Plastic Podcast Episode 15 - Plastic in Soil: Implications for Agriculture

Episode Description: In this episode of the Plastic Podcast, host Clark Marchese dives deep into the impact of plastic in our soil. Joined by experts Dr. Xuejun Liu and Dr. Kai Wang from China Agriculture University, the discussion explores the presence and effects of microplastics and nanoplastics in agricultural soils. We delve into the sources of plastic pollution, the current state of research, and the unanswered questions that drive future investigations.

Dr. Liu and Dr. Wang share insights from their studies on how plastics affect soil health, agricultural yields, and even the tiny earthworms and bacteria that play crucial roles in nutrient cycling. Learn about the complexities of detecting microplastics, the potential risks to human health through the food chain, and the ongoing efforts to develop biodegradable plastics as a solution.

Discover how this pervasive issue of plastic pollution in soil is being tackled and the importance of continued research in this field. Tune in for a thought-provoking discussion on the hidden impacts of plastics beneath our feet.

Episode Guests: Dr. Xuejun Liu and Dr. Kai Wang

More information about Dr. Xuejun Liu and Dr. Kai Wang here

Sino-UK agricultural plastics project link

More information about the episode and the Plastic Podcast

Episode Transcript  and more information on the Pine Forest Media Website

Follow Pine Forest Media on Instagram @pineforestmedia

Hosted, produced, written, and edited by Clark Marchese 

Cover art and PFM logo by Laurel Wong. 

Theme music by Tadeo Cabellos 

Transcript:

[00:00:09.640] - Clark

Hello, and welcome back to another episode of Plastic Podcast, the show that tells the science and the story of our relationship with plastic. Today we are getting our hands dirty, literally. Well, not literally. We're getting our words dirty. Lots of dirty talk today because we are doing plastic in soil. So much fun. I bet you cannot even wait. Okay, so by now, we know that plastic is basically everywhere. So it may not come as a surprise to you that scientists have started fighting microplastics and nanoplastics in the soil. The usual questions are, how much is there and where is it coming from? We can pretty much ask those questions about any place we find plastic. But there are a set of questions that are soil specific, which are important to ask, because even if we don't think about dirt all that often, we really do need it to grow things. For example, how do plastics and soil affect agricultural yields? What about the crops themselves? What about soil health and the little earthworms wriggling around down there that keep the entire system afloat? What about bacteria and nutrient cycling? How might plastics mess that up?

[00:01:21.470] - Clark

All of these things are very important to know, and you may have guessed it by now. If you've been listening to the show long enough, most of these questions are still unanswered. I found two scientists to talk to us today, both of whom are professors at the China Agriculture University. And they're going to tell us about the current state of knowledge about plastics and soil, basically what we do and do not know, and also what we would like to know. The first of these professors is Dr. Xuejun LIU, who is widely published on the topic, and his work focuses on microplastic pollution and impacts on food security, security, ecosystems, and human health. The second is Dr. Kai WANG, and his work focuses on understanding the behaviors of pollutants in the environment. And lucky for us, that includes microplastics and how they behave in the soil. So without further ado, let's get started. All right, perfect. Welcome to the show. Thank you both so much for giving your time today. The first question I have is if you could both introduce yourselves and tell us a bit about your research.

[00:02:30.520] - Dr. Xuejun LIU

Yes, my name is LIU Xuejun. LIU is a family name. Xuejun is my given name. I got my PhD in 1997. So my major research work is nitrogen seconding and atmospheric deposition and new pollutants sources and control agricultural ecosystems. We have some international collaboration. Our major work is the interactions between agriculture and the environment. Okay, so my colleague, Doctor WANG.

[00:03:08.290] - Dr. Kai WANG

Yeah, so I just introduced myself. So my name is Kai WANG. So WANG is very common family living in China I'm now working on the environment pollutant in the agricultural farmland and atmosphere as an associate professor at the China Agricultural University so I got my PhD in 2018 in Germany so my PhD is about the particle pollutants in the air after I enter the China Agricultural University, I started to work together with Xuejun on the plastic pollutions in the farmland, soils and as well as atmosphere deposition.

[00:03:49.560] - Clark

So doctor LIU, I know that you have a background in soil and plant health and doctor WANG, you have a background in pollutants of many kinds. What made you both interested to look at plastics specifically?

[00:04:00.500] - Dr. Xuejun LIU

To me it's only agricultural plastic issue, very important for Chinese agricultural production so that's a wide use of agricultural plastics in our agricultural crop production and some cash crops in China from south to north, from east to west so that's a very common practice in China using the plastic machine in the agricultural production but we also noticed there are some microplastics and the nanoplastic residues in soil profiles so there are some evidence to show there may some environmental risks with the macroplastic but we just started this work in the recent two or three years and there still need long term research in this area so we think we should balance the benefits and the environmental risks and the pollution issue equally so that's my major point ta, do you have some additional comments?

