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Bonding Over Science, Episode 7—Orchard Recycling: The Alternative to Burning Orchards

Microbiologist explains how grinding trees offers opportunity to improve soil health and function

Dawn Stringer |
Image of almond nut trees in an orchard

In the latest Bonding Over Science podcast episode, Amisha Poret-Peterson discusses how recycling orchards can improve soil health and function as well as offer an alternative process to burning trees. (Photo courtesy of iStock | Chris Boswell)

Listen to the latest Bonding Over Science podcast episode on whole orchard recycling as an alternative to biomass burning for orchards that are no longer producing crops.

What happens to orchards when they’ve reached the end of their lifespan and are no longer producing crops? Conventionally, orchards are burned. However, concerns about biomass burning and air quality have led to an alternative practice known as whole orchard recycling. Hear microbiologist Amisha Poret-Peterson on the Bonding Over Science podcast as she talks about using nuclear magnetic resonance at the Environmental Molecular Sciences Laboratory to learn about the role and efficiency of microbial nitrogen in soils in whole orchard recycling. Read more about her project in the EMSL feature article.


Dawn Stringer: What do you do when an orchard is at the end of its life? Generally burning the trees is the solution to clean up, but given the negative impacts it has on air quality, researchers say there’s a better solution. In fact, there could be an option that’s even beneficial to the environment. 

I’m Dawn Stringer with Bonding Over Science, let’s take a walk through aisles of aged orchards, and how they can be recycled for the Earth’s benefit. 


Amisha Poret-Peterson: My name is Amisha Poret-Peterson. I'm a research microbiologist with the USDA [United States Department of Agriculture] in Davis, California. So my research focuses on understanding the role of microorganisms in soils. I research organic amendments and practices that are aimed at improving soil and plant health. And my lab, we focus on two practices. One is called anaerobic soil infestation. And the other, which we'll talk about today, is called whole orchard recycling. 

Both of those approaches involve amending soils with organic compounds, essentially. And so my research focuses on understanding the effects of those amendments on microbial communities and how they can suppress plant pathogens and also improve soil health, such as organic matter accumulation. 

Dawn Stringer: And that brings me right into the topic I want to talk about today, which is orchard recycling. Can you explain what it is? 

Amisha Poret-Peterson: Sure. So whole orchard recycling is a practice that was developed by one of my colleagues who is the University of California farm advisor, Brent Holtz. So once orchards reach the end of their productive life, the typical practice is to remove them. And under whole orchard recycling, instead of removing the trees, they are actually knocked down, ground into woodchips, and then placed back into the soil. 

So if you think about the lifetime of an orchard which can be up to 25 years, depending on the crop, we're basically taking that investment over 20 to 25 years and retaining it in the orchard by putting it back into the ground. So this practice has been studied for over 15 years now. So whole orchard recycling has several benefits and these include building soil organic matter, increasing a characteristic of soils called microbial biomass carbon. 

It can increase soil moisture and water retention. It can improve a soil physical property called aggregate stability, which basically means that soils become less prone to erosion. It can improve water infiltration; it affects the activity of microorganisms in soil. So we can measure enzymatic activities in soils and we see clear differences between soils that have undergone whole orchard recycling and those that have not that indicate better nutrient cycling and whole orchard recycling soils, soils that have undergone whole orchard recycling. 

And this work was done in the laboratory of Amelie Gaudin at the University of California, Davis, along with Brent Holtz and the person who is a postdoc Emad Jahanzad. 

Dawn Stringer: Very cool. Now, is this a costly process and is this something that orchards are currently doing? 

Amisha Poret-Peterson: Yes. This is a current practice that occurs in orchards. The cost of it can vary. So it's between $1000 to $2000 per acre. But that's in comparison to what would have to be spent to remove an orchard. So it is more expensive, just removal alone. Most of this whole orchard recycling occurs mostly in California, specifically in the Central Valley of California. 

You can consider it the breadbasket of California, where over 400 specialty crops are grown. So those are crops that are basically grown nowhere else in the United States besides California. Whole orchard recycling is applied to mainly tree crops. And so we're talking about almonds mostly, but it has been used in walnuts, other stone fruits. There is potential to use it with pistachios. 

So that's probably acreage wise over 4 million acres of crops are planted with just almonds, walnuts, and pistachios alone, and with almonds, at least the typical removal rate of orchards is about 50,000 acres per year. And so it means that there is a demand for disposing of tree biomass, of basically removing orchards and to improve air quality in a region of the central valley called the San Joaquin Valley, there is a restriction in burning agricultural waste. 

