Podcast Transcript
[00:00:00] Nataliya Shcherbatyuk: Hello and welcome to the Mulch Matters podcast where we will explore the intriguing world of mulch and its impact on agriculture and the environment, as well as update you on the latest research about soil-biodegradable mulch and recycling options for plastic mulch. I am your host, Dr. Nataliya Shcherbatyuk, and I am a Research Associate working on the project, “Improving end-of-life management of plastic mulch in strawberry system”. In each episode, we’ll dive into the latest research, trends, news, and insights on why mulch matters and how we can improve plastic mulch end-of-life options. We’ll also branch out and discuss other plastics as well as talk to researchers, experts, and practitioners in the field who will share their insights and experiences on how to use mulch effectively in different settings.
[00:01:02] Nataliya Shcherbatyuk: Welcome back to another episode of Mulch Matters Podcast. We will be talking about microplastics today with our guest, who is a PhD student at UC Davis, Amanda Wong. Hi, Amanda. How are you?
[00:01:20] Amanda Wong: I’m doing great. Thank you, Nataliya, for having me here today
[00:01:24] Nataliya Shcherbatyuk: Well, thank you so much for your time to be here with us today. And what I would like to ask first is tell us a little bit about yourself. Tell us who you are, what you do, your background. Yeah, let’s just go s- straight from there.
[00:01:42] Amanda Wong: Yeah. Thank you so much. Name is Amanda. I’m a PhD student at the University of California at Davis. I’m in the ecology program in the Department of Plant Sciences, and I’m currently researching microplastics in California agriculture and how that affects particularly lettuce. So, I’m originally from Hawaii, and having done beach cleanups and things like that with my friends in undergraduate at the University of Hawaiʻi at Mānoa, that got me to study microplastics, and the relationship to how that affects plants.
[00:02:17] Nataliya Shcherbatyuk: That’s so exciting. I’m just—while you’re talking about Hawaii, I’m imagining, you know. It’s a dream of everybody probably.
[00:02:26] Amanda Wong: Yes.
[00:02:26] Nataliya Shcherbatyuk: Okay. So, let’s talk about microplastics. Can you explain a little bit about what microplastics are, and why they actually became such a concern in agriculture environment?
[00:02:48] Amanda Wong: Right. So, you might have heard microplastics in the news and things like that. So, microplastics are actually quite small. They’re usually one micron to five millimeters in size—so, think of, like, a rice grain—and smaller microplastics come from larger plastic items that then break apart, uh, and form, um, even smaller and smaller and smaller plastics.
Um, so these pollutants aren’t new, but they’re around now because we see that they persist in the environment, and they don’t go away. So not only are there larger physical pollution or litter, if you will, um, they are very diverse or different in there, how they look as well as their chemical properties. So, you not only have different sizes, shapes, but also chemical makeup. So, what their main chemical symbol, like what you see on recycled plastics, you have numbers one through seven in a triangle. So that’s their main chemical backbone. But they also contain a diversity of other additives such as dyes, BPA, and other things that aren’t disclosed to us as consumers. Um, so just the diversity of it makes it a real, um, concern and issue in the environment.
But in terms of why this might be a concern is that, you know, we use plastics in agriculture, and there’s not really a good way of recycling that. Um, and these plastics in agriculture, you know, when they break down from sun exposure, um, they can also release those chemicals within these plastics, as well as transporting other chemicals, uh, such as pesticides or fumigates, things like that. And that can have lasting effects on how the soil health, uh, and potentially have impacts on plant health as well. Um, and then when these plastics get even smaller, they can, um, be absorbed by the plants, and this is kind of where the research is right now, but trying to see how the microplastics are affecting the plants to then potentially affect crop growth and yield, and what potential impacts, that is currently being researched on how these microplastics might affect human health.
[00:05:15] Nataliya Shcherbatyuk: That’s definitely the concerning topic that you’re talking about, and also quite fascinating, I would say. You’re talking about the plant’s reaction and absorption of microplastic. And I also wanted to ask you what actually motivated you to focus specifically on microplastics in soil and their impact on food crops?
[00:05:38] Amanda Wong: Right. So going back to my time in Hawaii. So I was at the University of Hawaiʻi at Mānoa for my undergraduate degree, and I did a very broad, undergraduate program in global environmental science, and that gave me the opportunity to explore my passion for plants. Being within the oceanography department, I had a lot of friends that were interested in marine biology and coral and fish, so we’d do a lot of beach cleanups, as I previously mentioned. But this one particular trip with the YMCA to, umm, a more rural area, um, if you will. So this beach was called Moʻomomi Beach, and this is a protected place that regular people can’t go and access because this is land under the Nature Conservancy.
