Can Woodchips Help The Gulf Of Mexico’s Dead Zone?

16:43 minutes

a person pouring our woodchips from a measuring glass
Credit: Illinois Crop Sciences via YouTube

In the Gulf of Mexico is an ecological dead zone, caused by algal blooms at the mouth of the Mississippi River. Warmer ocean temperatures provide the perfect conditions for algae to grow out of control, suffocating seagrass beds and killing fish, dolphins, and manatees. Fueling this toxic algae’s growth is nitrogen. The Mississippi River empties into the gulf, and drainage water from farms along it carries fertilizer ingredients—straight into the marine ecosystem. 

While farmers have tried using practices to reduce fertilizer runoff, like cover crops, no-till farming and conservation buffers, for decades, the problem has only gotten worse. According to a new paper published in the journal Transactions of the American Society of Agricultural and Biological Engineers, a creative new approach involves denitrifying bioreactors—a system that allows bacteria to help convert nitrate in the water to harmless dinitrogen gas.

an aerial view of a large pile of woodchips and a trench of them with a tractor nearby. close to the trench is a small river
Credit: Illinois Crop Sciences via YouTube

“It’s a complicated name, but it’s really a very simple idea,” says Laura Christianson, assistant professor of crop sciences at the University of Illinois at Urbana Champaign, and lead author on the study. She talks with SciFri producer Katie Feather about how a simple system involving woodchips in a trench can help keep nitrogen out of drainage water from farms across the midwest. Katie also speaks to Shirley Johnson, a farm-owner from Peoria, Illinois, about why she adopted the bioreactor technology, and what farmers can do to help their downstream neighbors. 

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Segment Guests

Laura Christianson

Laura Christianson is an associate professor of Water Quality in the Department of Crop Science at the University of Illinois, Urbana Champaign in Urbana, Illinois.

Shirley Johnson

Shirley Johnson is a farm owner in Peoria County, Illinois.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Toxic algae blooms are a now-common sight in the Gulf of Mexico. Warmer ocean temperatures provide the perfect conditions for algae to grow out of control. The other ingredient fueling toxic algae growth is nitrogen. Drainage water from farms up and down the Mississippi River, which empties into the Gulf, is ferrying fertilizer ingredients straight into the marine ecosystem. And while farmers can’t solve warming ocean temperatures on their own, they’re testing out a new way to keep nitrogen out of the Mississippi River system. SciFri’s Katie Feather is here to talk more about the solution farmers are finding. Hey, Katie.


IRA FLATOW: All right, tell us what that solution is.

KATIE FEATHER: So the solution is called a denitrifying bioreactor. Can you guess what that is?

IRA FLATOW: A denitrifying bioreactor sounds like something out of Iron Man.

KATIE FEATHER: It does, doesn’t it? But a bioreactor is just a reactor where biology is doing the work– in this case, bacteria.

IRA FLATOW: So cool.

KATIE FEATHER: Yeah, it is, except it’s not happening in this big factory-type place. It’s basically just wood chips in a trench.

IRA FLATOW: You know, when you say wood chips in a trench, you know, I think I know what that is. I used to use that process in my saltwater fish tank to remove deadly nitrogen from the water. Tell everyone how wood chips in a trench plus bacteria can extract nitrogen from the water.

KATIE FEATHER: Yeah, it was kind of hard for me to visualize, too, but Dr. Laura Christianson, the lead author on this recent study about the bioreactors, had a really great way of describing it.

LAURA CHRISTIANSON: If a farm had the bioreactor, you wouldn’t see it actually in the field where we’re growing crops. We call this an edge-of-field practice. And so as you would imagine, a bioreactor would be on the edge of the field, catching the drainage water underground coming from the field. And then in some cases, you wouldn’t see anything at all, because the wood chip trench sometimes is covered up with a soil cap. And sometimes, you would actually see the wood chips coming up to the surface.