[00:05:10.950] - Dr. Kai WANG

Yeah, it's a question we know that plastic products like plastic mulch films actually contribute a lot to the food security, especially in developing countries but now we start to investigate microplastics in the soil so that's why we start our project and to give some solid evidence.

[00:05:33.220] - Clark

Okay, so you both are telling me something that I've heard a lot so far when it comes to plastic, which is firstly, there are some benefits that we cannot deny. However, there is growing concern about how plastics might be affecting us and we don't have the answers about how just yet so we need to go and find them so we've been finding plastics in the sea, in the air, in our bodies. What can you tell us about plastic and soil? Are we able to make any general statements about it?

[00:05:58.260] - Dr. Xuejun LIU

I think based on the data flow of a second national agricultural pollution source census in 2022, there is about more than 1 million tons of accumulated plastic residues in Chinese agricultural soils but the amount of plastic input into the soil is much higher than the real amount of residual plastic because we have not the complete investigation. Besides, the plastic can also be transferred between the soil, water and atmosphere between this flow based on water flow and air flow. So it's very completed so far. So we think the microplastic problem is an environmental issue, but we cannot make a clear conclusion on how this pollution affects ecosystems and human health. So that's the current state. To my opinion.

[00:07:01.260] - Dr. Kai WANG

From my side, a large amount of plastic residues actually exist in the farmland soils. But now we don't know the exact number of microplastics in the soil, but the variance between different sources is very big. And another thing, the analytical method for the. Quantifying the microplastic in the soils is still very limited. For example, for the nanoplastic, which means the particle size is less than one microliter, there's no good method for detection. So there's a lot of uncertainty on the number of microplastics.

[00:07:44.080] - Clark

Okay, so for the audience to visualize a microliter, if you were to imagine a water bottle, it would probably be around a liter. A Stanley cup holds 1.2 liters. So just a little bit less than a Stanley cup. Now, a microliter is one 1,000,000th of that. So very small. And it sounds like we don't have either an adequate method or a standard method to measure them. It also seems like there's some variation between the soil and the region. So it's hard to make many general statements at the moment. Now, this data that you're referencing came from a case study of Chinese agricultural farmlands. And you took samples from over 700 sites to sort of take stock of the situation. What else can you tell me about this case study?

[00:08:22.540] - Dr. Xuejun LIU

Yes, I think our publication showed there are some sources and distributions of microplastics in Chinese farmland soils. So far, as far as I know, the major source of these macroplastic is from agricultural plastic. We use the plastic mulch film covering on the soil surface or the greenhouse. We also use the greenhouse plastic cover to protect rainfall and also to increase the temperature and so on. These two sources are major sources of the soil residual plastic. We also found some macroplastics from the atmosphere, atmospheric deposition. And also irrigation and compost also provide some macroplastics.

[00:09:20.540] - Dr. Kai WANG

Yeah, but I have a slightly different opinion about the source, Professor LIU said it is the major source of the plastic film. But we haven't quantified all the sources.

[00:09:33.150] - Clark

Okay, so it seems like, at the very least, we can call films that cover the farmland a major source, although there are others, and there is still need for further investigation on this front. What about soil health? Do we have any data or information about how plastics might be affecting that yet?

[00:09:47.800] - Dr. Kai WANG

So the first is the definition about the soil hails. Just different, scientists say, have maybe several different definitions with scientists. I can give an example of that. So we have a colleague at Shenyang Agricultural University in China. So they have a very long term field experiment in more than 30 years. So they have two treatments. One is with plastic film, another without bloodstream. Actually, there's no significant effect on the soil properties, like maybe soil organic carbon or other parameters.

[00:10:23.970] - Dr. Xuejun LIU

But in sampled or simulated experience and laboratory conditions, if we add too much plastic, there are some reports to show there are some negative effects or some impact on the eastward activities and also some macro organisms. But those amount of addition. Plastic addition is very high. It's much higher than the real soil content. So we do not think those results can reflect the real situation in the current agricultural soils. So it's not very clear. But we think with the long term depreciation of west film, there will be some risks on the soil health.

[00:11:11.710] - Dr. Kai WANG

And that's why when we write our manuscript, we always add a potential in front of the risk, because now there's no solid evidence. But we think, yeah, in the future maybe we can have some direct evidence to confirm this effect here.

