Potential Impacts of Microplastics in Agricultural Soils with Dr. Yingxue Yu

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Puyallup Research & Extension Center
WSU Dept. of Crop and Soil Sciences

Contact Information

Dr. Yingxue Yu, yingxue.yu@wsu.edu

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Episode Transcription


Drew Lyon: Hello, welcome to the WSU Wheat Beat podcast. I’m your host, Drew Lyon, and I want to thank you for joining me as we explore the world of small grains production and research at Washington State University. In each episode, I speak with researchers from WSU and the USDA-ARS to provide you with insights into the latest research on wheat and barley production.

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My guest today is Dr. Yingxue Yu. Yingxue is a post-doc in the Department of Crop and Soil Sciences at Washington State University. She is located at the Puyallup Research and Extension Center in western Washington. Yingxue has a B.S. in hydrology and water resources from Shandong University of Science and Technology, and an M.S. in hydrogeology from China University of Geosciences.

She has recently completed her Ph.D. in soil science with emphasis on soil physics and vadose zone hydrology, under the supervision of Dr. Markus Flury. Her research focuses on fate and transport of micro- and nanoparticles in the terrestrial environment. Recently, she has been studying the environmental impacts of plastic pollution on terrestrial ecosystems, specifically how microplastics affect soil ecosystems’ functioning.

Hello, Yingxue.

Dr. Yingxue Yu: Hello, Drew.

Drew Lyon: Welcome to the WSU Wheat Beat podcast. I’m wondering what is the current understanding of microplastics in agricultural soils?

Dr. Yingxue Yu: That’s a very good question. So, [if] we want to understand microplastics in agricultural soil, I think we need to clear [up] what are microplastics. I want to give you a simple definition. So, in the science community, microplastics are commonly defined as plastic particles with size less than five millimeters. Which means most of them are invisible to our naked eyes. Our eyes can see up to 50 micrometers in size without any help. So, as you can see, microplastics are mostly invisible.

Microplastics can form during the breakdown of larger plastic pieces because these plastics eventually would undergo weathering and become fragmented. Or they can be manufactured into micrometer size. For instance, we have some plastic beads in our personal care products or plastic resin pellets as the stock for plastic products.

Now we are clear [as to] what are microplastics. And then we can think what are the sources of microplastics in agricultural soils.

Microplastics in agriculture mostly come from the application of biosolids, compost, [and] plastic mulching. For instance, biosolids have been estimated to introduce around 50,000 to 300,000 tons of microplastics per year in North America. And with that source, we can roughly estimate the concentration of microplastics in agricultural soils. And based on current reports, the reported microplastic concentration in terms of mass ranges from less than 0.001 to 0.1% weight by weight. And then when all these microplastics in soil–they are going to do something to the soil–because they are not natural stuff or natural substances in the soil, so there will be some potential impacts on microplastics.

These potential impacts include–they can affect soil physical, chemical, or biological properties, which can translate into crop performance, such as germination rate or biomass accumulation. Microplastics can also interact with terrestrial animals such as earthworms because they come to earthworm surfaces or be ingested by earthworms. Earthworms can also drag them around in soil, redistribute them. Microplastics can also impact plants because they can be accumulating around root surfaces or potentially be taken up by plants.

And these all impacts are concerning because agricultural soil is so important for our food production, and if microplastics negatively affect soil properties and negatively affect plant performance, then we would have some concerns about food security and safety.

Drew Lyon: Alright. So there’s a lot of potential negative consequences of microplastics in the agricultural soils, not to mention other environments. What are the major findings of your study on the effects of microplastics and on, say, soil physical properties? And what are the implications of those findings?

Dr. Yingxue Yu: Thank you for that question, it’s a very good one. Because of these concerns though, now we are looking at what microplastics would do to soils’ physical properties. So in our study we measured the response of several physical properties, including bulk density, hydraulic conductivity, [and] water holding capacity. What we did was to intentionally mix microplastics into the agricultural soil and then making all the measurements.

We did see that microplastics can decrease bulk density because microplastics make the soil fluffier and loosely packed, which means it could be a good thing for root penetration because the soil becomes fluffier. Because of the loose structure, we also see an increase in hydraulic conductivity. Hydraulic conductivity basically is how water moves through the soil, how fast it can move through the soil. And then with the increased hydraulic conductivity, we can see that water can filtrate much faster into the soil and the soil can be doing better with microplastics in there.

However, the loose structure caused by microplastics also means less water holding capacity. That’s a problematic thing for plants because less water is not good for plant growth. So overall we see that microplastics can affect soil physical properties.

But I also want to stress that these changes are only observed under very high plastic concentrations. These concentrations have never been reported so far in agricultural soils.

