153: The Role of Nematodes in Soil Health

Think most nematodes are parasitic? Actually, the majority are beneficial and can provide biological control for bacteria, fungi and other nematodes. Deborah Neher, Professor in the Department of Plant and Soil Science at the University of Vermont explains that the name nematode in Latin means roundworm. But do not confuse these worms with the common earthworm. They have a very simple anatomy that is purely dedicated to eating and reproduction. The microscopic, aquatic organisms live in water films that surround soil particles. Nematodes are the most numerous soil-dwelling animal and can live in extreme conditions. Listen in to learn how nematodes fit into a healthy soil system.

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Transcript

Craig Macmillan 0:00

And with me today is Dr. Deborah Neher. She's a professor at University of Vermont and has done some really, really interesting work on soil health in particular, micro organisms and what role they play. And so today we're gonna talk about nematodes. I'm excited.

Deborah Neher 0:15

Yeah, it's great to be here. nematodes get me excited too. Been working on it for like 30 years now, believe it or not.

Craig Macmillan 0:22

I believe it. First of all, let's drop back a second. And we're talking about healthy soil soil health, the role that these organisms play in that. What is your definition of a healthy soil, I think that's kind of a tough thing.

Deborah Neher 0:33

Sure. And I know everybody has a slightly different version. But just to keep it really short and succinct, it needs to be porous, it needs to be chemically balanced, as well as containing organic matter. Let me just elaborate on those briefly. We need a range of pore sizes in soil to help give it good structure and that also allows for a balance of water and oxygen, so that the plants and the microbes can live and have air to breathe. We need a chemically balanced both the nutrients as well as a pH. And as far as organic matter it plays a number of different roles. It can hold moisture and nutrients kind of like a sponge. And that's also where we have the biological activity happening. And organic matter has negative charges on its surface the nutrients have positive charges so they can attract you know like magnets with opposite charges. You know, organic amendments usually come with microbes as well as nutrients so they're bringing the life into soil and supporting that.

Craig Macmillan 1:31

And speaking of life, that's what a lot of your work has been the the soil microbiome plays a huge role and how that functions obviously and different organisms have different roles. So we have bacteria, fungi, we have protozoa, we have nematodes, am I leaving anybody out I don't want to leave anybody out.

Deborah Neher 1:49

Micro arthropods is another big one like mites is another one.

Craig Macmillan 1:53

Oh, yeah, that's right. When we do we don't talk about that much at least in my experience is the nematode part of it. What exactly is a nematode? First of all.

Deborah Neher 2:01

So the term nematode might be a bit foreign, it is a Latin word. If we translate it into English, it means round worm. And they are different than earthworms. They are different taxonomic phyla, so very different. One thing you will notice when you look at a nematode is it moves very differently than an earthworm. It has kind of a snake like S shaped movement, and that's because it only has longitudinal muscles, so it's not very coordinated. Nematodes are the most numerous of all soil dwelling animals. There's a great quote about how abundant they are. This quote is by a famous nematologist about 100 years ago, his name was Nathan Cobb, and he said, nematodes are so numerous that if you were to zap the earth, and just leave the nematodes representing the structure there, went outer space and looked back, you would be able to see see the Earth and the contours of the earth based on where the nematodes are. If you knew the associations of those nematodes and plants and animals well enough, you'd even be able to tell which plant and animal communities were wher. Back to kind of what they are there, you know, the very minute the soil nematodes are microscopic in size, one millimeter would be a very large one, you'd be able to see that kind of with your naked eye, but we usually have to look at soil nematodes, through a microscope. These tiny round worms, they basically are aquatic organisms, they live in the water films that surround soil particles. So I like to refer to them as the other aquatic organisms. And they're so tiny that they're forced to really navigate within the existing pore structure of soil within those water films. They are different from earthworm. Earthworms can move soil particles, these guys can't. Another kind of interesting thing, if you looked at their anatomy, it's very simple. They basically their whole anatomy is only about getting food and having sex, okay? They don't have any eyes, they don't have any appendages. They're determining where they're going based on kind of touch and smell chemical cues, as I'd like to say they've just got life down to the basics, just eating and having sex. And their whole whole body structure is based on that.

Craig Macmillan 4:28

They are relatively simple. What are some of the major categories of nematodes because there's a huge array of different even beyond genus just types and categories and do all kinds of different things. What are some of the major categories?