[00:11:28.610] - Clark

Yeah. So I guess soil health is actually quite a broad term and we could have probably asked a number of different questions there. But there are physical considerations like soil density, texture, and water holding capacity. There are biological factors like the root health and microbe activities, and also some chemical questions like ph, salinity, mineral concentrations. But I did hear you mention earthworms. Did the study find out anything about how plastics are potentially impacting earthworms?

[00:11:57.020] - Dr. Xuejun LIU

That's a good question. Actually, one of my PhD students, a lady, she is working with Professor Yan Changrong  in Chinese agricultural environmental protection and management. His study, she added a different rate of bast residues to the soil. So she found some differences on the number of earthworms in the soils and also the activities, their movement and so on. To see, with the small amount of plastic addition, there's even some improvement or some promotion of the earthworm. But with the increase of the macrophages in the soil, the earthworm was negatively affected. But this work is still ongoing. But his initial research shows some impact on the earthworm in the soil.

[00:12:54.610] - Clark

Yeah, so we just defined a couple of categories to consider when it comes to soil health, sort of physical, chemical and biological considerations. And earthworms play a role in all three. Their  burrows help aeration and water infiltration through the soil. Their excrement promotes microbe activity and they even regulate ph and contribute to the soil's ability to sequester carbon. And these are just a few examples. So we really need those earthworms to be doing okay with gas.

[00:13:21.830] - Dr. Xuejun LIU

The plastic film may enter the earthworm digestive system. So they're eating the soil, and if they carry it too much, I think the blood system and also their digestion system may be broken down or something like that. I'm not very sure. Yeah, but I think that there should be some impact if they are too much in their body.

[00:13:50.960] - Clark

What about the even smaller guys though? I know that bacteria in soil are sort of the base of a nutrient cycling system. Are we seeing any interference with microplastics?

[00:14:00.080] - Dr. Xuejun LIU

The nutrient availability. Yeah, actually macroplastics are carbon based materials. So normally there can also be some conditions that can be used by macro organisms. So this macro plastic will affect the nitrogen cycling and some carbon cycling. Yes, there are some interactions with the nutrients and carbon.

[00:14:29.010] - Clark

I'm curious about water. Do we see plastic in soil affecting the soil's ability to hold water?

[00:14:35.770] - Dr. Xuejun LIU

No, we do not observe such evidence. But we find if the macro, not macro, micro plastics in the soil profiles, that will negatively affect the root growth in the deep soil. And then these macroplastic residues also affect water flow in the soil profile. For example, if there, in the deep soil to the surface soil, if there's too much microplastics, that will affect the water movement in the soil and the root growth.

[00:15:11.590] - Clark

Okay, so it's not so direct of an impact. It's not as though the presence of microplastics themselves impact the water carrying capacity of soil. But the other effects that these plastics are having can sort of indirectly relate to water through the roots, for example, which are important for water retention. At the risk of sounding silly, are there any ways to remove microplastics from the soil or to take them out?

[00:15:34.450] - Dr. Xuejun LIU

How to take them out? We cannot take them out. So actually if they enter into the soil, it's very difficult. That becomes smaller and smaller. And then from mackerel to mackerel to manure, let's check it.

[00:15:51.970] - Dr. Kai WANG

It's very difficult to remove the microplastic from the soil. And that's why in China the government tried to increase the thickness of the plastic mulch film. So if it's thick enough, then it's very easy to recycle the plastics residuals. So this is important thing I would say, and I think the manufacturers and they try to develop some biodegradable plastic materials, since it is very easy to be decomposed in the soil by its organisms.

[00:16:26.220] - Clark

Okay, so it seems like there are many unanswered questions and a large need for future research. If you could suggest some areas of research or research questions that you think should be addressed with the most urgency, what would they be?

[00:16:39.180] - Dr. Xuejun LIU

Yeah, that's very important actually in the future. I think first of all we need a monitoring system network to know why the hotspot is in soils, in other environments and so on. If we know the exact distribution, we can take exact actions to control the pollution on the import and so on. So this monitoring network is very important. And we also have to share the data from different regions, different countries and different climate and soil conditions. So second, I think we can develop biodegradable plastic materials. If the biodegradable plastic films or materials are really biodegradable, then it becomes CO2 and water, then it will be safe. And also for the traditional plastic film, we have to change or to improve the thickness of this plastic film. And also we need some policy regulations for these materials, the quality and the quantity, and also the use areas for different specific crops. If we have the standard regulation, those regulations can get the farmers to use widely and properly. Otherwise there will be some problems.