So, with this YMCA group that I went with we are basically the goal was to pick up plastic pollution. And we weren’t only talking about microplastics. This whole beach was covered with plastics. It looked like confetti, if you will. There were toilet seats tumbling in the waves. Um, there were also just large plastic baskets, bottles, bags, basically anything you can imagine, and this really inspired me because I was really interested in how these coastal plants were affected by these microplastics on this beach. And at that time—this is a little bit before the pandemic—a lot of the research on plants and microplastics was done in Europe and Asia and really not so much done in United States. So, what got me thinking about what the connection between plastics, plants and humans is, and that led me to agriculture. So that led me to apply to graduate school and be interested in how those three things connect, and that led me here to UC Davis to study how plants and microplastics interact in our environment in California agriculture.
[00:07:41] Nataliya Shcherbatyuk: And Amanda, you wrote a review where you look at over 50 studies, correct?
[00:07:47] Amanda Wong: Yes.
[00:07:48] Nataliya Shcherbatyuk: Okay. That, well, first of all, that’s impressive. There are a lot of studies to cover. And I wanted to ask you, based on this review, what are the most, I would say, consistent effects that you saw, uh, from those studies of microplastics? What kind of effect do they have on plant growth and development?
[00:08:09] Amanda Wong: Yeah, that’s a great question, so we saw that there were mostly germination and some more long-term experiments being two to four weeks. And at least for germination several studies saw that there was a delay and even reduced germination overall. And this is because microplastics can block the seeds, as- also preventing water uptake that’s delaying germination. In the more long-term studies where the plants are able to grow out more, they have found signs of oxidative stress, as well as reduced chlorophyll content, photosynthesis, biomass as well shoot and root biomass. So really these microplastics are showing not only effects in germination, but more long-term as well. Um, but again, these studies are quite on the short term and not really looking at when these plants are flowering and growing fruit, if you will.
[00:09:12] Nataliya Shcherbatyuk: I also read that it mentioned something about stimulatory effect at lower concentration of microplastic. Can you expand it a little bit? What do these positive responses look like and why they might occur?
[00:09:28] Amanda Wong: Yeah. So, this is probably quite shocking to you, but in the field of toxicology, this is kind of a known response. It’s called hormesis, so basically for toxicants at low dose, there might be a beneficial effect or a stimulation effect, but when you get up to a higher dose, this becomes toxic to the plant. So, in terms of, like, positive responses, as I mentioned to the contrary, some studies have seen faster or more germination, but usually this, um, manifests as increased biomass. Um, so this one study in wheat seedlings, done in 2020 using a hydroponic system, they didn’t find any differences in the germination of the wheat seedlings, but they did find that there was increased root elongation, in these wheat seedlings that were exposed to microplastics. And they attributed this to something called a nano-priming effect.
So those small, uh, microplastics or nanoplastics in this case, they stimulated, I guess, a slight stress response that got these, uh, seedlings to, um, grow faster, if you will. Um, and then you… in terms of other positive responses, so in soil that’s probably more degraded or compacted, um, microplastics could improve soil conditions. So being that plastics are lighter and less dense, um, these microplastics can, um, decrease the soil bulk density and as well as increase porosity and aeration, and also have effects on soil microbial communities, um, by altering, um, who’s there, as well as potentially affecting soil nutrient cycling and water dynamics. So at least for this one study in Germany using wild carrots, they saw that these microplastics, um, increase, um, root and shoot biomass, but, um, decrease soil aggregation and microbial activity.
So, the positive might be that the shoots and roots of these wild carrots increase, but I don’t think it’s really great that microbial activity changes. Um, so even though there are short-term positive effects in these experiments, that doesn’t say that, you know, these microplastics should be applied into our agricultural systems or anything like that. Right. Um, but, you know, these are experiments that are done in a controlled setting. So, we’re really trying to figure out, you know, with these short-term experiments, maybe at certain concentrations there are stimulation effects, but what does that actually look like for long-term effects and how does that affect us as humans?
[00:12:21] Nataliya Shcherbatyuk: And you also noted that monocots, for example, may be less sensitive to microplastic if you compare that with dicots. So, what kind of evidence support this, and what might explain the difference?
[00:12:37] Amanda Wong: Yeah, so that’s a great question as well. Um, so, you know, monocots and dicots are different in their overall plant growth and form as well. So, this one study from 2024 looked at four different species. They looked at two monocots, so barley and wheat, and two dicots, so carrot and lettuce. And they looked at several different concentrations of polyethylene microplastics expose, exposing these four different plants to these microplastics. And they found that there was decreased lettuce root length as well as decreased carrot shoot length. But there was no difference in the monocots, so wheat and barley. But this was an acute study where they only exposed these plants for a very short time through germination and initial growth after that. But they attributed these effects to having smaller seeds for dicots compared to monocots.