Deborah Neher 4:41

Yeah, I like to think about the categories we call them kind of feeding groups or trophic groups would be a way to think about it. Most people when they think of nematodes they automatically think about plant parasites, and that is an important, you know, group of nematodes but that's usually a minority. The majority of nematodes are actually beneficial. And they can be categorized into groups. One group would be bacterial feeding nematodes, another one that feeds on fungi. Another group that's actually predacious. And those can actually feed on protest or other nematodes. So they could have like a biological control component. And then we have some that we call omnivores, meaning they eat a variety of food sources. But those predators and omnivores are kind of higher up on the food chain than those that are feeding on the microbes. So you kind of almost have a whole food web just have nematodes.

Craig Macmillan 5:40

Wow, that's a that's pretty, pretty amazing. They also though, were parasitic to animals, their animal parasitic nematodes as well, right?

Deborah Neher 5:48

Yes, there are animal nematodes. These tend to be much larger than the ones you see in soil. They're not you know, restricted to going between the particles of soil. They're actually in the cavities, you know, the digestive cavities of animals. So examples like intestinal roundworms, pinworms, hookworms, those would be examples of nematodes that can be inside vertebrates, and those are very serious parasites.

Craig Macmillan 6:11

Yeah. And so that kind of where I was going for, which you've been very helpful was it's a type of organism that is incredibly diverse, all aspects of life on Earth are connected to nematode in some fashion. Now going back to soil specifically, so what are the roles that different kinds of nematodes play in the functioning of the healthy soil?

Deborah Neher 6:31

I'd like the fact that you pitch that as what is the function, because this is something I think about is it's really more important thinking function than just the specific organism itself. And that's why I study them. And that two major ecosystem functions in soil one is decomposition. And the other is nutrient cycling. Nematodes are really tightly linked with nutrient cycling, and particularly nitrogen, okay, and they're not feeding on, you know, the decaying organic matter, they're feeding on the microbes right. Their link in with nitrogen cycling is kind of both direct and indirect. Directly, they actually the food they take in any extra nitrogen they have, they excrete it in the form of ammonium as a byproduct. And that happens to be a nutrient or form of nitrogen in that plant can take up. So that's one way. And the second way indirectly is by feeding on bacteria and fungi, they can, you know, decompose the tissues, the body tissues of those microbes. Let me back up one moment, because bacteria and fungi are all especially bacteria, very important in nitrogen cycling, all the different stages. And nematodes can are a predator on those. And when you have a predator on a prey like that, you know, it kind of culls out or removes the sick, the weak, the old and keeps the most active ones going. And it ends up by that grazing activity on those microbes, that it makes nitrogen cycling more efficient, they can release more nitrogen per unit of carbon. So they don't take as much energy to kind of keep this nitrogen cycle going. So they're kind of keeping that bacteria in a healthy state, you know, in terms of their role in nitrogen cycling, but as I mentioned before, they also excrete ammonium. So we've estimated based on in my research that they contribute about eight to 19% of the nitrogen mineralization in soils, which is much higher than it's been reported in the past. So very important.

Craig Macmillan 8:46

That's incredible. And so what they're doing is they're consuming something and they're digesting it. And so it goes, it comes in in one form and leaves in another. Yes, it makes it in a plant available form.

Deborah Neher 9:00

Yeah, so you know, when they're ingesting bacteria, fungi, those are all their proteins and their bodies are comprised of amino acids. And when you digest an amino acid, one of the products is ammonium. And when the nematode gets more nitrogen than it needs for its own maintenance, the excess is then excreted as an ammonium. And that's a form plants can take up.

Craig Macmillan 9:26

And again, like they like that ammonium is going to be in solution in water on a soil particle and a root is going to grab that water and pull it up into the plant.

Deborah Neher 9:37

That's exactly right. Yeah, any nutrients have to be in water solution to be able to be transported into the plant through the roots.

Craig Macmillan 9:46

And this just occurred what are the conditions, soil conditions that promote different types of nematode populations and what are so conditioned that maybe limit them?

Deborah Neher 9:56

So that's a great question. What are the reasons that I got and to study nematodes in the first place is because they are distributed everywhere in all kinds of ecosystems, all types of vegetation. So that by saying they're everywhere means that they can also withstand some extreme conditions. I mean, we find them in extremes for temperature for cold for dry. Some groups of nematodes have an ability to go what they say in a kind of a cryobiotic state, or if it's for dryness and anhydrobiotic state, meaning when it's super dry, they can change into kind of a suspended animation or a dormancy where they can just kind of shut down, they survive, but their metabolism goes way, way down. And they have a way to kind of change their chemistry within their body so that they can stay alive and not damage their tissues. You know, I've studied nematodes in desert soils, where it's 60 degrees Celsius, so like, you know, over well over 100 degrees, and they're hanging out in there. Now, some of them are gonna get triggered into this other stage when it gets harsh. Okay, so one thing that would not be good for nematodes is if you would deplete the oxygen, they do require oxygen for survival. So if you had a situation where it was say flooded, for a long period of time, and all that oxygen got used up, that would be that would be very harmful to nematodes. The other thing that we can see is if you use very intense, general biocides, let's just say application of methyl bromide, for example, that pretty much wipes out everything.