But it would be a pit 3 or 4, maybe 5 feet deep, filled with 2, 3, 4 feet of wood chips. And then the bioreactor itself, I would say, varies in length and width between probably about 10 to 15 feet wide, generally, and usually anywhere from 40 to about 80 or 100 feet long, depending on how much drainage water is coming into the bioreactor.

KATIE FEATHER: So why do farmers have all this runoff happening in their fields in the first place, especially in the Midwest?

LAURA CHRISTIANSON: Good question. So if you think about arid environments or out in the western parts of our country, they might have irrigation systems that are very essential for them to be able to grow crops and grow food. But here in the Midwest, we have plentiful rainfall. We’re very fortunate for that. But our soils, our field, stay too wet. Rather than having not enough water, we have too much water.

And so a critical part, an essential part, of how we do agriculture is the practice of tile drainage. The tile drains allow our fields to dry out, especially in the spring, which is important, especially right now as we’re hot and heavy into planting season here in Illinois. We need to be able to get our tractors into the field to be able to plant. And obviously, if the field is too wet or too muddy, we can’t do that.

So tile drains help dry our fields out in the spring so that we can get into the field to plant. And then tile drains also allow the soil and the fields to be dry enough for plants to grow, so our crops don’t get drowned out during our wet springs. But they also serve as a conduit for moving nutrients, like nitrogen and sometimes phosphorus, from our field, where we need those nutrients for plant growth, to downstream waters, like you mentioned, the Gulf of Mexico.

KATIE FEATHER: So how do the wood chips extract nitrogen from the water? Tell me about that process.

LAURA CHRISTIANSON: Such a good question. And the way you worded it– wood chips extracting nitrogen– that’s not exactly how bioreactors work. Bioreactors work by enhancing a natural process, a natural part of the nitrogen cycle, called denitrification.

Now, you mentioned those bioreactors sounded very complicated, like something out of Iron Man, but it is actually simple, because we’re just enhancing a natural process that’s been happening all around us for millions and millions of years on its own. And that’s the process of denitrification. The process of denitrification is done by natural little denitrifying bacteria that are good bacteria living in the soil all around us, like I said. And these bacteria convert nitrate, which is a form of nitrogen, into dinitrogen gas, which is a stable, benign gas that makes 78% of our atmosphere.

And so in a bioreactor, we have these denitrifying bacteria that are native and natural in our soil. These denitrifying bacteria live on the wood chips. They eat the carbon in the wood chips as their food. So we’re providing them, like, a super buffet of carbon to eat and fuel them. And then as the nitrate in the water flows by them, they convert that nitrate to nitrogen gas.

KATIE FEATHER: And the bacteria– they live on a certain type of wood chip, or they’re just hanging out on all wood chips?

LAURA CHRISTIANSON: The amazing thing about this process with these wood chip bioreactors is that there are many bacteria that do this process of denitrification. And so they’re not picky, and so we generally use, really, any kind of wood chip. The actual physical properties of the wood chip, like how big the wood chips are, tend to be a little bit more important for us compared to the kind of tree that the wood chip is from.

KATIE FEATHER: And so there’s a reaction happening here. Are there any byproducts of this reaction that we should be concerned about?

LAURA CHRISTIANSON: Well, you’re right. It is a reaction, and the reaction is the conversion of nitrate, which is a form of nitrogen, into dinitrogen gas, which is into– which again, forms 78% of our atmosphere. Dinitrogen gas is a stable, benign gas. It’s not a greenhouse gas.

But I will say there is a byproduct that we do talk about a lot when we talk about bioreactors. And that potential byproduct is nitrous oxide. So sometimes when the bacteria maybe aren’t as happy, maybe when the water is flowing too fast for these denitrifying bacteria to do their job, or if the water is especially cold– under certain conditions, these bacteria are a little less happy, I would say, and there’s the risk that we produce slightly more nitrous oxide than we would otherwise.

So nitrous oxide, as your listeners will know– nitrous oxide is a greenhouse gas. And so we are concerned about the potential production of greenhouse gas from bioreactors. But the good news is that a number of research groups have looked at this question– are bioreactors just transforming a water pollution problem into an air pollution problem? And the resounding results from a number of groups looking at that question is that no, we’re not producing a huge amount of nitrous oxide from our bioreactors.

