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FIGURE 4.28. Vertical distribution of astigmatic mites in conventional and no-tillage agroecosystems. Arrows indicate autumn and spring dates for mowing, tillage, and planting. Numbers increased under conventional tillage following autumn tillage, but not following spring tillage (from Perdue and Crossley, 1990).

described a buildup of astigmatic mites following pipeline construction in Ontario, Canada. The mites were associated with accumulations of residue under moist conditions. Philips (1990) provided keys to families and genera of soil inhabiting Astigmata.

Most of the soil Astigmata are microbial feeders. Those with chelate chelicerae are able to chew vegetable material, fungi, and algae (Philips, 1990). Members of the Anoetidae have reduced chelae; their palpi are highly modified strainers for filter feeding on microbial colonies (Philips, 1990).

Occasionally, species in the family Acaridae become pests in microbiology laboratories, where they reproduce rapidly on agar plates. They are readily cultured on Baker's yeast; a few grams of yeast left untend-

ed in a collembolan culture will soon become infested with acarids. We have found astigmatic mites as contaminants in some Tullgren extractions. When fresh agricultural products are stored in the laboratory, large populations of Astigmata may develop, and may wander into Tullgren funnels during extractions. Similar population excursions of prostigmatid mites (family Cheyletidae) have also yielded extensive contamination of Tullgren samples. It is good practice to operate some empty, "control" funnels to check for the possibility of wandering microarthropods in the funnel room.

Other Microarthropods

In addition to mites and collembolans, Tullgren extractions contain a diverse group of other small arthropods. Although not numerous in comparison to mites and collembolans, Tullgren extractions may have abundances of several thousand per square meter. Collectively, the "other" microarthropods have relatively small biomasses and probably have no major impact on soil ecology. Such a judgment may be premature, in view of the general lack of information about their ecology.

Small spiders and centipedes, occasional small millipedes, insect larvae, and adult insects occur in soil cores extracted on Tullgren funnels. Most of these are better sampled as macroarthropods using hand sorting or trapping methods. Some insects (small larvae of carabid and elaterid beetles, thrips, pselaphid beetles, tiny wasps) are sometimes numerous enough to be effectively sampled from soil cores. Social insects, such as ants and termites, require special sampling considerations.

Protura

Proturans (Fig. 4.29) are small, wingless, primitive insects readily recognized by their lack of antennae and eyes (Bernard, 1985). Seldom as numerous as the other microarthropods, proturans occur in a variety of soils worldwide, often associated with plant roots and litter. Keys to families and genera of the Protura were published by Copeland and Imadaté (1990) and by Nosek (1973).

Numbers reported in the literature range between 1000 and 7000 per square meter at best (Petersen and Luxton, 1982). They penetrate the soil to surprising depths (25 cm), considering that they do not appear to be adapted for burrowing (Price, 1975; Copeland and Imadaté, 1990). Their feeding habits remain unknown. Observations that they feed on mycorrhizae (Sturm, 1959) have not been verified, but their occurrence in the rhizosphere of trees with mycorrhizae would support Sturm's observations.

FIGURE 4.29. Proturan.

Diplura

Diplurans are small, elongate, delicate, primitive insects. They have long antennae and two abdominal cerci. Most diplurans are euedaphic, but some are nocturnal cryptozoans, hiding under stones or under bark during the day. They occur in tropical and temperate soils in low densities. In Georgia Piedmont agroecosystems, the authors have sampled dipluran populations with Tullgren extractions, finding populations of approximately 50 per square meter.

Two common families of diplurans are found in soils, readily separable by their abdominal cerci. Campodeidae (Fig. 4.30) have filiform cerci; Japygidae (Fig. 4.31) have cerci modified as pinchers. Keys to families and subfamilies were provided by Ferguson (1990a). The japygids are predators on small arthropods (such as collembolans), nematodes, and enchytraeids. The cerci are used in capturing prey. Campodeids are predators on mites and other small arthropods, but also ingest fungal mycelia and detritus (Ferguson, 1990a). These animals are adapted for life in the soil by their elongate narrow form, sensory antennae, and sensory cerci.

FIGURE 4.30. Campodeidae. FIGURE 4.31. Japygidae.

Microcoryphia

The jumping bristletails (family Machilidae) were formerly included in the order Thysanura with silverfish and relatives, but now are placed in the separate order Microcoryphia. They are closely related to one another (Ferguson, 1990b). Machilids, when disturbed, can leap a distance of 10 cm (Denis, 1949). They feed on a wide variety of primitive plant materials such as lichens and algae. We have observed Machilids (presumably Machilis sp.) on rocky cliff faces in Georgia. They emerge at dusk from cracks in the rock surface on Stone Mountain and other granite domes on the Georgia Piedmont, and may reach densities of 50 per square meter. They return to their crevices at dawn. They fall prey to spiders (Pardosa lapidocina) on these outcrops (Nabholz et al., 1977).

