Microplastics—tiny plastic beads, fragments, and fibers released from personal care products, plastic waste, and laundering of synthetic fabrics—have long been understood to contaminate the ocean. Only recently have scientists learned they’re also prevalent in the soils human society depends on.

In our latest Insight, Maxwell Helmberger of Michigan State University talks about his new Review paper: Towards an ecology of soil microplastics, and what led him to write it.

What is the background behind your paper?

For as long as microplastics have been a topic of scientific research and public concern, they’ve been primarily thought of as a problem for oceans and ocean life. Within the last several years, however, we’ve realized they’re no less ubiquitous in fresh water, soil, and even the air. Microplastic research in soil is still new to the scene; the first paper addressing the topic was published in 2012 (Rillig, 2012, Microplastics in terrestrial ecosystems and the soil. Environ. Sci. Technol.) and the first primary research articles came out in 2016.

How did you come up with the idea for it?

This paper started as the literature review for my dissertation. When coming up with my research proposal (which at my institution is a formal document you have to defend to your committee), I had to consider the types of questions I wanted to investigate. There’s been a fair bit of work surveying different kinds of soils around the world, developing methods to better extract and identify microplastics from soil, and ecotoxicological assays testing the effects of microplastic exposure on important soil organisms like earthworms, springtails, and even microbes. But I’m not an analytical soil chemist or ecotoxicologist. The people doing that work now are doing a much better job of it than I could ever hope to do. I have neither the ability nor desire to quantify oxidative stress in a nematode’s intestines, so I’ll leave that work to those who do. Both my interests and past research experience lie in more explicitly ecological questions, in how species respond to and interact with their abiotic and biotic environment. Thus, while combing through the soil microplastic literature early in my degree program, I was primed to pull out these kinds of questions, such as how microbial colonization of microplastics affects their palatability to animal detritivores. I realized that the review I was writing could make for a valuable publication if I also framed it as a perspectives piece on what I saw as missing from the soil microplastic literature.

We don’t know if and to what extent microplastics will harm terrestrial biota. There are too few ecotoxicological studies with too many methodological differences between them for us to generalize about microplastics’ effects on soil organisms with any confidence.

About the research

What is the relevance of microplastic pollution to the terrestrial environment?

At the moment, uncertain. We don’t know if and to what extent microplastics will harm terrestrial biota. There are too few ecotoxicological studies with too many methodological differences between them for us to generalize about microplastics’ effects on soil organisms with any confidence. Also, until we’ve spent more time surveying soils around the world and characterizing the microplastics within, we won’t know what kinds of microplastics (shape, size, polymer composition, etc.) will be most realistic to include in these ecotoxicological assays either. Regardless of the uncertainty, we need to further understand and address this issue so that any negative effects at least don’t catch us by surprise.

What are the challenges associated with studying microplastics in soil?

Several reviews have touched on this topic. Microplastics, especially the really small ones, are always challenging to extract and/or identify from environmental samples. New soil microplastic methods are being developed and published on, but since soil microplastic research is relatively new, there are currently no standardized methods to quantify their abundance and describe their size, shape, and polymer composition. Many methods require expensive laboratory equipment and each comes with distinct downsides. The one I’m working on to detect microplastics in terrestrial invertebrates is no exception!

Aboveground, we have the classic example of MacArthur’s warblers foraging on different parts of the same spruce tree. A dynamic like that could easily exist for, say, predatory mites on and inside large soil aggregates, especially given their high species richness, but good luck ever observing that in situ

Beyond microplastics, ecological questions in general are usually harder to answer in soil than in other systems, and so we know less about basic ecological interactions and processes in soil than we do aboveground. Consider something like niche partitioning. Aboveground, we have the classic example of MacArthur’s warblers foraging on different parts of the same spruce tree. A dynamic like that could easily exist for, say, predatory mites on and inside large soil aggregates, especially given their high species richness, but good luck ever observing that in situ. Soil is obviously a cryptic environment, but even if it wasn’t, the small size and difficult taxonomy of many key soil organisms makes it hard to observe the system without disturbing it in the process.

About the author

How did you get involved in ecology?

I grew up an only child in an off-the-grid log cabin in the northern Minnesota woods, so it shouldn’t surprise anyone that the natural world has always interested me. Turning over rocks was one of my favourite pastimes as a kid. As I started college, I initially wanted to be a wildlife biologist, but after taking a soil science class, working in an entomology lab, and delving into the soil ecology literature, I decided I wanted to get into soil ecology instead.

What are you currently working on?

I’m currently developing a method to detect microplastic presence in terrestrial invertebrates and their faecal material, a necessary first step for my future research. I plan to use it to both survey for microplastics in field-collected soil animals and answer in the lab some of the questions brought up in my paper. I’m also engaged in science communication, having created a series of education clay animation videos on soil ecology and entomology topics. I’ve also designed an educational card game on the roles of soil organisms in maintaining soil health, which I’m in the planning stage of formally evaluating in a classroom setting.

What do you do in your spare time?

Creative writing is my main hobby. I’m over 200,000 words into a fantasy fiction novel that I’ve been working on in some form since age 12, with many overhauls and redrafts. I also enjoy hiking, insect collecting, and strategy games.

You can read the paper in full here.