In general, because we’re designing these bioreactors very specifically to do this one very specific job of converting nitrate into dinitrogen gas, in general, we have denitrification operating very efficiently. And so we’re not producing a lot of nitrous oxide. And so that’s a really good story. There’s more work to be done, more research to be done, on that, but we know with good confidence that we’re not producing a huge amount of nitrous oxide with this water-cleaning technology.

KATIE FEATHER: What about the impact of these bioreactors? How much nitrogen are they removing? If a bunch of farmers were to adopt these bioreactors, would it totally take care of the problem? Like, how many farmers would you need to do that?

LAURA CHRISTIANSON: That’s such a good question. The scale of our water quality challenges across the Midwest, across the upper Mississippi River basin, is going to require every conservation practice that we have in our tool book. And it’s going to require at least one conservation practice being applied to every acre that we have. The scale of water quality challenges is an immense challenge for us in the Midwest.

So bioreactors remove anywhere from 20% to about 40% of the nitrogen in the water that you would otherwise send downstream. Our goals for the Mississippi River basin to reduce the hypoxic zone in the Gulf of Mexico are to reduce the amount of nitrogen and phosphorus that we send downstream by 45%. And so a bioreactor– even if every single farm, every single field, had a bioreactor, we still wouldn’t be meeting that goal.

And so that’s why I say we need every single practice in our toolkit to make significant headway towards meeting those ambitious 45% nitrogen and phosphorus loss reduction goals in the Midwest. And so we’re talking about things like improved fertilizer management, making sure we soil test, making sure we do cover crops. That’s one of our best in-field practices.

We also have practices that have been in our toolbox for a long time, like wetlands or constructed wetlands. And those are some of the best practices that we have. But every practice is different. Every practice has its own costs, benefits, limitations. And so bioreactors is certainly one of our favorites, but it won’t work for everyone.

KATIE FEATHER: I want to bring in someone you’ve been working with, a farm owner, Shirley Johnson, who just implemented her wood chip bioreactor late last year. Shirley Johnson, welcome to Science Friday.

SHIRLEY JOHNSON: Thank you, Katie. I really appreciate this.

KATIE FEATHER: How did you find out about a denitrifying bioreactor?

SHIRLEY JOHNSON: So my sister and I own the farm together. We actually inherited the farm from my father, who passed away in 2019. And we are both environmentalists, and we want to be good stewards of the land. That includes not only soil health, but also the water exiting the farm. We wanted to make sure that we were not adding more pollution in the form of nitrates to our local streams and then ultimately the Illinois River, the Mississippi River, and the Gulf of Mexico, where there’s a big dead zone, as you know.

So we were looking around for water conservation practices. And we found several, and we weren’t exactly sure which was appropriate for our situation. So we contacted the NRCS, the Natural Resource Conservation Service, and told them that we were interested in trying to clear nitrates from the runoff from our fields. And NRCS advised a bioreactor, and that was one of the things that we had found when we were looking into this as well. And we were recommended that that was the best thing for our situation. So we had a bioreactor installed.

KATIE FEATHER: And how was the implementation process for you? A pit with wood chips in it seems very straightforward, but I gather it’s not as simple as just digging a pit and putting some wood chips in it.

SHIRLEY JOHNSON: That’s right. That’s right. So the NRCS actually did the design. They have engineers skilled at doing designs like this. And so I found a contractor who had actually built one before. I was happy to have someone with experience building bioreactors. And so he came out and built it, and it does involve control valves and hooking into the existing tile line. But the main part of it is the pit with wood chips in it that the water runs through.

KATIE FEATHER: How do you think more traditional farmers would feel about a denitrifying bioreactor? Do you think someone like your father would have been into the idea or shied away from it because of the investment?

