Benjamin Rasmussen for Nature
Early on a cold spring morning, Diana Wall is trying out a tool normally used to make holes on golf courses — and she can't contain her excitement. Her team has always used more laborious methods to take samples of soil and its resident organisms. “Oh, that's a beautiful core,” she says as one student bags a sample filled with tiny roundworms. “Hello, nematodes!”
Wall, a soil ecologist and environmental scientist at Colorado State University in Fort Collins, has come to this site about an hour east of the campus to collect data for one of her latest experiments. She and her colleagues are creating an artificial drought in a patch of grassland by covering it with temporary shelters. They expect that predatory nematodes will die or enter a type of suspended animation, leaving the parasitic nematodes that prey on plants to dominate the ecosystem. “How do plants respond below-ground to drought?” she wonders.
Wall has been asking — and answering — similar questions about soil for decades. She has become one of the most celebrated and outspoken experts on the hidden biodiversity in dirt, having studied soils and their inhabitants in nearly every corner of the world. She has a special fondness for Antarctica, which she has visited almost every year since 1989. It was there that she and a colleague made a landmark discovery, demonstrating that the soil in one of the driest spots on Earth is home to some animal life and not sterile, as many had thought.
The same drive to challenge orthodoxy also helped her to advance in a field in which women were once rare. “Many times, I felt like I was hitting the glass ceiling and got discouraged,” she says, before emphasizing how things have improved. “Today, I love seeing so many women in Antarctic and other research.”
Alongside her own experiments, Wall has become an ambassador for soil science and conservation — at a time when soil ecosystems are being devastated by forces such as erosion, pollution, pesticides and climate change. Soil degradation over the past two centuries or so has released billions of tonnes of stored carbon into the atmosphere, and this discharge could accelerate, speeding up climate change. Beyond that, says Wall, the threats to soil could jeopardize food production, water quality and the health of humans, plants and animals. The current path, she says, “leaves our terrestrial biodiverse world as we know it very uncertain”.
The efforts of Wall and other scientists to raise the profile of soils have been making an impact. The United Nations declared 2015 the International Year of Soils, and in May, Wall travelled to Nairobi to launch the Global Soil Biodiversity Atlas — a compendium of information developed by a team of more than 100 scientists, which she helped to lead.
David Montgomery, a geomorphologist at the University of Washington in Seattle, says that Wall has inspired many other researchers in their science and outreach on topics important to society. “We need more first-rate scientists willing to speak in those arenas.”
Wedded to the ice
This month, Wall is busy planning for her next trip to Antarctica, which will come, as usual, just after Christmas. Her colleagues joke that those journeys keep Wall young because she often crosses the International Date Line on her birthday, essentially erasing the day from the calendar. Assuming that she passes her physical — for which she is swimming and cycling — this trip will be her 27th to Antarctica.
Wall is 72 and has seemingly boundless energy. Tall and thin, she speaks quickly and picks up the pace as she describes the zoo of organisms in soils, from nematodes to the vast array of microbes. She emphasizes how bacteria and other microorganisms provide services that humans take for granted: filtering water, stabilizing soil, improving air quality and recycling nutrients that enable crops to grow. “I like to think of it as this factory underground,” she says.
Wall credits her mother, a biology teacher, with helping to spark a lifelong interest in biology. Raised in Lexington, Kentucky, Wall got her PhD in plant pathology from the University of Kentucky in her home town. In 1972, she left for the US west coast to pursue postgraduate research in nematology — convinced that nematode parasites had a lot to reveal about how life behaves above ground.
California was a shock at first. “That was eye-opening to me, because I had never crossed the Mississippi River, and it was — oh my god, where are the trees?” she says. But she ended up liking it there, and the University of California, Riverside, remained her home for much of the next two decades.
She strung together a series of grants to keep her work going, confident that soil microorganisms were more significant than most researchers realized. “Originally, I was just convinced these all make a difference and I was waiting to be proven wrong,” she says.
Wall focused at first on nematodes in deserts and arid croplands, conducting the bulk of her research in southern California, New Mexico and Michigan. By the late 1980s, she was seeking ways to understand a species' impact on an ecosystem. “If you want to find out how a plant parasite has an effect on a root or a predator, how do you exclude everything in the soil except that?”
She tried chemicals to kill off species, but they also harmed what she wanted to study. Then a colleague suggested that Wall go somewhere without plants, where the food web was simpler. “I tossed around a number of places,” she says. “And we ended up in Antarctica.”
She and her colleague Ross Virginia from Dartmouth College in Hanover, New Hampshire, decided to collect samples in the McMurdo Dry Valleys, a series of ice-free basins near the US McMurdo research station. The valleys receive no snow or rain, and humidity is so low that researchers have found the mummified remains of seals that made their way into the valleys thousands of years ago. Previous researchers had discovered nematodes and other life near glacier-fed streams that trickle during summer, but experts thought that the dry soils making up most of the valleys were barren.
On one of Wall and Virginia's first visits to the Dry Valleys, they had just six hours to collect as many samples as possible before the helicopter returned to pick them up. They found nematodes in about 65% of the samples. “I couldn't believe it,” she says. Ultimately, this showed that life can thrive even in the most inhospitable underground environments, revealing that major ecosystems were being overlooked.
Wall has returned to the Dry Valleys every field season except 1992, when she didn't receive funding for the trip. To recognize that long-running research, the US Geological Survey named valleys there after Wall and Virginia.
