persistent and are likely to affect soil structure, water flow, biology and chemistry. In wilderness areas, endemic and vulnerable plants and endangered organisms may be at risk."
A growing body of toxicological research has revealed that acute exposure to microplastics has adverse effects on the neurological, endocrine, immune, reproductive and digestive systems of organisms. Similarly, it can affect plant biomass, tissue composition and root systems.
However, somewhat surprisingly, the impacts of recreational visitors and their microplastic-shedding footwear on wilderness areas has not been investigated before - something Matt and Nicola highlight in an upcoming issue of Geoderma, one of the world's premier soil journals.
They are now busy trialling new techniques for sampling soils for microplastics, and about to begin "dosing up" plants in the UNE glasshouses with different levels of microplastics to test root responses. The pair will also assess erosion and the movement of plastic particles during simulated storms.
"In some places the signatures of plastics seem to be quite high, but we are trying to tease out how much is from one major recreational event (like an organised trail run) and how much is from long-term deposition," Matt says."In the grand scheme of things, this stuff is not as widespread as plastics in the ocean, but I get the feeling it could be one of those insidious issues that we don't really notice until we start to see the effects of it."
Microplastics vary in size from 1 micron to 5 millimetres. They also vary in shape (from linear microfibres to microbeads) and chemical composition (shoe soles, for instance, contain rubber and plastic, zinc and other trace elements, and a variety of organic and inorganic additives).
Outdoor recreation is increasingly popular in nature reserves, wilderness and ecologically protected areas. Several thousand people can participate in organised trail running events that extend deep into wilderness environments, like the popular Ultra-Trail Australia.
"As recreation in wilderness areas increases, microplastic accumulation in environmentally sensitive areas is likely to accelerate unless we introduce guidelines to ensure the recreation is sustainable," Matt says. "I would also love to see footwear companies make a more concerted effort to develop shoes made of degradable or more abrasion-resistant materials."
The Danish Environmental Protection Agency estimated the total annual abrasion from shoe soles in 2015 in Denmark at between 100 and 1000 tonnes, and a German study calculated that shoe sole wear and tear (109 grams per capita per annum) was the seventh biggest contributor of microplastics in that country.
With many wilderness events cancelled in Australia in recent months due to COVID restrictions, Matt and Nicola hope to test their methods before and after this year's Duval Dam Buster Trail Run in Armidale, scheduled for September.
So what if they find microplastics are accumulating in Australia's wilderness areas?
"We may need to make management or monitoring recommendations for areas that are most susceptible and vulnerable," Matt says.