Antibiotics in Soils + Practical Takeaways with Dr. Jane Lucas
Image description: photo by Evan Gora. Dr. Lucas is smiling in front of a mountain view. She's wearing a red sweater and gray scarf.
In mid-May, I attended a virtual conversation on soil health with Dr. Jane Lucas, Community Ecologist with the Cary Institute for Ecosystem Studies in Millbrook, NY.
I learned a lot from Dr. Lucas about antibiotics and antibiotic resistance in soils, and I wanted to explore this topic further. I also watched Dr. Lucas’ February 2023 talk, “Soil Health in the Age of Antibiotics,” hosted by the Dutchess Land Conservancy.
In this article, I’ll share my main takeaways on antibiotics in soils—including a few hopeful fun facts, and things we can do in response to this issue.
Antibiotics in Soils: A Natural Phenomenon
I mainly hear about antibiotics in the health world. I hadn’t considered that antibiotic compounds are a common natural occurrence.
*Folks around the world have used herbs with antimicrobial compounds as remedies for millennia. But to my knowledge, people didn’t conceive of bacteria and antimicrobial compounds the way modern/Western medicine and science do.
Humans have only understood and knowingly used antibiotics for a century.* But organisms have been creating them for millions of years for defense, communication, and maintaining access to limited resources.
One example can be found in bean-loving bacteria. Gardeners may be familiar with legumes’ relationship with nitrogen-fixing bacteria. The bacteria take up residence around a plant’s roots and provide it with nitrogen in a usable form. In beans, this beneficial bacteria was found to produce an antibiotic compound, helping to protect the plant from pathogens. Dr. Lucas noted that this is a relatively new discovery. We may find many more antibiotic compounds in root systems in years to come.
So, What's the Problem?
Though antibiotic compounds aren’t foreign in the natural world, human activity has significantly increased their presence in soil. About 80% of human-introduced antibiotics are administered to animals in livestock production. Antibiotics also pass through human bodies to our water system via toilets, and they leach from landfills after disposal.
Neither animals nor humans process antibiotic drugs very efficiently. Depending on the antibiotic, cattle may poop out anywhere from ~15% to 95% of an antibiotic (or a metabolite(s) of the drug that also possesses antibacterial properties).
Image description: Kelly, Highgate Farm Dog and Sentinel, watches over chard and lettuce beds (Marshall, NC)
These drugs have made a significant impact on the soil ecosystem. Where livestock are given regular doses of antibiotics (for disease prevention and treatment, and also to increase their size), a 25-50% decrease in active soil microbes has been recorded.
Soil invertebrates—mites, worms, nematodes, spiders, insects, etc—are a vital component of the soil community, helping to store water and recycle dead material into fresh earth. Antibiotics change the makeup of invertebrate populations. They decrease the presence of invertebrates that feed on microbes—whether directly, and/or just by killing their food.
Interestingly, antibiotics can increase the populations of predator invertebrates, like spiders and predatory mites. (Killing a bunch of their food creates an easy feast.)
The soil microbes that survive antibiotic exposure operate less efficiently. They have to work harder to survive, and they release more carbon dioxide in the process. The soil's ability to hold carbon decreases, which has negative implications for climate change.
What Does This Mean for Our Garden Soils?
According to Dr. Lucas, antibiotics can create “big shifts” in the function of our soils—from nutrient cycling to water retention.
With fewer microbes, soils can’t hold as much water. Soils become less drought-tolerant, and we need to water more intensively.
Image description: Kohlrabi bulbs are ready to harvest on mulched soil.
Antibiotics change nutrient availability in soils. Once antibiotics enter the soil, there’s an initial increase in nitrogen—probably from the sudden death of many microbes. (Some antibiotics also contain nitrogen.) This nitrogen quickly leaches out of the soil.
Soil bacteria play an important role in making nutrients accessible to plants. In addition to the nitrogen-fixing bacteria mentioned above, "many bacteria secrete enzymes in the soil to make phosphorus more soluble and plant-available” (source). I’m interested in learning more about how antibiotics affect plants’ ability to utilize vital nutrients.
