Drilosphere
The drilosphere is defined as the whole soil volume that is under the influence of earthworms, including the earthworm's burrow as well as the body and internal structures of the worm that come in contact with the soil. (2) It generally includes the burrow of the earthworm and the area 2mm away from it, but some effects are known to extend to 4 to 8mm away from it. (4) As ecosystem engineers, earthworms have the ability to change the soil, both physiochemically and biologically (2). The drilosphere has the characteristics of a self-organized system, completely different from the surrounding soil. (1)
The term drilosphere was first coined in 1975 by M.B. Bouche. (4)
Earthworms and Soil Properties
Earthworms have the ability to influence macroporosity, infiltration rates, air permeability, aggregate stability, and water holding capacity. (4) One experiment found a positive relationship between earthworm burrow length and and saturated hydraulic conductivity, or the ease with which water can move through pore space. Earthworms are known to increase the formation of macroaggregates. (4) Another study found that earthworms reduce surface crusting by increasing aggregate stability and infiltration rates. (4) This is caused by the aggregation of organic matter and sand, silt, and clay particles into structural units through the digestion and secretion of soil by the earthworm. (4) The drilosphere was found to have lower pore diameter and specific pore volume than bulk soil. (4) Water content at field capacity, carbon mineralization rates, and the ratio of carbon mineralization rates to total carbon were greater in the drilosphere than bulk soil. (4)
One study done by Andriuzzi that added 13C and 15N to plant litter around earthworm burrows found that, after 45 days, earthworm activity had increased nutrients in the drilosphere, as well as up to 50-75 mm from the soil. (4)
A study in 1999 done by Tiuov and Scheu found that organic carbon, total nitrogen, pH, basal respiration, microbial biomass, and bacterial volume were significantly higher in the drilosphere than in the surrounding bulk soil. (4)
Castings
After digestion, the earthworm releases castings as waste. (1) These casts are very fertile, and contain the digestive plant and soil material, as well as bacteria from the intestine of the earthworm. Ammonium, phosphorous, and other nutrients are found at high concentrations. The casts can be globular or granular. (1)
Globular Casts
Globular casts are comprised of flattened units. (1) These are formed by worms that excrete small, independent pellets that rarely stick together.
Granular Casts
Granular casts are formed by small, fragile, and fine-textured pellets. (1) This makes them susceptible to runoff during rain events.
Geophagous Worms
Geophagous worms are worms that feed on soil. Ingesting soil frees dormant bacteria from tight pore spaces and restarts the complete enzymatic capacities of the bacteria. (1) The digestion system of earthworms allows them to make use of soil that may be poor or deplete in resources, because of their mutualistic relationship with bacteria that live in their digestive tract. (1)
Digestive System
The digestive system of worms consists of the pharynx, the esophagus, the crop, the intestine, and the gizzard. (3) The soil is ingested by the worm and swallowed by the pharynx. When the soil reaches the esophagus, the worm releases calcium carbonate to ensure that their is not excess calcium in the worm. (3) The food then moves into the crop, where it is stored, and then into the gizzard. (3) In the gizzard, stones eaten by the worm help grind up the food. The ground up food then moves down into the intestine, where gland cells release fluids to aid in digestion. The walls of the intestines contain blood vessels, which help to absorb and transport nutrients. (3)
References
(1) Cuddington, Kim, et al. Ecosystem Engineers: Plants to Protists. Academic Press, 2007.
(2)Brown, George, et al. Regulation of Soil Organic Matter Dynamics and Microbial Activityin the Drilosphere and the Role of Interactionswith Other Edaphic Functional Domains. European Journal of Soil Biology.
(3) https://www.sas.upenn.edu/~rlenet/Earthworms.html
(4)Johnson-Maynard, Jodi L., and Daniel G. Strawn. “Linking Physical and Biogeochemical Properties and Processes in the Drilosphere.” Soil Science, vol. 181, no. 3/4, 2016, pp. 126–132.
(5) https://kollathdesign.com/portfolio/worm-burrow-diagram/