So, when you have a smaller seed, you have a higher surface area to volume ratio, so more surface area for these microplastics to interact with the seed. Um, so that’s what they attributed this to. But there are a growing number of studies looking at comparing monocots and dicots. So, this one study actually that was published last month looked at 100 different studies in a meta-analysis. So, taking all the data from these 100 studies and bringing them together to see what the effects were across these studies. And they saw that there was a lower root dry biomass and length in monocots. So, this is opposite to what your question was from the review. They also found lower shoot fresh biomass in dicots than monocots. So overall, they found that there was a negative effect of microplastics on plants across these 100 studies. But being that monocots and dicots are a vast group of plants, you know, they all have their own individual differences. So really we need to have more studies to figure out what, what’s actually going on.
[00:14:53] Nataliya Shcherbatyuk: Yeah, definitely more studies would be helpful, and also more studies on the long term. That’s missing gap. And let’s talk a little bit about the project you’ve done in greenhouse experiments with what you call realistic microplastics. And, well, first of all, tell us what it is what you did, how you did it, and why using these realistic types of microplastic is important for research.
[00:15:20] Amanda Wong: Yeah, that’s a great question. So, I just completed a greenhouse experiment here at UC Davis, and through this experiment, I collected plastic mulch film that has been sitting out in the school’s agricultural fields. And I brought those back to the lab, washed them off, cut them into smaller pieces and then ground them up into a powder, if you will. And I used this powder of microplastic mulch and put them into these pots. And then, I grew lettuce from them to see how different types of lettuce respond to exposure from these microplastics.
So, this is quite different from other former studies because a lot of microplastics in plant studies, specifically looking at microplastic uptake, they’re using fluorescent nanoplastic beads at really high concentrations, that are not representative of the actual environmental conditions. So, you in nature you’re probably not gonna have fluorescence at all. And furthermore, you’re not gonna have perfect pristine beads, that these plants are exposed to.
So earlier in my PhD, I looked at microplastics in California agriculture. And at the highest end of these microplastics in California agricultural soil, we found a percentage of about 0.01% in California agricultural soil. So, we used the same concentration to conduct my realistic microplastics experiment with lettuce. So, we put 0.01% microplastics by weight into our potting mix, and then grow our lettuce with that. And having these more realistic microplastics experiments are very beneficial because we can actually see how these microplastics are currently affecting crops in today’s conditions. Um, a lot of these former experiments have been using 1%, 5%, and I even saw one experiment with 10%, and I really hope we don’t get to that anytime soon.
[00:17:31] Nataliya Shcherbatyuk: Well, you’re doing a great job, and it’s extremely interesting. So how much longer do you have in your PhD?
[00:17:38] Amanda Wong: Yeah, so I’m wrapping up my PhD. I’m writing and publishing, trying to figure out what my next steps are. Yeah. I’m really excited to see what the next part of my journey will be like.
[00:17:48] Nataliya Shcherbatyuk: Yeah, me too. And you were proposing a protocol which includes standards for microplastic experiments. Can you tell us more about this protocol, and what are the key principles of it?
[00:18:05] Amanda Wong: Yeah, so for this protocol that we outlined in the, this review paper as well, uh, we wanna try to, um, bring together research on this topic because, um, as I mentioned how not only the plastics are diverse in terms of their chemical makeup and size. So, um, but a lot of experiments are also maybe using different pristine or degraded plastics, maybe different sizes, um, types. Um, initial studies really looked at, um, using plastics from, like, scientific stores that they bought and used that in their study, and that’s not really realistic. So, we proposed to use different principles in our review, um, and there’s four of them.
So, the first one being using a standardized plastic mixture. So not only incorporating one size, but also maybe combining microplastics and nanoplastics of different sizes to be more representative of what we find in an environment. Um, and then as I mentioned as well, the second one being that we want to use more realistic concentrations. Um, so not only looking at, um, kind of what we might see in the future, but what are the current conditions now in the environment. So, like how I used 0.01% in California agricultural soil, but also looking at how those agricultural soils might differ from urban soils that would have more microplastics.
And the third principle for this is using toxicology, trying to have a dose response experiment. So, looking at different doses and exposing plants to certain concentrations so we can try to find at what point do these plants have a more shift from maybe their positive response to a negative response, or what is that inflection point where we see these negative effects? And then the fourth one being, as we’ve talked about through this whole podcast, is prioritizing more long-term experiments. So not only for several weeks, but months, if not to when these plants start to flower or their end of their life. And seeing as well as, um, in greenhouse studies, but more field-based studies as well. So just trying to be a little bit more realistic in our experiments and trying to see what’s currently going on now.