Pseudoscorpionida

Pseudoscorpions (Fig. 4.32) are minute copies of their more familiar relatives, the scorpions, except that they lack tails and stingers. They occur throughout the terrestrial world except for Arctic and Antarctic regions. Pseudoscorpions are small cryptozoans, hiding under rocks and bark of trees, but they are occasionally extracted from leaf litter samples

FIGURE 4.32. Pseudoscorpion.

in Tullgren funnels. They are predaceous on small arthropods, nematodes, and enchytraeids. Keys to families and genera were provided by Muchmore (1990).

False scorpions are found in a number of habitats but not in large numbers. They can move readily through small spaces and crevices. Wallwork (1976) notes that two important habitat features for pseudoscorpions are high humidity and the availability of small crevices. Forest leaf litter provides both of these features, as do bark of decomposing logs, caves, nests of small mammals, and similar habitats.

Hand collecting is successful but Tullgren extraction is the usual means of sampling pseudoscorpions (Hoff, 1949; Muchmore, 1990).

Symphyla

Symphylids (Fig. 4.33) are small, white, eyeless, elongate, many-legged invertebrates that resemble tiny centipedes. They differ from centipedes in several characteristics, but superficially symphylids have but 12 body segments and 12 pairs of legs, whereas centipedes have at least 15 pairs of legs, the first pair modified as fangs. Edwards (1990)

FIGURE 4.33. Symphylid (Dindal, 1990).

provides a partial key to genera, and notes that the North American fauna of Symphylids is badly in need of further revision. Symphylids are part of the true eudaphic fauna, occurring in forest, grassland, and cultivated soils. They are omnivorous and can feed on the soft tissues of plants or animals (Edwards, 1959). Some species reach pest status in greenhouse soils where they feed on roots of seedlings (Edwards, 1990). Symphylids have silk glands near the end of the abdomen. The function of silk strands for these soil dwellers is obscure.

Pauropoda

Pauropods (Fig. 4.34) are tiny (1.0-1.5mm long) terrestrial myriapods with 8-11 pairs of legs and a distinctive morphological feature—branched antennae (Scheller, 1988). They are white to colorless and blind; these characteristics make them members of the true eudaphic fauna. Pauropods occur in soils worldwide but are not well known. They are commonly collected in Tullgren extractions but are seldom numerous, usually fewer than 100 per square meter. In forests, they inhabit the lower litter layers, F-layers, and mineral soil; they also occur in agricultural soils. It is generally assumed that pauropods are fungus feeders, but they may also be predaceous. Little information has been accumulated about their biology or ecology (Scheller, 1990). The taxonomy of the group is in need of revision. Although considered to be poor in species, probably less than 20% of extant species have been described. For example, Scheller (2002), working in the Great Smoky Mountains National Park, has found more than 30 species of Pauropods previously undescribed in that region, with seven or eight of them new to science.

Enchytraeidae

In addition to earthworms (discussed in the next section), another important family of terrestrial oligochaeta is the Enchytraeidae. This group of small, unpigmented worms (Fig. 4.35), also known as "pot-worms," is classified within the "microdrile" oligochaetes and consists of some 600 species in 28 genera. Species from 19 of these genera are found in soil, the remainder occurring primarily in marine and freshwater

FIGURE 4.34. Pauropoda (Dindal, 1990).

FIGURE 4.35. Anterior end of Mesenchytraeus solifugus and Enchytraeus albidus. (a) End view of M. solifugus displaying head pore (hp), and sensory structures (ss) at tip of prostomium. The mouth (m) and one cluster of setae (s) are visible in the lower region of the image. (b) Ventral view ofM. solifugus displaying mouth, head pore, and sensory structures. (c) Ventral view of E. albidus displaying mouth, sensory structures, and four setal clusters. Scale bar = 50 |im (from Shain et al., 2000).

FIGURE 4.35. Anterior end of Mesenchytraeus solifugus and Enchytraeus albidus. (a) End view of M. solifugus displaying head pore (hp), and sensory structures (ss) at tip of prostomium. The mouth (m) and one cluster of setae (s) are visible in the lower region of the image. (b) Ventral view ofM. solifugus displaying mouth, head pore, and sensory structures. (c) Ventral view of E. albidus displaying mouth, sensory structures, and four setal clusters. Scale bar = 50 |im (from Shain et al., 2000).

habitats (Table 4.7) (Brinkhurst and Cook, 1980; Dash, 1990; van Vliet, 2000). The Enchytraeidae are thought to have arisen in cool temperate climates where they are commonly found in moist forest soils rich in organic matter; interestingly, Tynen (1972) described the occurrence of "ice worms," which were enchytraeids that emerged onto snow- and ice-covered ground in British Columbia. Various species of enchytraeids are now distributed globally from subarctic to tropical regions.

Taxonomic organization of the European Enchytraeidae was definitively treated by Nielsen and Christensen (1959, 1961, and 1963); much less work has been done in other parts of the world. More recently, keys to the common genera were presented by Dash (1990). Identification of enchytraeid species is difficult, but genera may be identified by observing internal structures through the transparent body wall of specimens mounted on slides (Fig. 4.36).