[00:18:12.400] - Dr. Kai WANG

Yeah, and I have actually two points to add. So one is that for the, I think for the scientists, I think we actually need to establish a standard method to measure the microplastic. Because as I mentioned, there's different, different methods that will give different results and they have very big variations. And so first we need a standard method. And another thing, I think the easiest way to reduce pollution. So like in our daily life, we try our best to not use plastic products. For example, when you go to the supermarket, you just use a cloth bag, right? So it's the industry and for the public.

[00:18:55.550] - Dr. Xuejun LIU

Yeah, actually I have a chinese natural sense foundation project. This project is just as Kai mentioned, to make a standard approach to quantify the microplastics in the soil. In addition, I think in the future they are also important for us, for our scientist communities and the policymakers and the farmers working together to have some agreement about how to use their plastic.

[00:19:30.780] - Clark

Okay, those are all great to keep in mind. As we start to approach the end of the episode, I want to ask if there's anything that we missed, anything that we didn't talk about today that you think is important to mention when we're talking about plastics and soil.

[00:19:44.220] - Dr. Xuejun LIU

Yes, I think we actually do not talk about the micro plastics in the food chain. This is important. Actually we found that some groups reported their micro, or even the nanoplastics can enter the root system and transport to the shoot and grains of cereal crops. So if this is the true situation, we think there will be direct evidence to show the negative impact on human health. Then we have to make stronger actions to reduce these kinds of residual in the soils. So this is important because human health always ranks the lumber wall in different kinds of environmental issues.

[00:20:39.680] - Clark

Okay, just to clarify, do I understand that plastics and soil is a way for plastics to transfer into the crops? Is that what I'm hearing?

[00:20:47.560] - Dr. Xuejun LIU

Yes, two years ago that's a publication to shield the micro plastic can transport from the soil to the root system.

[00:20:56.650] - Dr. Kai WANG

Or wheat?

[00:20:57.730] - Dr. Xuejun LIU

Yeah, wheat, cabbage. Cabbage, then to the consumable parts of the crop. So that means the human can get this microplastic from their food and if this crop can, you can also be used for animal feed, then can transport to the animals, then to humans two ways. So I think this issue should be emphasized or stressed in future studies. 

[00:21:27.130] - Clark

Okay, we'll definitely keep an eye on that. The last question is where can listeners find you and follow your work?

[00:21:32.660] - Dr. Xuejun LIU

Yes, we have a home page.

[00:21:34.970] - Dr. Kai WANG

I think you can find it. If you are interested, you can find more information about our project because it's an international project and we have a homepage for this project. So this project involves six countries.

[00:21:46.110] - Dr. Xuejun LIU

Make us send the linkage to you afterwards.

[00:21:49.340] - Clark

Perfect. I will include that in the episode description for anyone who might be interested. And this is the point where I say thank you both for giving me your time today. Thank you for teaching me about plastics in the soil and thank you for your very important research in this space.

[00:22:01.450] - Dr. Xuejun LIU

Thank you. We are very glad to know you, to see you and talk to you about the macro first issue. Very good.

[00:22:22.200] - Clark

Alright, a major thank you to Dr. Xuejun LIU and Dr. Kai WANG from the China Agriculture University. It seems like there are still a lot of knowledge gaps on this subject, so I'm glad that there are researchers like our guests today working on it. And to any future researchers out there who might want to get their hands dirty, it seems like there is space for you here. One thing that makes me feel a little bit relieved is if you can remember all the way back to I think it was our fourth episode with Doctor Winnie Cortin Jones who taught us about biodegradable plastics. We learned that they are not the magic bullet solution that we were hoping they would be. But I remember when I asked her about any unproblematic applications that they could have, she gave me the exact example of plastic films on agricultural farmlands because we can watch the full cycle of their decomposition and that's one area where I think we can actually have a big impact. So I think that's always encouraging. However, we know that these films are not the only source of plastic in the soil, so we can't just rely on these alternative agricultural products either.

[00:23:19.640] - Clark

Alright, I do believe that is all I have for you today. So thank you for playing in the dirt with me and I'll see you next week. You've been listening to Plastic Podcast. You can find more information about this week's guests and links to their work in the episode description. Cover art for the show was done by Laurel Wong and the music you're listening to was done by Tadeo Cabellos. I am your host, Clark Marchese, and this episode was produced and engineered by me. So if you found it interesting, send it to someone you know. Plastic Podcast is part of a larger network of sciency podcasts called Pine Forest Media. You can find more information about us in the episode description as well or on our website@pineforestpods.com, and we are also on Instagram and TikTok @PineForestMedia. We've got some exciting podcasts coming out this year, and a five star rating across platforms and review on Apple Podcasts is one of the best things you can do to help science communication like this reach more people and for our entire network to grow.