SHIRLEY JOHNSON: No, I think he would have been interested in it. And the reason I say that is Dad was always a very progressive farmer. So for example, he was one of the first farmers in the area to do no-till. And that, of course, is a way to help reduce soil erosion. So he was always interested in the latest, the newest practices. And one thing my sister and I wanted to do was carry on his legacy. But if he were alive today, I’m sure he would be happy to see the farm being used for something that hopefully has a bright future for everybody.

KATIE FEATHER: This is Science Friday from WNYC Studios. I’m Katie Feather, talking today with my guests Dr. Laura Christianson, assistant professor of water quality in the Department of Crop Sciences at the University of Illinois at Urbana-Champaign, and Shirley Johnson, a farm owner in Peoria County, Illinois. What about other people in the ag world? Do they know about this? When you bring it up in conversation, are they confused by it, or dismissive of it, or excited by it?

SHIRLEY JOHNSON: That’s an interesting question. So I don’t think it’s that well known. But it’s interesting, because we sited the bioreactor right next to the road, and the NRCS engineer was there looking at it one day, and he said that several farmers pulled off and said, hey, what’s going on here? So people are curious.

They see it there. They don’t necessarily know what it is for sure, but they are curious. And I think that once we get more data and once there’s more information from these bioreactors, it will be able to be something that can be implemented on more farms.

But I would emphasize that, for me as a farmer, it’s not going to increase my yield. It’s not going to improve my soil health. But what it does is, my downstream neighbor is going to be getting cleaner water. The streams, the Illinois River, the Mississippi River, the Gulf of Mexico, the dead zone in the Gulf of Mexico– the idea is to be able to reduce the damage downstream. So I feel that I need to take responsibility as a farmer for the effluence from my farm.

KATIE FEATHER: Well, thanks so much for joining us today.

SHIRLEY JOHNSON: Thank you so much.

KATIE FEATHER: Shirley Johnson is a farm owner in Peoria County, Illinois. Laura, I want to get back to you. If we could get the implementation of the bioreactor to be simple enough, then the barrier of entry for farmers would be really low, and hopefully they’d be more likely to use one, right?

LAURA CHRISTIANSON: Yeah. I think there are a number of barriers facing our implementation, and really our ability to scale the adoption of not just bioreactors, but a variety of conservation practices right now. For bioreactors in particular, I think one of the first things that we’re dealing with still is that there’s somewhat of a distrust or a lack of understanding that tile drainage does contain nitrate.

We’ve made a lot of headway on doing education about the fact that tile drainage does contain nitrate. We are losing nitrogen in the form of nitrate from our fields through our tile drains. But one of the challenges there is that nitrate is a dissolved pollutant, and so it’s clear. So tile drainage water often tends to be very, very clear. It looks very clean, but in fact, it does contain nitrate. And so that’s really where we’re starting, just talking about our nitrogen loss, where nitrogen is coming from in our fields and how it gets moved through tile drains.

You know, and then with bioreactors in particular, it is still a relatively new practice. I would say we’ve been promoting bioreactors for roughly a decade. And so in terms of, we’ve been talking about things like conservation tillage for 20, 30, maybe 40 years. And so it takes some time just for education about the practice and how the practice works.

KATIE FEATHER: Let’s say a farm owner is out there listening to this and is really interested in getting a bioreactor on their land. What’s the best way for them to go about seeing if this is the right thing for them?

LAURA CHRISTIANSON: The best way, the best first step, is to go to your local Natural Resources Conservation Service, USDA NRCS, office. The Natural Resources Conservation Service is our premier federal agency that handles agricultural conservation practices. And so there is cost share or incentive payments available for getting some of the bioreactor paid for through USDA environmental quality conservation programs. And so starting with your local NRCS office is absolutely the best place to start.

KATIE FEATHER: We’ll have to leave it there. Thanks so much for coming on and sharing more about this with us.

LAURA CHRISTIANSON: Oh, it’s my pleasure. Thank you so much.

KATIE FEATHER: Dr. Laura Christianson is assistant professor of water quality in the Department of Crop Sciences at the University of Illinois at Urbana-Champaign. For Science Friday. I’m Katie Feather.

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