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Do earthworms affect groundwater?

Do Earthworms Affect Groundwater?

Earthworms, nature's master tillers, can enhance plant growth by making soil porous so it drains and aerates well.

But do the underground channels they create speed downward movement of agricultural chemicals? If the answer is no, at least for some worm species, what should be done to foster their good works?

The North Application Experimental Watershed at Coshocton, Ohio has been the scene of possibly the longest no-till experiment in the country: 27 years and still going strong.

No-till - that is, planting crops without first plowing to clear and bury the residue of the previous crop - was designed to reduce surface runoff of soil and agricultural chemicals in water that flows into takes and streams - not specifically to preserve groundwater quality, says William M. Edwards, leader of conservation tillage research at Coshocton.

The absence of plowing and the abundant crop residue left on the surface does create a favorable environment for many earthworms, he says. But he's not convinced earthworms play more than a very small role in the transport of chemicals to groundwater.

Edwards did find that vertical burrows can provide flow paths to carry nitrate and other agricultural chemicals down into the soil. But just because the chemicals may go down a few feet doesn't mean they will end up in groundwater, he says.

To further put the problem in perspective, he notes that in one summer less than 4 percent of rainwater that infiltrated soil at a Coshocton site went into burrows at nightcrawlers (Lumbricus terrestris).

Nightcrawlers, usually 4 to 8 inches long and about 3/8 inch thick, feed on crop residues near the soil surface mostly at night. They build permanent burrows to the surface that go down as far as necessary to escape winter cold and daytime summer heat - in Ohio, perhaps 3 feet or more.

Seven other earthworm species at Coshocton were identified by ARS entomologist Edwin C. Berry of the National Soil Tilth Laboratory, Ames, Iowa. Adults of these species were usually 2 to 6 inches long and half the diameter or less of nightcrawlers.

Most of their activity is in the first foot of topsoil as they make temporary burrows, passing soil containing organic matter - all the food they need - through their bodies.

Small horizontal burrows may help water to soak into the soil to be used by plants without moving it anywhere near the groundwater, says Edwards. Increased infiltration also means less surface runoff is available to carry fertilizers and pesticides to takes and streams.

Moreover, the burrows provide soil aeration to accelerate microbial action, speeding the breakdown of pesticides.

In the last couple of years, Edwards has turned most of his attention to pesticide movement. For his studies, he brings cubic-foot blocks no-till cornfield topsoil into the lab and exposes them to artificial rainstorms of various intensities. During and after each rainstorm, water drains through the soil and is collected in 64 funnels below each block.

Edwards found the amount of atrazine and alachlor herbicides collected in funnels below nightcrawler burrows depended greatly on the sequence of rainstorms. If the initial rainstorm was minor, it carried the herbicides just far enough down to mix with and bind to organic matter. Very little moved downward if a major rain then followed. But if a major storm came first, more of the herbicides moved downward.

Although ARS studies in Coshocton, have come up with some answer, what applies in Coshocton doesn't necessarily apply elsewhere.

In an ARS study at St. Paul, soil scientist Dennis R. Linden and his colleagues also saw relationships between organic matter from crop residues, earthworms, water infiltration, and chemical movement in the soil.

Soil from no-till plots where the researchers had removed cornstalks for 5 years supported only inconsequential earthworm populations. But plots where residues were left on the surface or where residues were incorporated into the soil by tillage supported active populations of common field worms. With increased burrowing, more water infiltrated the soil, says Linden.

Mixing cornstalks into the soil resulted in more infiltration, but the burrows were less continuous with the soil below, resulting in less chemical movement deep into the soil.

In further studies, the St. Paul scientists grew corn in several ways: in soil with no crop residue; with incorporated residue or surface residue, and with or without a common earthworm - Lumbricus rebellus - that lives near the soil surface. Then, to see how the earthworms might affect the movement of pesticides through the soil, the scientists applied potassium bromide to the soil surface and slowly applied simulated rainfall.

"We found surface residue provided a major food source for worms, helping them to form stable burrows that were open to the surface," Linden said. These burrows allowed rapid water infiltration, but 95 percent of the bromide was retained in the top 6 inches of soil. In soil with incorporated residue, that same amount of bromide went down 5.3 inches as more of the water was soaked up near the soil surface.

In a 1988 springtime survey of earthworms in Minnesota soils, Dennis J. Fuchs, then a graduate research assistant, and Linden found the rate at which water was conducted through the top foot of soil increased in proportion to the earthworm population.

The common field worm, Aporrectodea tuberculata, was the most populous earthworm species in the survey. Five other, less abundant species also stayed mostly in the upper foot of soil. The scientists also found:

* Alfalfa and forage grasses supported the highest earthworm populations (up to 830 per cubic meter of soil).

* Medium-textured soil types that were neither extremely acidic nor alkaline supported high populations; coarse-textured soils supported almost no earthworms.

In an ongoing 5-year study at the National Soil Tilth Laboratory, scientists are studying the effect of various life forms, including several earthworm species, on movement of plant nutrients and pesticides toward groundwater. During each season, the researchers measure distributions of roots, microbes, earthworms, and insects in several Iowa soils with differing tillage and residue management regimes.

"We plan to use these measurements to begin developing computer models that will help us predict pesticide breakdown, groundwater quality, and changes in soil tilth," says Jerry L. Hatfield, laboratory director. - By Ben Hardin and Don Comis, ARS.

William M. Edwards is at the USDA-ARS North Appalachian Experimental Watershed Research Unit, P.O. Box 478, Coshocton, OH 43812. Phone: (614) 545-6349. Dennis R. Linden is at the USDA-ARS Soil and Water Management Research Unit, 454 Borlaug Hall, University of Minnesota, St. Paul, MN 55108. Phone: (612) 625-6798. Jerry L. Hatfield and Edwin C. Berry are at the USDA-ARS Soil Tilth Research Laboratory, 2150 Pammel Drive, Ames, IA 50011. Phone: (515) 294-5723.

PHOTO : At the Soil Tilth Research lab in Ames, Iowa, technician Ken Ford uses a video camera adapted for microscopic work to study earthworm cocoons and newly hatched earthworm larvae. (K-4024-1)
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Title Annotation:includes related article
Author:Hardin, Ben; Comis, Don
Publication:Agricultural Research
Date:Apr 1, 1991
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