Agricultural Management Effects on Earthworm Populations
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SOIL QUALITY – AGRONOMY TECHNICAL NOTE No. 11 United States Department of Agriculture Agricultural Management Effects on Natural Earthworm Populations Resources Conservation Service Introduction: Earthworm habits and their effects on soil Soil Quality Many people consider earthworms to be an indicator of soil quality because they Institute respond to and contribute to healthy soil. For earthworms to be abundant, a field must meet several conditions that are also associated with soil quality and agricultural sustainability: moderate pH, surface residue for food and protection, and soil that is not waterlogged, compacted, droughty, or excessively sandy. Not all healthy soils will have earthworms. Worms are not common in sandy soils, in drier regions of the southern and western United States, and in local areas where earthworms have not yet migrated or been introduced by human activities. Earthworm species vary in how they get food, and thus inhabit different parts of the soil, and have somewhat different effects on the soil environment. They fall into three distinct ecological groups based on feeding and burrowing habits. Epigeic (litter dwelling) earthworms live and feed in surface litter. They move horizontally through leaf litter or compost with little ingestion of or burrowing Technical Note No. 11 into the soil. These worms are characteristically small and are not found in low organic matter soils. Lumbricus rubellus is an example of epigeic species. June 2001 Endogeic (shallow dwelling) earthworms are active in mineral topsoil layers and associated organic matter. They create a three-dimensional maze of burrows while This is the eleventh note in a series of consuming large quantities of soil. The genuses Diplocardia and Aporrectodea Soil Quality- have endogeic life habits. Anecic (deep burrowing) earthworms live in permanent, Agronomy technical notes on the effects nearly vertical burrows that may extend several feet into the soil. They feed on of land management surface residues and pull them into their burrows. Lumbricus terrestris is an on soil quality. example of an anecic species (Coleman and Crossley, 1996). Once established, earthworms contribute to soil function. They: Series written by: • Shred residues, stimulating microbial decomposition and nutrient release; Soil Quality Institute 411 S. Donahue Dr. • Produce casts rich in N, P, K, and other nutrients; Auburn, AL 36832 • 334-844-4741 x177 Improve soil stability, air porosity and moisture holding capacity by burrowing and aggregating soil; http://soils.usda.gov/sqi • Turn soil over and may reduce the incidence of disease by bringing deeper soil to the surface and burying organic matter; • Improve water infiltration by forming channels and promoting soil aggregation; and • Improve root growth by creating channels lined with nutrients for plant roots to follow. Both endogeic and anecic species are tainted residue before it has a chance to important in contributing to these functions leach down through burrows. Earthworms in agricultural systems. The shallow may act as plugs in their own burrows, and, dwellers improve topsoil porosity and the in the case of the herbicide atrazine, deep burrowing earthworms improve earthworm-feeding activity may actually infiltration and drainage. change the chemical to reduce its mobility (Farenhorst et al., 2000). In addition, Earthworms may have undesirable impacts earthworm activity improves soil structure if they remove too much surface residue and therefore reduces runoff of chemicals and leave the soil surface unprotected, or if into surface water. Another concern is that their burrows open into surface irrigation fresh earthworm casts on the surface are furrows. There is also some concern that unstable and may lead to higher soil earthworms may enhance the “preferential erosion and nutrients in runoff. Earthworm flow” of herbicides and other pollutants casts stabilize as they age so that the risk of down burrows and into groundwater, erosion is greatly reduced. In summary, causing water to pass too rapidly through earthworms affect soil function in multiple the soil matrix. When their total effect is ways. In specific situations they may have considered, however, earthworms are undesirable effects, but predominately they unlikely to have a significant negative contribute to improved soil quality and are impact on water quality. Where they are a sign of a healthy, properly functioning active, earthworms may bury herbicide- soil. What determines earthworm abundance? The number of earthworms in an • Moisture holding capacity and internal agricultural field is influenced by the drainage – Earthworms need moist but intensity and number of soil disturbance