So the traditional method for disposing orchards would have been to burn them and that results in particulates in the air and decreases air quality. So whole orchard recycling is an alternative practice to burning. 

Dawn Stringer: Now, since you mentioned the lifespan of an orchard is about 25-ish years, is it inevitable that an orchard will have to be recycled and is this environmentally better than the alternative, which is burning? 

Amisha Poret-Peterson: I wouldn't say that it's inevitable that an orchard has to be recycled. Farmers have an option of going into another crop after planting an orchard. They might decide to fallow the land. So there are all sorts of options for what to do with an orchard that has basically ended production. They could even switch to another tree crop. So there are options. 

But whole orchard recycling is becoming essentially one of the only options for tree disposal because if burning is restricted, you're still going to have to remove the trees. They'll likely be chipped. And so the other option would be to take that biomass and transport it off of the land, that actually used to be an option. So the traditional ways of disposing of tree biomass was either to burn in place or to basically turn the trees into woodchips and then transport that biomass to energy cogeneration plants. And it would essentially be burned for energy production. And with the cogeneration plants closing, that option has been foreclosed. As far as being environmentally beneficial, we're still studying it. This practice was developed about 15 years ago. We have field trials that we've been conducting and the longest one has been in place since 2008. So that's a 15-year-old trial, and we're still learning things from it. 

So the study that I mentioned earlier was conducted when the orchard was 10 years old, and we saw the beneficial effects of increasing organic matter microbial biomass, carbon soil, aggregate stability, and other indicators of soil health. I think the data is pointing that there are some environmental benefits to this process, but studies are still ongoing. We do have some questions about the practice. 

So one of the first observations that occurred when you're recycling, doing whole orchard recycling, is that you're putting biomass into the soil that has a high carbon content. So woodchips are 50% to 60% just carbon. And so when you do that, you can imbalance carbon to nitrogen ratio in soils and that has an effect on the availability of nitrogen to plants. 

So initially when whole orchard recycling was performed, there was an observation that trees were stunted, likely due to nitrogen not being available for plant growth. And so we're interested in understanding the processes involved in nitrogen cycling in these soils. And we are still trying to figure out, does whole orchard recycling in the long term actually result in nitrogen becoming more available due to microbial processes and physicochemical processes in the soil? 

We have questions about greenhouse gas emissions from whole orchard recycling. And so when you put a lot of carbon into the soil, you actually get a lot of CO2 production or respiration. And so that is something that my colleague Mae Culumber who is with the University of California Agriculture and Natural Resources and another colleague at the USDA in California, Suduan Gao, they're researching the greenhouse gas dynamics in these systems to figure out what the impact of whole orchard recycling is on CO2, on carbon dioxide, and another greenhouse gas called nitrous oxide. We also have some questions about the effect of recycling on the potential to introduce plant pathogens because you are grinding up trees and orchards that are often removed because they become less productive, because of the impact of plant disease. And so anecdotally, we don't see orchards that have undergone whole orchard recycling seeming to have more disease pressure, but we haven’t conclusively proven that through trials or other experiments, there are still some questions. 

But I would say overall, it does seem to be a beneficial process. It builds soil organic matter, which is important for having downstream impacts on soil health. That alone probably makes it something worth doing as a practice. 

Dawn Stringer: Absolutely. And you covered so much just then, explain how you became interested in the research of orchard recycling. 

Amisha Poret-Peterson: So I was actually doing other field trials that we had for anaerobic soil infestation and met a researcher, Dr. Mae Culumber, and she mentioned that she was working on whole orchard recycling. And we got to talking in the field and she's researching greenhouse gas emissions, in particular carbon dioxide and nitrous oxide emissions and at that time, the issue with tree stunting was not quite worked out yet and how to prevent that in recycled soils and how to prevent that under whole orchard recycling. 

And so we were talking in the field and I mentioned that I was interested in nitrogen cycling, microbial communities. And so we decided that, oh, it would be nice to look at the microbial communities at the same time that she was sampling for production in orchards. And so my role in the project became to essentially do a lot of profiling of microbiomes in soils under a whole orchard recycling. 

Dawn Stringer: Now you are working with EMSL on your project. Can you explain what you're hoping to learn from that and explain your project a little bit? 

Amisha Poret-Peterson: The project at EMSL is looking at [a] parameter in soils called microbial nitrogen use efficiency. And so this parameter basically represents the amount of organic nitrogen that is taken up by microorganisms versus [what] is released back to the soil or mineralized. And so this is a parameter that is increasingly looked at in soils because it can influence how we view soil organic matter accumulation. 