Antibiotics can increase the pH in soils. For folks whose soil pH is already at 7 or above, this can make some nutrients less available to plants.
This article notes additional negative impacts that antibiotics can have on plants, including delayed germination and stunted growth. In that study, veggies and other herbaceous plants were more negatively impacted by antibiotics than grasses.
Antibiotics in our Lawns
Many seeds and cropping systems use antibiotics. Grass seed is often coated with an antifungal substance, which lands in our lawn soils and beyond.
Interestingly, human-tended lawns have shown the greatest rates of antibiotic resistance--even more than farms. (More on antibiotic resistance below.)
Like a good scientist would, Dr. Lucas didn’t draw conclusions about why lawns are problematic in this way. The data she referenced doesn’t speak to this. And, I imagine that avoiding grass seed treated with antimicrobials, as well as conventional fertilizers and herbicides that alter the soil microbiome, could make a difference in preventing antibiotic resistance in our lawns.
Whatever the case, it’s worth noting that our most antibiotic-resistant soils may be the ones closest to home.
Image description: Two individuals flip sod to begin creating a vegetable garden.
Antibiotic Resistance in Soils
According to the World Health Organization, antibiotic resistance is one of the biggest threats to food security and global health. We don't know much about natural or “normal” levels of antibiotic resistance in soils. We should! Millions of people have died from antibiotic resistance. It costs us trillions of dollars.
Most antibiotics break down relatively quickly--some within a week. (Some break down into metabolites that also have antibacterial properties.) While the antibiotic compound may not stick around, antibiotic resistance can last “a long time”—especially when soils face additional challenges, like drought.
Where humans interact with an ecosystem heavily, research has shown a 47% greater rate of antibiotic resistance.
We don’t know much about the correlation between antibiotic resistance in soils and humans in close proximity. Dr. Lucas would love to link up with human health researchers to start drawing these lines. More funding is needed in this area.
We know that antibiotic resistance can move from manure to crops planted in it. But Dr. Lucas noted that this doesn’t seem to translate to antibiotic-resistant infections in humans very often.
Image description: a stand of sunflowers in full bloom.
Things We Can Do to Prevent + Remedy Antibiotic Resistance:
Add healthy compost to soils.
Earthworms! They magically eliminate antibiotic resistance somehow. Pretty cool. (They're not native here in the US, but they still do us good.)
Look for grass seed that isn’t coated in fungicides. Better yet: choose native grasses to plant in your yard, and/or replace monocropped lawns with diverse perennials, shrubs, and trees. Natives are best for our local ecosystems.
Know where your meat comes from. Find out how antibiotics are used. Are they only given to treat illness? Or are they given on a regular basis to prevent disease in close-quartered animals, and to grow larger animals?
Farmers and gardeners: Don't spread fresh manure on fields.
Wash your produce to avoid contact with antibiotic-resistant bacteria.
Rethink antibacterial soap. A dear friend of mine has a Ph.D. in microbiology. When she worked in a lab, she and her peers used “regular old soap” to clean supplies—never antibacterial soap. ‘Regular soap kills bacteria just fine,’ she noted. ‘I saw for myself under the microscope.’
Bring unused pharmaceuticals to hospitals that accept drop-offs, rather than tossing them in the trash or toilet.
I’ll add this one to our list: choose compost products that don’t contain biosolids (treated sewage sludge). According to this journal article, “Though some wastewater treatment processes can degrade antibiotics, there is notable variability in antibiotic removal rates. This can be attributed to differences in treatment processes, such as nature of influent, treatment plant capacity, and the type of technology use.”
Finally, Dr. Lucas encouraged us to “engage with your policymakers. Encourage them to use informed regulations—in particular, ones that consider a One Health perspective to ensure that we’re using antibiotics in the best way possible.” She noted that we should consider humans, animals, and ecosystem health together. The reality is that we’re tightly linked.
Watch Dr. Lucas’ talks here:
Stressed Out Soils: In Conversation with Dr. Jane Lucas
Soil Health in the Age of Antibiotics
Learn more about Dr. Lucas’ work at the Cary Institute here.
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