The Johns Hopkins Gazette: April 28, 2003
April 28, 2003
VOL. 32, NO. 32


Tilling Takes Toll on Soil's Tiny Creatures, Student Finds

By Michael Purdy

Johns Hopkins Gazette Online Edition

It's a long journey from going into college unsure of what you want to study to presenting original research in a congressional office building, but Sarah Placella made the trip in a little less than four years.

Placella, a senior from Glenwood, Md., presented research on the effects of agricultural land management practices on soil organisms at the Rayburn House Office Building in Washington, D.C., on April 1. She had won a place in a competitive national poster session for undergraduate researchers, Posters on the Hill.

"When you do research, you learn so much that you can't learn in the classroom," Placella said about her study. "It can be very intense."

Sarah Placella on a sampling expedition to gather soil creatures for her study of three farming systems and their effects on the ecological health of the land.

Placella found evidence that farmers who use conventional techniques, which involve frequent tilling and leaving the soil bare for extended periods of time, may be decreasing the long-term ecological health of soil by disrupting the small creatures whose activity normally restores and rejuvenates it.

The strongest indicator of concern so far is a sharp decrease in earthworms in a tilled plot versus other plots where tilling occurs less frequently and cover crops are used in between plantings.

"Earthworms are really important because they make burrows, creating niches where other organisms can live," explained Placella, author of the report and a senior in Earth and Planetary Sciences in the Krieger School of Arts and Sciences. "Earthworms take organic matter from the top of the soil, and they pull it through the profile, mixing it with mineral soil. The burrows also help with infiltration, allowing water to go deep into the ground."

Placella remembered her path to Earth and planetary sciences as unpredictable but oddly direct in retrospect.

"I've always been interested in a lot of different things, and I wasn't very good at picking one particular thing that I wanted to major in," Placella said. "The great thing about Earth sciences is that it's not just geology, it's not just chemistry, it's not just physics or biology; it's all of them."

Not too long after she first checked into the Earth and Planetary Sciences Department and found "the friendliest department at Johns Hopkins," Placella met her mentor, Kathy Szlavecz, a senior lecturer who specializes in soil organisms.

Szlavecz helped Placella successfully apply for a Research Experience for Undergraduates grant from the National Science Foundation. With funding from that grant, Placella conducted studies on three experimental farming systems at the U.S. Department of Agriculture's Research Center in Beltsville, Md. One of the three systems was tilled using conventional methods, another was not tilled, and a third was farmed with organic techniques that included heavy use of cover crops, plants grown over the winter to protect the soil from erosion and provide soil-building organic matter.

"The USDA has a sustainable agriculture project set up in Beltsville. It was a really great opportunity for me because the long-term field study is all set up, with all the wrinkles ironed out," Placella said. "I'm finding with my senior thesis project [I'm doing now] that because my work is outside, there are many variables that I can't control and problems that arise. But the scientists at the USDA have been working on this for a long time; they've already fixed the unexpected problems."

"With the exception of the weather," Placella's mentor Szlavecz interjects later. "You can't fix that."

Placella placed traps in the three farming systems she studied in spring and summer 2001 and classified and counted the organisms caught in the traps, including ants, spiders, crickets, millipedes, centipedes, daddy longlegs and isopods (more commonly known as pill bugs or roly polys). This process involves taking a mass of creatures caught in a trap, pulling them out one by one with tweezers, and sorting them into cups or other containers depending on their species.

She also sampled earthworms in the area by extracting them with mustard solution.

"[Sorting and counting] takes forever, but it's interesting," Placella said. "When you look at the cups, sometimes you can tell what treatment the soil is undergoing just by looking at them, if you've done it enough."

Scientists like Szlavecz have previously established that many species of soil organisms are non-native immigrants to the United States. They get transported here accidentally, in the potting soil of decorative plants or, in the early days of colonization, in soil used as ship ballast. All the earthworms Placella found in the three farming systems in Beltsville were non-native species. For her senior thesis project, a study of 72 samples of soil organisms from forests around Baltimore, Placella is currently finding a similar predominance of non-native species.

The main focus of Placella's earlier research was a comparison of the diversity of soil organisms present in the three USDA systems. She found the fewest earthworms in the till system. She speculated that this might be because the tilling physically damages both earthworms and their burrows, and the lack of organic matter provided by a winter cover crop provides less food to the earthworms. Studies in other regions of North America and Europe have previously shown similar results. Earlier studies also have linked earthworm population and potential nitrogen mineralization, which refers to the transfer of nitrogen, an essential plant nutrient, back from decaying organic matter into the soil. Scientists think this link may hinge on the microbes that live in earthworm burrows, which are believed to play a pivotal role in mineralization processes.

In addition to her earthworm finding, Placella observed that ant populations increased in tilled soil. She offered two potential explanations: Tilling may have made it easier for the ants to tunnel in the soil, or the physical disturbance of soil provided the ants with additional food, such as seeds.

"Soil organisms respond to soil management practices," Placella said. "A lot of the variations could be due to the fact that there was different material in and atop soil according to management practice. Generally, though, the organic farming method supported a greater diversity of organisms."

Szlavecz said Placella's results had helped her win a grant from the Center for a Livable Future at the Johns Hopkins Bloomberg School of Public Health.

Placella, who expects to graduate in May, plans to take a year or two off before returning to graduate studies in Earth sciences.