I'm interested in understanding microbial nitrogen use efficiency because it can give us insight into the question about, you know, how much nitrogen is immobilized in recycled soils or under whole orchard recycling. It can tell us about potentially about how much nitrogen is available to plants. It can potentially tell us about essentially nitrogen accumulation in soils. 

Dawn Stringer: What instruments are you taking advantage of for this project? 

Amisha Poret-Peterson: And so we are using NMR. So we measure nitrogen use efficiency. We, we do so using a stable isotope of nitrogen. So we label soils with nitrogen 15. And we're interested in tracking where that label goes. And so when we do the nitrogen use efficiency measurements, we essentially look at, we label the soils with N15 and we look how that label becomes diluted over time to get at how, how basically what is the production of amino acids from soils. 

But there's also a potential for us to see where that nitrogen ends up within soil microorganisms because adding amino acids, some microbes are taking up that nitrogen. And so that nitrogen can end up being mineralized, so released as nitrate or ammonium or maybe that nitrogen from the amino acids is incorporated into biomass. And all of this occurs through particular metabolite or metabolic processes. 

So what the NMR is hopefully going to allow us to do is track those metabolites. 

Dawn Stringer: Is this helping you progress this project faster than you could have without the EMSL capabilities? 

Amisha Poret-Peterson: 

Yes. So EMSL has state-of-the-art instrumentation, the 15 NMR methods are very difficult and so scientists have been very patient with us and working diligently to get the methods working on our particular soils. 

Dawn Stringer: Yeah, that's what we're here for. That's what we like to hear from our users for sure. Is there anything else that you want to add about your orchard recycling research that I didn't ask you about yet? 

Amisha Poret-Peterson: 

Well, one thing, when we think about the environmental benefits of whole orchard recycling, this is a practice that is usually done before planting of a new orchard. But there is potential to integrate orchard recycling with other practices. Whole orchard recycling and stacking other soil health practices on top of it is something that I'm interested in researching further so we can think about, you know, what happens if you do compost commitment to orchards that have undergone whole orchard recycling because that's a very common practice. 

One thing, what about repeated applications of woodchips to the soil? So they potentially may become a source of woodchips just with orchards being removed. And so, you know, maybe it's not put back into the ground that the trees are removed from, maybe it's used for other orchards. So what does that basically ramp up or increase soil organic matter accumulations in soil? For almonds and walnuts because they are harvested from the ground currently, there's not a lot that can be done in the alleyway of orchards, but if there are systems designed to basically do something called catch harvesting, so the nuts do not hit the ground, that opens the opportunity to use cover crops in almond and walnut orchards. And some growers actually do use cover crops in these systems already. Essentially, if whole orchard recycling becomes the only option for tree disposal or the primary option for tree disposal, how does whole orchard recycling interact with or integrate with other management practices? 

Dawn Stringer: Like all research projects, lots of questions to be answered, but a lot of progress being made as well, which is really exciting. And as this progress continues to happen, where can listeners learn more about your work in this field? 

Amisha Poret-Peterson: Sure. So we do have a website through the USDA. If you Google Crops Pathology and Genetics Research Unit, you can find me listed under there and my contact information. So I am also on LinkedIn. My colleagues and I have done some public outreach, so presentations and articles and trade magazines, so those are available for people who are interested in whole orchard recycling. 

But one way would be to directly contact me. 

Dawn Stringer: Very cool. Well, thank you so much for meeting with me today. This is really interesting information and I'm excited for people to hear about it. 

Amisha Poret-Peterson: Thank you for the invitation. 


Dawn Stringer: While there’s still a lot of work to be done with Amisha’s research, whole orchard recycling is proving to be a benefit to the Earth as farmers rotate trees. In addition, burning in California could be phased out by 2025 in hopes of improving air quality. 

This research shows us just another way EMSL’s users are finding climate smart options for existing processes. 


Dawn Stringer: Thank you for listening to Bonding Over Science, I’m Dawn Stringer for the Environmental Molecular Sciences Laboratory.  

We don’t have time to cover it all, so don’t forget to check out EMSL-DOT-PNNL-DOT-GOV for a full article on this topic featuring who I spoke with today. And don’t forget to follow us on all social media platforms for the latest and greatest news coming from EMSL! 


Dawn Stringer: EMSL is a Department of Energy, Office of Science national user facility that accelerates scientific discovery and pioneers new capabilities to understand biological and environmental processes across temporal and spatial scales. EMSL leads the scientific community toward a predictive understanding of complex biological and environmental systems to enable sustainable solutions to the nation’s energy and environmental challenges. If you’re interested in working with EMSL, learn more at, that’s E-M-S-L-DOT-P-N-N-L-DOT-G-O-V.