Sun. Oct 2nd, 2022
Multicolored autumn mountain slope.
enlarge If a rotting cadaver releases a deluge of nitrogen into the soil, it can change the appearance of nearby trees. At least that’s the theory.

Since 1980, the Forensic Anthropology Center at the University of Tennessee has probed the depths of the most macabre of sciences: the decomposition of human bodies. Popularly known as the Body Farm, scientists here investigate how donated cadavers decay, such as how the microbiomes in us get confused after death. That microbial activity leads to bloating and eventually a body will puncture. A rich fluid of nutrients, especially nitrogen, flows out for plants on the Body Farm to absorb.

That gave a group of researchers at the University of Tennessee, Knoxville, an idea: What if that explosion of nutrients actually changes the color and reflection of a tree’s leaves? And if so, what if law enforcement agencies could use a drone to scan a forest, looking for these changes to find deceased missing people? Today, in the journal Trends in Plant Science, researchers formally put forward the idea — which, to be clear, is still theoretical. The researchers are just beginning to study how a plant’s phenotype — its physical characteristics — might change if a human body composes nearby. “What we’re proposing is to use plants as indicators of human decomposition, to hopefully be able to use individual trees in the forest to help determine where someone died, to aid in body recovery,” says UT Knoxville- plant biologist Neal Stewart, co-author of the new paper.

While a large mammal such as a human decomposes in a forest, its decomposition changes the soil in a number of ways. The body’s “necrobiome” — all the bacteria that were inside it when it was alive — replicates like crazy in the absence of an immune system. This necrobiome mixes with the microbes in the dirt. “The soil microbiome will change and of course the plant roots will feel changes too,” Stewart says. But, he adds, “we don’t really know what those changes are.”

It’s also still unclear how the gases emitted from a body might affect plants in the area. In addition, a cadaver attracts a horde of opportunistic critters which further complicate the dynamics in the game. Scavengers such as vultures can peck at a body, while flies can lay eggs that incubate carnivorous maggots. These larvae can be in such numbers that another group of researchers found that they form meandering rivers around pig carcasses left in the forest. Visiting animals can also contribute their urine and feces to the harmful mix. That is all to say that while the microbial community in the soil will change, the animal community above will also change, with as yet unknown effects on plants in the surrounding area.

One factor Stewart’s team is considering is the cadaver’s role in supplying nitrogen to the soil. As the body decomposes, it floods the soil with the chemical — perhaps even too much nitrogen for some plant species like grasses, which initially die around a cadaver. In the longer term, this nutrient helps plants grow so that the later vegetation springs back. But how plants can bounce back — whether those plants look different because of this infusion of nutrients — is unclear. “Surprisingly, no one I can tell has ever really studied this in a systematic way,” Stewart says.

Looking for a response

So Stewart and his colleagues set out in June to do just that. “Right now, for the Body Farm study,” Stewart says, “we basically just take the trees and shrubs that grow naturally in our lots, put donors — as they call them — and look at the responses from the leaves, reactions of plants, at different distances.” Because this work only started a few months ago, the researchers don’t have any data to share yet. But next year they plan to take soil exposed to decomposing human bodies and apply it to plants in a greenhouse to see if it changes the appearance of their leaves. “A controlled study, a repeat study,” Stewart says.

There are a few ways a leaf reaction can occur. For starters, the leaves may reflect more or less light or change color, for example turning greener or even red. “We’re definitely looking at green, but we’re also looking at all the wavelengths within visible light, including those on either side of the visible,” Stewart says. For example, the chlorophyll in plant leaves fluoresces and emits light that special optics can capture. This signature is invisible to the human eye, but not to existing technologies. It is known as hyperspectral detection. These devices can actually see across the electromagnetic spectrum, including visible and infrared light. Fluorescence is specifically an indicator of what is in the cell walls of a plant leaf. Stewart and his colleagues think that an amino acid like phenylalanine, which is released from a dead body, can cause leaves to fluoresce in a certain way.

Bring on the cannabis!

Because different types of plants fluoresce differently, authorities are already using this technology to detect cannabis and poppy cultivation. Farmers and scientists also use it for other types of agriculture, as chlorophyll fluorescence is an indicator of productivity. “Hyperspectral imaging has been used to correctly classify pork and other animal fats and is also used for nutrient soil management in detecting phosphorus and other nutrients,” said Mississippi State University microbiologist Heather Jordan. (She does the aforementioned work on pig carcasses, but was not involved in this new study.)

But to use this technology to find the remains of missing people, Jordan adds, “it would be necessary to distinguish the effects of human decomposition from those of the decomposition of other animals.” After all, man is not alone among the large mammals. Pigs, deer, bears – you name it, and it rots there in the wild, flooding the soil with the same nutrients as a human cadaver.

However, there can be subtle differences in how plants respond to the decomposition of different types of mammals. For example, we humans have different microbiomes and a different fat content than, for example, a bear. “I think once we find our key spectral signatures, we can think about a comparative study with other large mammals,” Stewart says.

Another challenge is the variability of decay. It’s one thing to fly an airplane over a forest and spot a cannabis crop with hyperspectral imaging, but “that’s quite another to a randomly localized death decaying at an unknown rate,” says Mississippi ecologist Brandon Barton. State University, which partners with Jordan. † Any number of variables affect how a dead organism releases the nutrients and microbes that can affect surrounding vegetation, such as how hot it is or how much moisture there is. All this, he says, is “a lot of background noise.”

“When a coyote comes by”

And in Barton’s experiments with pig carcasses, the body often doesn’t hang around for long — at least not the meat — so eventually not all the nitrogen stored in a body will be available as fertilizer. “The chance that all that nitrogen will end up in the soil is quite small,” he says. “Because flies are going to consume tons of it. If a coyote comes by, it will eat a lot of it. Vultures, crows, all these organisms will remove it.” If a plant’s leaves change only slightly in response to a smaller dose of nitrogen than they would have received if a body had been left undisturbed, that could make it challenging for searchers to find a signal.

Since the researchers are just starting to collect data on how plants might respond to human cadavers, this is all still theoretical. But imaging technology will only become more sensitive over time. Even if this system can’t tell humans apart from other large mammals, if authorities generally know where to look for a missing person, they can scan a few square miles, get some hits on where plants might change, and investigate every spot. Some spots may turn out to be covering the remains of deer, but one could direct them to a person. And if it works, for the families of those people, this science could one day bring the end.

This story originally appeared on wired.com.

By akfire1

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