well-aerated soil. events like tillage and traffic, the • Rainfall and temperature – Climate abundance and quality of food sources, the affects the soil environment and food chemical environment of the soil, and the sources (plant biomass) for soil microclimate. Important factors of the earthworms. soil environment include: • Predation and parasitism. • Organic matter (food sources) – Higher inputs of fresh organic matter • Earthworm introduction – Even where are associated with greater earthworm environmental factors are favorable, populations. earthworms may not have migrated and established populations. • Soil type – Populations are highest in medium textured soil. When all other factors are equal, the availability of plant litter and organic • Depth to a restrictive layer – matter is usually the most important in Earthworms prefer deeper soil. determining earthworm abundance, but any • Soil pH – In general, earthworms will of the other factors may override the not thrive in a soil with a pH below 5 influence of organic matter. (Edwards and Lofty, 1977). 2 Many management practices affect of management: (1) tillage, (2) crop earthworm populations because they rotations and cover crops, (3) fertilizers, (4) change one or more of the environmental pesticides, (5) irrigation and drainage, and factors listed above. This technical note (6) worm seeding (inoculation). will examine the effects of six components Tillage As the number and intensity of tillage Research has found the following: operations increase, so does the physical destruction of burrows, cocoons, and the • Earthworms were reduced by 70% earthworm bodies themselves. Less compared to previously undisturbed sod after five years of plowing intensive tillage systems that leave residues (Edwards and Bohlen, 1996). on the surface throughout the year improve the environment for earthworms. The • After 25 years of conventional tillage residues provide food, insulate earthworms crop production earthworm populations from weather conditions, provide cover to were only 11-16% of what existed in protect them from birds and other surface the original grass field (Edwards and predators, and protect their burrows. Bohlen, 1996). Decreased tillage disturbances particularly • Edwards et al. (1995) reported up to 30 benefit night crawlers (L. terrestris), which times more earthworms in no-till move in the same burrow between deeper systems compared to plowed fields. soil layers and the soil surface in search of food. When tillage destroys the burrow, • In Nigeria, researchers found 2400 some earthworms will not have the energy earthworm casts/m2 in no-till plots 2 reserve to form a new burrow to their food compared to 100 casts/m under source. Endogeic (shallow dwelling) conventional tillage (Edwards and earthworms will tolerate annual tillage Lofty, 1977). because they continually form new burrows • In a Georgia experiment, no-till fields and acquire a greater proportion of their had an average of 967 earthworms/m2 food from the soil rather than surface litter. compared to 149 /m2 in conventionally No-till and other methods of conservation tilled fields (Coleman and Crossley, tillage such as chisel plowing and ridge 1996). tillage can increase populations of both types of earthworms (Edwards and Bohlen, Table 1. Earthworm Populations 2 1996) (Table 1). (No./yd ) as influenced by amount of surface residues at Langdon Research Although a single tillage event will not and Extension Center, ND (Deibert and drastically reduce earthworm populations, Utter, 1994). repeated tillage over time will cause a 40 - 45% 80 – 90% decline in earthworm populations. Residue Residue Earthworms 71 106 Cocoons 204 514 Total 275 620 3 Crop Rotations and Cover Crops Tillage affects decomposition and rootworms and anhydrous ammonia availability of surface residue, while choice fertilizer (Kladivko, 1993). of crop determines the quantity and quality of the residue as a food source for Crop rotations with pasture or hay greatly earthworms. Earthworm populations will increase earthworm numbers. There is a decrease to very low numbers under an strong correlation between earthworm exhaustive cropping system of plowing, numbers and years in grass and legumes. crop residue removal, and no additions of For example, a crop every third year in manure or other organic inputs. grass rotations will have greater earthworm numbers than a crop every two years or There is a strong correlation between annual cropping. Dick Thompson (Boone, earthworm numbers and the amount and Iowa) followed a 6-year rotation of corn, quality of residue returned to the soil. soybeans, oats, and 3 years of pasture (Table 3.) Generally, cereal crops such as (alfalfa, red clover, grasses, and other wheat (especially if straw is returned to the forages). He reduced tillage and used soil)