Illinois Natural History Survey - University of Illinois

Illinois Earthworms: Indicators of Soil Health?

Soil is one of Illinois' most valuable natural resources. It supports biomass production (including food, fiber, and energy); it filters, buffers, and transforms environmental pollutants; and it provides a biological habitat and genetic reserve for a vast number of organisms. The ability of soil to perform these functions is called soil quality or soil health. In recent years, concerns about the sustainability of food production and the off-site impacts of soil degradation have given impetus to efforts to develop objective criteria for assessing and monitoring soil quality. Although the initial focus in developing soil quality criteria has been on chemical and physical properties, scientists recognize that many important functions of soil, such as decomposition and nutrient cycling, are mediated by the organisms living in soil, and that an objective, comprehensive assessment of soil quality should include biological parameters. The soil is home to a complex community of organisms that includes bacteria, fungi, protozoa, nematodes, potworms, earthworms, mites, springtails, millipedes, centipedes, and other arthropods, so there are many potential candidates for biological indicators of soil quality.

Earthworms are the largest and most familiar soil invertebrates in Illinois soils. They are important members of the soil community because they can be the most abundant invertebrates by weight, reaching up to 1,000 lbs live weight per acre in some agricultural soils, and even more in unmanaged soils. Furthermore, because of the ability of earthworms to redistribute large amounts of soil and organic matter (roughly as much as their own body weight each day), they may shape the rest of the soil community and important ecosystem processes such as water infiltration, nutrient cycling, and decomposition. In Illinois, about 35 species of earthworms have been reported. Of these, roughly half are native species about which we know very little. The remainder are mostly European species that probably came to North America in soil used for ship ballast or on the roots of nursery stock.

During May-June 1997, as part of the Illinois Soil Quality Initiative (ISQI), Survey scientists sampled earthworm populations on 12 Illinois farms in conjunction with sampling of physical, chemical, and microbiological properties by University of Illinois soil scientists. Farms were clustered in groups of three that had similar hydrology and soil type but different management systems: 1) no-till corn-soybean rotation, 2) conventional tillage corn-soybean rotation, and 3) conventional tillage with a longer rotation including a perennial hay. We collected samples from cornfields on each farm (nine samples per field) as well as from uncultivated grassy border areas of each field (three samples per field). For each sample, we dug soil from a pit 12 inches x 16 inches x 8 inches deep. We hand sorted the earthworms from each sample, then counted, weighed, and (in the case of adults) preserved them for identification. We kept juvenile worms in lab culture for identification when they matured.

burrow.gifA nightcrawler in its permanent vertical burrow.

Because of our method of sampling, we collected mostly endogeic earthworms--worms that tunnel back and forth through the topsoil and feed on buried decaying organic matter and organic matter-rich soil. We did not collect anecic earthworms, such as the nightcrawler, Lumbricus terrestris,that form permanent vertical burrows 3-9 feet down into the soil and pull plant residues from the soil surface into their burrows to feed.

The most striking observation was the difference in abundance of earthworms between cultivated fields and their uncultivated grassy borders. Across all farming systems we estimated an average of 148,000 worms per acre within the fields (up to 292,000 worms per acre in one field) and almost four times as many, an average of 589,000 worms per acre, in the grassy border areas. In one of the grassy border areas, we estimated an abundance of 1.54 million worms per acre. The difference between in-field and grassy border populations was even greater when we considered the living weight of the earthworms: an average of 78 pounds per acre within fields, with a maximum of 179 pounds per acre, and almost 30 times that weight--an average of 2,263 pounds per acre and a maximum of 5,909 pounds per acre--in the grassy borders. Thus, topsoil-dwelling earthworms were not only less abundant within fields than in borders but they were also smaller. In contrast to the difference between fields and grassy borders, we were unable to detect a difference in the number or weight of topsoil-dwelling earthworms among the fields with different management practices.

dime2.gif
Endogeic earthworm that, unlike a nightcrawler, burrows back and forth rather than vertically through topsoil.

What caused the difference in earthworm populations between grassy borders and cropped fields? There are several possible explanations, keeping in mind that we sampled only topsoil-dwelling earthworm populations. During the winter, the thick layer of grass on the soil surface in the grassy borders may afford some protection to the soil; researchers have shown that bare soil freezes more deeply, resulting in higher earthworm mortality. The grassy borders may also provide a more abundant, stable, and nutritious food supply to support larger populations of earthworms. Agricultural practices and chemicals may result in higher mortality of earthworms within the field than occurs in uncultivated border areas. Finally, native earthworm species may not tolerate soil disturbance; we found them in the grassy border areas, but not within the fields.

We hope to relate our observations on earthworm populations to information about soil physical and chemical characteristics and soil management practices. By identifying factors that affect the abundance and structure of earthworm populations in agricultural soils, we hope to develop biological criteria for assessing and monitoring the health and quality of agricultural soils in Illinois.

Ed Zaborski, Center for Economic Entomology



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