Microplastics could pose a threat to plants

There’s been plenty of media attention on the threat plastics, especially microplastics, pose to animal life. A paper in New Phytologist suggests that there’s more to this. Plants might be suffering from plastic in the environment too.

Rillig and colleagues have outlined a series of threats plastic poses to plant life. At the moment the risks are unproven. This is because it’s so much easier to spot plastic in aquatic systems than soil. The authors point out that particles of plastic might not seem important when soil is so particle-rich. However, they cite plenty of reports showing microplastics in many terrestrial ecosystems.

The team argue that not all microplastics will have the same effect. Beads are a problem in the food chain in marine environments. But Rillig and colleagues only see this as a potential minor effect in soil in terrestrial environments. Fibres might even boost plant growth in changing soil density. Not all effects are expected to be positive though. These same changes in density could affect the microbe community. It’s unknown if the physical effects of microfibres will outweigh the chemical and biological effects on plant symbionts.

The relative importance of these pathways is expected to differ as a function of microplastic types
Concept diagram showing the various mechanisms via which microplastic particles could affect plant growth. Source: Rillig et al. 2019.

Plastics can change the soil chemistry. Films could increase water evaporation, drying out the soil. Plastic surfaces could allow toxic substances to accumulate in ways that they couldn’t in organic soil. Interestingly, one of the big problems the authors highlight are biodegradable plastics.

If you thought biodegradable plastic was emphatically good, you might want to think again. Large biodegradable plastics breaking down become microplastics and food for microbes. Rillig and colleagues say these are a rich source of carbon. This is good for microbes, which need large amounts of carbon to build cells. They don’t just need carbon though. They also need other nutrients like nitrogen. If a plastic feast provides the carbon, but not enough other elements, then the microbes grab this from elsewhere in the soil. This leads to ‘Nutrient immobilization‘. When microbes grab the nutrients, they’re no longer available for plants to use.

Rillig and colleagues also point out that breaking down microplastics will create nanoplastics. When plastic particles are smaller, there’s a greater chance of uptake by roots. Will these nanoparticles be toxic? The authors say “The existence of nanoplastic particles in soil has never been demonstrated, since current extraction and quantification methods either miss them, or do not deliver any size information. However, it seems quite likely that nanoplastics are present in the environment, if microplastic particles fragment into smaller pieces.” The team also cite work in the lab showing that microplastics become nanoplastics as they break down. It seems unlikely this doesn’t also happen in the wild.

The only possible gap in the paper I see is there’s no discussion of thermal properties of microplastics. I’m not sure if this is an issue or not. To some extent, the differences in the thermal properties of soil are likely to be caused by physical effects anyway.

Microplastics as trigger of combined physical or chemical‐like effects.
Microplastics as trigger of combined physical or chemical‐like effects. Source: Machado et al. 2018.

In another recent paper, Rillig and Machado working with other colleagues concluded: “In isolation, microplastics might not be the single most toxic (lethal or sublethal) environmental contaminant. However, there are consistent past, present, and future trends of increasing a near‐permanent plastic contamination of natural environments at global scale.” As microplastics beget nanoplastics, that will mean there’s a long-term problem of contaminants in the soil. The difficulty of finding and quantifying that threat means that microplastics aren’t likely to be a solved problem in botany any time soon. The paper is an important reminder that it is rare for problems to simply vanish.