Craig Macmillan 11:38

That'll take care of it. Yeah.

Deborah Neher 11:44

I mean, there are different species that that are adapted to different kinds of conditions. So if you have, you know, a tropics versus agricultural land in the temperate zone versus a wetland, they're gonna have different species just because they're adapted to those unique conditions.

Craig Macmillan 12:01

So you can tell a lot about a location. You can almost guess location like like constant I'm guessing that particulars species or perhaps genus, I'm not sure what level would be the important one, but you probably could or should identify these specific organisms, these specific types of organisms. How do you do that? We're talking about microscopic things that are distributed around soil. How do I find these little guys? What how did nematologists do it?

Deborah Neher 12:31

Yeah, no, that's a great question. First of all, address your species versus genus query. One is when we're looking to plant parasites, everybody identifies them to species, and sometimes even more precise than that there's actually races or, you know, subspecies, we would say. When we're looking at free living nematodes, we're happy if we can get to genus. We don't even have the knowledge to go to species for many of those. And part of it, there's so many different kinds, because I might find 50 to 100 different genera in one soil sample. So how do I get there, you get there. The first step is you have to get them out of the soil. And what we do is make a water slurry. So we'll take the soil, and we'll mix in water and stir that. And that will allow the nematodes to kind of swim out of that soil and into that water. And then we will run those through a series of sieves. So we start with kind of a coarse or large opening, go through that one first and make it successively smaller and smaller, until we can collect just we can get rid of the soil particles, and just have the nematodes on the final sieve. So that's one approach. But there are other methods that can be used to in terms of like cleaning them up as we can. I've used techniques where you can put them in a tray that has some kind of filter holding it up and having water and they'll swim out of that and you can collect them in an outer tray. And another cool technique is we can use a sugar flotation when you put this liquid into a tube with sugar, and spin it in a centrifuge, they're going to float because they're less dense than sugar. So if you're trying to concentrate them, you can use this method so that you can just pour the top part of that onto a serve. Now one tricky thing with that technique is you can't leave the sugar on very long, you got to get them rinsed off or they're just gonna shrivel. It's gonna make identification impossible, and it's also just a sticky mess to work with. So, people have been steering away from that. So I think of kind of a sieving methods giving a water slurry. The method I described with the sieving and flotation method is the one that I use for looking at entire soil communities. If one was interested in only the plant parasitic nematodes, there's another technique that's called an an illustrator, or semi automated method, and faster for like a diagnostic lab. And this is a technique where they use a sieve and they run water over it, and then there's some movement of that water and they can collect it, it's just a way they can try to process multiple samples. An illustrator is is a faster method, kind of what we call semi automated will allow a lab to process more samples within a day. The one downside with it is it's not as efficient with clay soils can miss some of the nematodes and under represents these predators, and the omnivores. It works great when you're just looking for a particular plant parasite, for example, that's fine, because you kind of know which needle in the haystack you're looking at.

Craig Macmillan 15:58

At least you know that.

Deborah Neher 16:02

Once you get the sample out, then it's usually in a water suspension. And we usually just let those settle for a little while and ends up that nematodes fall with gravity about one inch per hour, wait till they settle and we can concentrate them and then we'll first of all count how many nematodes we have in that volume of sample. And then we'll take a what we call a mass mount slide a slide that has a cavity, some depth, put some in there, and then we'll do the identification of those and we have to go to about 200 times magnification using a light microscope. Features that one looks at are some of the mouth features those that are at the feeding so you can understand the feeding, but also the the esophagus, within it and nematode you can see straight through there transparent. So and think about it is that you don't need the pigments if you're in soil because you don't have light. So these organisms are you don't need sight you're not seeing and they're you know, they're transparent. And so you can see straight through them. One of the challenges is, they are 3d. So sometimes you have to do a lot of focusing up and down takes a really good microscopists to do that. Some people will actually try to fix them or you know, kill them to hold them still. But sometimes you lose characters like how they move and swim can also be a character sometimes that helps to identify.