Their work in Antarctica dovetailed with discoveries that Wall had previously made about how nematodes cope with extremely dry conditions in the US Southwest. In the Chihuahuan Desert, Wall and her colleagues showed that the worms rely on anhydrobiosis1: they shed most of their water and put metabolic activity on hold. Wall says that the nematodes end up looking like Cheerios, the ring-shaped dry cereal.
With one eye focused on tiny nematodes, Wall kept the other on the bigger picture of how these creatures fit into ecosystems. This was all part of her ever-growing desire to understand and highlight the importance of life underground — something routinely ignored by many researchers until roughly the past decade. Studies that tracked the decomposition of fallen leaves and other organic materials, for example, tended to overlook the role of soil organisms.
Wall says she grew tired of that limited perspective. “We wanted to show that animals are important in these processes.”
So in 2001, she started a global, multiyear project to measure the impact of soil animals. Her team sent mesh bags filled with hay to colleagues at more than 30 sites around the world. Placed in various locations, the bags attracted worms, beetles and other types of soil invertebrate, while control bags excluded them. Wall's team then analysed the carbon content in each bag and compared the rates at which the organic matter decomposed with and without the soil animals. The results supported Wall's point: soil fauna increased decomposition rates significantly in many regions4. A follow-up study5 found that excluding soil fauna reduced decomposition rates by a global average of 35%.
Those studies helped to convince researchers to pay more attention to life in soil (see ‘Soils under siege’). “We now understand how key these organisms are to many ecosystem processes,” says Amy Austin, an ecologist at the University of Buenos Aires.
Source: Global Soil Biodiversity Atlas
The litter finding means that there could be big changes in how carbon moves throughout ecosystems as forces such as climate change alter soil communities.
Wall and her colleagues have seen some of this up close during their most recent field season in Antarctica. In as-yet-unpublished work, they found that the dominant nematode in the Dry Valleys, an endemic genus named Scottnema, has been declining in number, whereas a nematode that lives in wetter soils, Eudorylaimus, has been increasing, thanks to the melting of ice and permafrost. “It looks like there's going to be a species shift,” she says. “It's a fight for habitat.”
Scottnema is Wall's favourite nematode. “It's living in this harshest environment, mostly by itself, and it's just so recognizable,” she says. But that's not the only reason that she has concerns about the species' decline.
The two nematodes feed on different carbon sources in the soil, and population changes could alter the rate at which underground carbon escapes into the atmosphere. If so, the carbon-storage potential of the soil in Antarctica — a crucial region for absorbing carbon dioxide from the atmosphere — could change. Shifts in soil biota elsewhere on the planet could also affect how much carbon remains locked up, she says.
Emissary for soil
In August this year, Wall found herself at the White House talking about soils with other experts and policymakers as part of a national effort to prevent erosion and promote soil health. It was the latest scene in a role she has increasingly embraced over the past 15 years — to bring soil health to the global stage.
As Wall's research career blossomed, she took on more leadership positions. She served as president of the American Institute of Biological Sciences in 1993 and the Ecological Society of America in 1999. By that point, her involvement in these organizations was making her think bigger. “I'd been pretty concentrated on the Antarctic research,” she says. “I thought I should be doing more.”
She began participating in and leading initiatives that were increasingly global in scope — chairing, for example, the International Biodiversity Observation Year starting in 2001, which funded research projects to highlight the importance of biodiversity around the world. In 2011, Wall became the founding science chair of the Global Soil Biodiversity Initiative, the group behind the soil atlas that was launched in May. Looking forward, Wall wants to integrate data on soil health and biodiversity into global policies for mitigating large-scale environmental challenges. And she's talking to colleagues about launching a big US experiment to unravel the relationships between soils, biodiversity and health. “Conservation and protecting species is a very old idea, and so is soil conservation. But only now are these two ideas coming together,” she says.
While campaigning for soils, Wall has also been a champion for women in science. When she was starting out, there weren't many role models for women in her field. And when she made her first trips to Antarctica, she made do with men's long underwear and boots, and endured eight-hour flights on military aircraft that lacked sit-down toilets.
Wall was initially turned down for a tenure-track position at the University of California, Riverside, in the late 1980s — a decision that she and others suggest was related to her gender. Jill Baron, director of the North American Nitrogen Center at Colorado State University, says that how Wall recovered from that rejection is emblematic of her character. “She moved on into this stellar career,” says Baron. “And she's been working to make sure that other young women who come in don't have to ever have that again.”
That kind of drive makes a big impression on people just entering science. Ashley Shaw, a PhD student studying under Wall, recalls their first meeting. “She was just so enthusiastic about her science and what she was working on,” says Shaw. “I walked away feeling like I could save the world.”
Wall joined Colorado State University in 1993 to become director of the institution's Natural Resource Ecology Laboratory. There, colleagues say, she attracted interdisciplinary, accomplished scientists, which elevated the stature of the lab both on and off campus. She now serves as founding director of the university's School of Global Environmental Sustainability.
There, in an office covered in photos and paraphernalia from Antarctica, she talks eagerly about her goals for the future — and takes offence when people ask her if she plans to retire. “Whether I pass my physical to go to Antarctica or get too old and have to have a wheelchair dropped for me from the sky,” she says, “I want to keep working on the issues.”
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