[00:20:35] Nataliya Shcherbatyuk: That’s interesting, and it’s very important. And if you don’t mind, can we come back to the study you’ve been also working on? It’s in California and microplastic, and can you tell us more about what you’ve done?
[00:20:48] Amanda Wong: Yes. So, this research was for my dissertation, and the kind of setting the stage for this experiment. So, for this project looking at microplastics in California agriculture we went across the Central Valley and sampled conventional and organic fields, as well as from crops containing tomatoes, almonds, and leafy greens being lettuce and basil, and a couple of other herbs. And we wanted to see if there’s differences in this organic versus conventional systems as well as different crops. So, tomatoes and leafy greens are one season, whereas almonds are long growth or they stay there for a long time. So, seeing if there’s differences in these different systems. And we’re working on publishing that information right now, so stay tuned to those results.
[00:21:42] Nataliya Shcherbatyuk: Fantastic, and speaking about publishing, I will add the link to the review in the podcast description and transcript. So, our audience will be available as well.
[00:21:53] Amanda Wong: Oh, thank you so much.
[00:21:55] Nataliya Shcherbatyuk: And I wanted to finish up with what I think is quite interesting but also important question, what do you think? For example, if you could recommend only one, but it’s a major shift in how the scientific and research communities’ approach microplastic research, what would it be?
[00:22:24] Amanda Wong: Oh, that’s a tough question. But if I had to pick one I know I would say long-term experiments. Definitely, you know, the theme of this whole podcast is not only going for there’s several weeks but definitely the longer term until some of these crops, you know, flower, fruit, or end up how, when, at what point, when us humans would interact with these certain food crops. So, like when the tomatoes start to be ripe or when the lettuce is ready to be harvest, things like that, so that we can try to bridge that gap between microplastics and agriculture and how that might translate to impacts on human health. So really seeing, how these plastics are affecting these crops when we’re ready to harvest them. That, I think that’s really where we need to focus our attention as the scientific community
[00:23:15] Nataliya Shcherbatyuk: Yeah, that’s quite interesting, and I think I would probably name it as a top as well if I would ask this question.
[00:23:24] Amanda Wong: Yeah.
[00:23:24] Nataliya Shcherbatyuk: Because I think that at this point in the long term, it’s the gap that’s missing here in United States for the research of microplastics, and it’s not only like you said in the beginning of the podcast, that plastic gets to the soil and it’s, it’s not going anywhere. It’s there. Some pieces are there. So, it’s not only what comes yearly with use in agricultural plastic and how it affects annual plants, but also the effect on the long term for perennial plants when they’ve been exposed to microplastic in the soil through not only one year and harvest, but also through many years. Because when we think about annual crops, you planted them in the soil conditions, you harvested it, and it’s, it’s gone, right? So, that’s it. But for the perennial, you planted it and it’s exposed to the soil conditions for years if it’s not for decades, and it’s exposed for soil changes, conditions, including whatever happens with the plastic in the soil and microplastic, and how that impacts what kind of impact it has on the plants through the decades. I think that would be very interesting. Hope we will be increasing research on the long term. Obviously it needs time.
[00:24:46] Amanda Wong: Yeah.
[00:24:46] Nataliya Shcherbatyuk: And see more studies on effect on the plants, effect on the soil, uh, interaction soil, plant, human. I think that’s quite fascinating, quite interesting, because I always think that plants are resilient and they have their own way of figuring out any conditions that they face. But the response is, for me, it’s always fascinating just to see the response of plants to something they don’t know.
[00:25:14] Amanda Wong: Right. And it’s so diverse even across the same species.
[00:25:19] Nataliya Shcherbatyuk: Well, Amanda, thank you so much for your time. I think it was quite interesting, and important topic that we covered today. And we will be staying tuned to your next publication, and good luck with your PhD.
[00:25:34] Amanda Wong: Thank you for having me today.
[00:25:36] Nataliya Shcherbatyuk: That’s it for today and until the next episode. You can find more information by following us on Instagram and LinkedIn by @mulch_matters and going to our websites (www.smallfruits.wsu.edu) and choose ‘Mulch Technologies’. This work is supported by Specialty Crops Research Initiative Award 2022-51181-38325 from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed on this podcast are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.
Intro and outro music credit to Zakhar Valaha from Pixabay. Other sound effects and background audio were sourced from Descript’s media stock library. AI assistance was used.