Craig Macmillan 17:31

And that quantification and identification has been important to another topic you work on, which has been around the idea of nematodes, as bio indicators might tell you something about what's going on in the environment, just whether they're there or not. And who. Can you tell us a little bit about that work, because fascinating stuff.

Deborah Neher 17:47

Sure. First, let me just give you a perspective, I consider myself a community ecologist. So I use a community ecology approach. So I'm looking at the community structure, or the community composition, and how that changes under different land management practices. The type of index that I found works the best is one that really looks at what the life history characteristics of different species. That means how sensitive or tolerant they are to disturbance, you know, how many offspring they have with their generation time is. So just to give an analogy, you know, it's kind of like comparing rats and elephants or carp and trout. Rats are the kind of what we call early stage, you know, they come in, they can tolerate a lot of stress where an elephant is going to be very sensitive, they have a long generation time. So if we apply that in nematodes, we have those that are early colonizers and those that are later in succession. We can tell the type and severity of disturbance based on the composition of that community. And as we learn more than we'll be able to even tell, you know, was that disturbance due to cultivation? Or was it disturbance due to heavy metal contamination, or perhaps even just a lot of additional a lot of fertilizer, that can actually shift a community to a very early successional stage. It's kind of an indicator of ecological succession. So you know, if you're in a forest, and you have a clear cut, and you want to progress gradually to a mature stand, old growth, you know, there's a lot of changes in species composition. So if there's no disturbance, you'd get to the old growth where there's, you know, the ultimate would be if you clear cut it, you can see the same kind of patterns in nematodes. So you can tell the kind of disturbance and where it's at on their trajectory. So that can be helpful to know the level of disturbance. It can be a tool for monitoring if you're trying to restore an environment as well. Now one thing that I just want to kind of address a myth is the idea of diversity. Most people think more diversity is better. And yeah, in general sense, but we have to be a little careful because diversity is just a mathematical equation. And it doesn't tell you who is there. You could have a diversity of invasives. Right? So we need to get to know who is there, it's just not how many different kinds are there. The other thing, if we're thinking of that analogy of a clear cut to an old growth is the most diversity is actually in the middle in the intermediate, because you're kind of in a transition between an early phase and a late stage. So you have overlap of species. For both of those. When you're at either extreme of the continuum, you actually have lower diversity, we tend to like to think about biodiversity, but we have to be a little bit careful in terms of that. And that's why I prefer an index that just looks at the community structure where it is on the whole continuum, rather than limiting it to just diversity.

Craig Macmillan 20:59

So is this something that a grape grower could use over time to see I'm using these practices? Is the community changing? is a community changing? In a good way? Are there things that I do that have suddenly you know, like you said, knock backwards and successional progress? Is that Is this the kind of tool? Is this the kind of measurement I can use to make decisions?

Deborah Neher 21:21

That is exactly it, it works best if you're doing it through time, you need to start somewhere and get a baseline. And then it's how is that changing through time, and it gives you feedback back on, am I on the right track or not? It's using this index where I really learned the cultivation or the physical disturbance of soil has the greatest impact on the soil foodweb. And it will set that back further than say, applying, you know, whether it's a chemical disturbance, whether that's a pesticide or fertilizer, for that matter. So the physical disturbance is really the most destructive to soil foodwebs.

Craig Macmillan 22:00

And that actually brings up another question, when we think about soil foodwebs. Are we talking about the first four inches? Two inches, six inches? Is there stuff happening in a foot that I should be interested in? Is there stuff below that, that I should be interested in? Because I got roots that are down at you know, 3, 4, 5, 6, 7? Feet? Where should I be putting my attention Deborah tell me please? Where should we put my attention?

Deborah Neher 22:20

Well, that's a great question, because so much sampling focuses on the top eight inches, because that's the plow layer. But that's a problem. Because really, the biology is going to be as deep as the roots. You know, the biology tends to be the most abundant near the surface and decreases as you go with depth. The other thing with roots is you have to think where are the actively growing roots. And when you're dealing with trees and woody plants, the biggest roots are not necessarily where all the activity is, it's really where the new roots are. And that's where the most biological so thinking about where is that happening? That would be the right place to look. Right. And usually that's a little closer to the surface, but there could be some of it deeper. So I usually like to think think about where the roots are. And that's where you should be sampling.

Craig Macmillan 23:07

Well that is some really good advice. And thank you for taking a topic that I personally have found very confusing over the years and giving it some clarity for me and also helping me see how we can what it means but also how we can use it and and how we should monitor it, which I think is great. So I want to thank you for being our guest, Dr. Deborah Neher Professor University of Vermont and plant soil science.

Transcribed by https://otter.ai