And then back to the sources of these microplastics–biosolids, compost, plastic mulchings–these practices are all beneficial for our food production. So overall, we can see that these practices are still beneficial and their benefits are not compromised by microplastics coming from these sources.

Drew Lyon: Okay, so actually not all bad and possibly to some extent good. How far short of–what are you finding in the soils just naturally versus what they need to get to before we start seeing these negative effects? Are we a long ways away or is it is it something that needs to be closely monitored because it could we could reach those levels in a few years?

Dr. Yingxue Yu: Actually, that’s a very good question. That’s like our point in this study. As I mentioned, we need to look into the sources of microplastics and currently agricultural soils receive plastics from biosolids, compost, and plastic mulches, and these sources are not high enough in microplastic concentration to trigger the effects that we can see from our study, which means we’re still a long way from seeing negative impacts in the field.

And then biosolids [and] compost, they are good amendments for soil. We see a lot of benefits coming out from them. But the problem is we are continually putting microplastics into the soil. If we don’t do anything in the future because we see benefits from biosolids—we continually apply biosolids into the soil–and that would mean an increasing concentration of microplastics in the soil.

And you mentioning we may reach the concentration where we can see these negative impacts. So although we haven’t seen these negative impacts yet, but eventually we would if we don’t do anything to curb microplastic input into the soil.

Drew Lyon: Okay, so, what other questions are still unresolved regarding microplastic pollution in agricultural soils?

Dr. Yingxue Yu: That’s a good question too, because as I mentioned before, microplastics can have different negative impacts, but these impacts are dependent on their properties, including their size, shape, concentration, polymer type, etc. We still don’t know how these properties of microplastics can evolve in soil because microplastics can interact with different substances in soils, so their properties are very dynamic and changing all the time.

How fast they can change and what would these changes mean for the overall microplastics impacts on the environment is still unknown.

Drew Lyon: Okay, so still much to learn there. I don’t think microplastics are going away any time soon, so plenty of opportunity just to try to figure all that out. Do you have any suggestions on how to deal with microplastics in agricultural soils?

Dr. Yingxue Yu: Yes. So I would say the best way to deal with microplastics in agricultural soils is to not introduce any microplastics into the soil because, as I mentioned, microplastics are so small, they are invisible and it’s very difficult to remove them once they are in the soil.

Then we need to think about how to curb the input. So the input includes biosolids, compost, [and] plastic mulchings. For bio solids, they come from wastewater treatment plants, so there we can come up with some filtration techniques to filter out microplastics from the biosolids. And then for the compost, because we know plastic waste is a problem in compost, we need better techniques to sort out plastic waste from compost feedstocks and to avoid any plastic waste going into the compost. With plastic mulchings we can use biodegradable plastic mulchings because biodegradable plastic mulchings are designed to undergo soil degradation in years–then there will be no plastic residues left after the degradation period is over. Then we don’t introduce any microplastics into the soil.

Drew Lyon: Okay, so the “don’t get it in the beginning and then you don’t have to deal with it” is kind of the message.

Dr. Yingxue Yu: Yes, exactly. Because once they are there, it’s really difficult to get them out.

Drew Lyon: And I believe they last in the soil quite a long time, don’t they?

Dr. Yingxue Yu: Microplastics, they don’t degrade easily. So once they are there it probably will take centuries for them to go away because they are foreign substances for soil microbials. So microbials don’t know how to deal with them [so] that means they are persistent for long times in soils.

Drew Lyon: Okay, so your advice to not put it there in the first place is the best advice we have.

Dr. Yingxue Yu: Yes, exactly.

Drew Lyon: Yingxue, thank you for joining us today on the Wheat Beat podcast. Your work in microplastics is very interesting and I wish you well in future research to help us understand how we deal with this issue going forward.

Dr. Yingxue Yu: Thank you, Drew. Thank you for having me today. I’m very honored to be here.

Drew Lyon: Thanks for joining us and listening to the WSU Wheat Beat podcast. If you like what you hear don’t forget to subscribe and leave a review on iTunes or your favorite podcast app. If you have questions or topics you’d like to hear on future episodes, please email me at drew.lyon — that’s lyon@wsu.edu — (drew.lyon@wsu.edu). You can find us online at smallgrains.wsu.edu and on Facebook and Twitter @WSUSmallGrains. The WSU Wheat Beat podcast is a production of CAHNRS Communications and the College of Agricultural, Human, and Natural Resource Sciences at Washington State University.

I’m Drew Lyon, we’ll see you next time.


The views, thoughts, and opinions expressed by guests of this podcast are their own and does not imply Washington State University’s endorsement.