Microplastics: In Our Soils Too?

By Dr Alison Blyth & David Evans

Most environmentally conscious citizens are already well aware of the problems microplastics create for our waterways and oceans. But what we think we know about microplastics now might only be the tip of the iceberg.

New research is uncovering the impact microplastics may be having on our terrestrial environments and food chains.

What we know:

Microplastics are ubiquitous in the oceans, and have been found in 73% of North Atlantic fish studied.  Detectable amounts have been found in tap-water.

What we are learning:

Microplastics are not just a threat to our oceans. They are finding their way into our soils and may be working their way up the terrestrial food chain as well.

Our Growing Plastic Predicament

Plastic pollution is one of the greatest environmental challenges we face. Plastic production has skyrocketed since 1950 and it is estimated that by 2050 there will be more plastic in our oceans than fish.

Since plastic does not decompose naturally, most of the plastic we have produced to date is building up in our landfills or our natural environment. Only 12% of plastics have been disposed of by incineration, while a shockingly low 9% are recycled. While plastic pollution from everyday plastics like disposable cups or bags are an eyesore and pose their fair share of environmental issues, microplastics pose a more challenging threat.

Microplastics are classified into two groups:

Primary microplastics are things like plastic beads used in the cosmetic industry.

Secondary microplastics are fragments that have broken down from larger objects – although plastics don’t rot away like organic materials, they do physically break into smaller pieces with exposure to light and weathering.

Both of these microplastics share a common threat: the smaller plastics are, the more difficult they are to identify, track, and clean up.

Steps are now being taken to ban sources of primary microplastics. Multiple countries including Canada, the UK, Australia, New Zealand, and Italy, have banned microbeads for cosmetic and personal care use, while many major companies such as Unilever, Proctor & Gamble, L’Oreal have pledged to phase them out. However, secondary particles are an inevitable by-product of the plastic pollution in our environments, and will continue to be created for as long as plastic pollution exists

Research on microplastics is a growing field, with multiple groups working on these pollutants in the ocean, and increasingly in freshwater contexts. Until now little consideration has been given to the accumulation of microplastics on land, and especially in our soils, but this is an area of concern that we need to pay more attention to.

Microplastics in Our Soils

Is our farmland at risk of microplastic pollution?

The answer lies in two agricultural practices: the use of plastic mulches and the application of sewage sludge to fields.

Plastic Mulches

Plastic mulching is the process of using plastic sheeting to cover soil, which improves temperature control and water loss, as well as supressing weed growth. It is a cheap, low tech way to manage the soil, and has become extremely popular across the world.

Unfortunately, it also means that a large amount of plastic is left lying in the open, exposed to UV and weathering. Even with responsible disposal at the end of the mulch’s lifetime (which is often only one year), exposure to sunlight and the elements means that plastic sheeting can become fragile and start to break up into microplastic fragments.

Sewage Sludge

Whereas plastic mulch as a source of microplastics is self-evident, the risks of using sewage sludge as a fertiliser might not be. How does applying waste organic matter to the soil lead to microplastic accumulation?

The issue lies in how waste-water treatment works.

Many microplastics, including ubiquitous fibres from synthetic clothing, enter the environment via domestic and industrial waste-water streams. In a treatment plant, these streams are filtered, removing most microplastics and other solid contaminants (although there is a question mark about how effectively fibres and the smallest microplastics are filtered out).

However, when microplastics are filtered out of the water in a treatment plant, they are accumulated into the sludge. This is the same sludge that is then applied to farmlands as a biosolid fertiliser.

Through this process we are effectively removing microplastics from the aquatic environment, and dumping them back onto our soils.

The extent of this problem depends on biosolid use in different countries. Some countries, such as Switzerland, ban the use of human biosolids as fertilizer due to concerns about pathogens and heavy metal contamination. Other countries, including much of the EU actively encourage the use of properly treated sludge as agricultural fertilizer, and cite strict regulations governing treatment and potential pollutants to demonstrate its safety and eco-credentials.

None of these regulations consider microplastic pollution.

As this is an under-researched area, the fate of microplastics deposited in our soils, and their implications for the terrestrial environment are largely unknown. We don’t even know at this stage what volume of microplastics are being deposited on agricultural soils, although one study estimated that between 63,000 and 430,000 tons of microplastics are added annually to European farmlands ( tons for North America).

Plastics in Our Food Chain?

An obvious question about microplastics in soils is where do they go next?

This is exactly what scientists are trying to find out. In a Frontiers paper in 2017, soil biologist Professor Matthias Rillig suggested that land management practices, soil composition, climate, and soil biology will all influence microplastic fate.

The chemical composition and physical characteristics of microplastics will also play a role. One issue identified is that experimental research to date has focused on microplastic beads. However, with an increasing number of countries banning the use of these beads, the interaction of other types of microplastics with environmental factors is likely more important.

Microplastic fibres are now believed to be one of the most common forms of microplastic pollution, and these will respond differently within soils than do microplastic beads.

In particular, can they enter the food chain, water resources, or animals?

Research on microplastics in aquatic environments does give us some clues. We do know that as soon as microplastics enter the environment, they start to interact with the surrounding chemistry and biology, and build up what scientists have named an ecocorona.

This is where the plastic particle becomes a nucleus for a film of organic matter, chemical compounds, and microbial growth. A key implication of this is that the presence of an ecocorona increases the likelihood that plastic will be eaten by animals – from the point of view of a scavenger or grazer in the water or soil, it is not a piece of inedible rubbish, but a ball of food.

In soil environments, research has already shown that microplastics are ingested by earthworms, reducing their biological fitness and lifespan. In a 2017 paper in Nature Scientific Reports, researchers from Mexico showed that microplastic particles increased in number up the food chain from soil to earthworms to chickens. These initial findings suggest that more research is urgently needed to find out the extent of microplastic contamination in our farmlands and their animals.

Microplastics – an unfolding mystery

Microplastics are a known threat to our oceans but we are just not uncovering their potential threat to terrestrial environments.

Recent research has shown that microplastics are affecting animals even in the remotest corners of our planet. Now we must also investigate their impact much closer to home. Is it possible that microplastics will find their way back to us via our own farms?

 

Dr Alison Blyth is a geochemist working in earth and environmental sciences.

David Evans is an environmentalist and the co-founder of Tern Goods –A reusable alternatives company specializing in recycled and repurposed shopping bags invented to replace single use plastics.

 

Image Credit 1) ensoplastics.com 2) downtoearth.org