The Enchytraeidae are typically 10-20mm in length and they are anatomically similar to the earthworms, except for the miniaturization and rearrangement of features overall. They possess setae (with the exception of one genus), and a clitellum in segments XII and XIII, which contains both male and female pores. Sexual reproduction in enchytraeids is hermaphroditic and functions similarly to that in earthworms. Cocoons may contain one or more eggs and maturation of newly

TABLE 4.7. Enchytraeid Genera and Their Occurrence in Various Environments

Genera occurring in soil

Other genera

Environment

Achaeta

Bryodrilus

Buchholzia

Cernosvitoviella

Cognettia

Enchytraeus

Enchytronia

Fridericia

Guaranidrilus

Hemienchytraeus

Hemifridericia

Henlea

Isosetosa

Lumbricillus

Marionina

Mesenchytraeus

Oconnorella

Stercutus

Tupidrilus

Aspidodrilus

Barbidrilus

Enchylea

Enchytraeina

Grania

Pelmatodrilus Propappus Randidrilus Stephensoniella epizoic on earthworms fresh water only found in Enchytraeid culture marine marine epizoic on earthworms fresh water marine marine

From van Vliet, 2000.

FIGURE 4.36. Morphological characters of an enchytraeid worm. amp., ampulla: an. sept., ante-septal; br., brain; d.bv.o., dorsal blood vessel origin; ec.g., ectal gland; eff.dt., efferent duct; e.op., ental opening; es., esophagus; es.int.tr., esophageal intestinal transition; m.pha., muscular pharynx; neph., nephridia; oc., oocyte;pha., pharynx;p.b., penial bulb;pepneph., peptonephridia;p.sept., postseptal; se., setae; sept.g., septal gland; sm.v., seminal vesicle; sp., spermatheca; sp.dt., sperm duct; sp.f., sperm funnel; t., testes (Soil Biology Guide, Dindal, D. L., ©1990, John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.).

FIGURE 4.36. Morphological characters of an enchytraeid worm. amp., ampulla: an. sept., ante-septal; br., brain; d.bv.o., dorsal blood vessel origin; ec.g., ectal gland; eff.dt., efferent duct; e.op., ental opening; es., esophagus; es.int.tr., esophageal intestinal transition; m.pha., muscular pharynx; neph., nephridia; oc., oocyte;pha., pharynx;p.b., penial bulb;pepneph., peptonephridia;p.sept., postseptal; se., setae; sept.g., septal gland; sm.v., seminal vesicle; sp., spermatheca; sp.dt., sperm duct; sp.f., sperm funnel; t., testes (Soil Biology Guide, Dindal, D. L., ©1990, John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.).

hatched individuals ranges from 65 to 120 days, depending on species and environmental temperature (van Vliet, 2000). Enchytraeids also display asexual strategies of parthenogenesis and fragmentation, which enhance their probability of colonization of new habitats (Dosza-Farkas, 1996).

Enchytraeids ingest both mineral and organic particles in the soil, although typically of smaller size ranges than those ingested by earthworms. Numerous investigators have noted that finely divided plant materials, often enriched with fungal hyphae and bacteria, are a principal portion of the diet of enchytraeids; microbial tissues are probably the fraction most readily assimilated, because enchytraeids lack the gut enzymes to digest more recalcitrant soil organic matter (Brockmeyer, 1990; van Vliet 2000). Didden (1990, 1993) suggested that enchytraeids feed predominantly upon fungi, at least in arable soils, and classified a community as 80% microbivorous and 20% saprovorous. As with several other members of the soil mesofauna, the mixed microbiota that occur on decaying organic matter, either litter or roots, are probably an important part of the diet of these creatures. The remaining portions of the soil organic matter, after the processes of ingestion, digestion, and assimilation, enter the slow-turnover pool of soil organic matter. Zachariae (1963, 1964) studied the nature of enchytraeid feces and found that they had no identifiable cellulose residues. In addition, Zachariae suggested that so-called "collembolan soil," said to be dominated by collembolan feces (particularly low-pH mor soils) were really formed by Enchytraeidae. Mycorrhizal hyphae have been found in the fecal pellets of enchytraeids from pine litter (Fig. 4.37) (Ponge, 1991). There is also the strong likelihood that enchytraeids consume and further process larger fecal pellets and castings of soil fauna such as collembolans and earthworms (Zachariae, 1964; Rusek, 1985).

Enchytraeid densities range from less than 1,000 to more than 140,000 individuals per square meter in intensively cultivated agricultural soil in Japan and a peat moor in the United Kingdom, respectively (Table 4.8). In a subtropical climate, Coleman et al. (1994a) reported enchytraeid densities of 4,000 to 14,000 per square meter in agricultural plots in the Piedmont of Georgia, whereas van Vliet et al. (1995) found higher densities (20,000 to 30,000 individuals per square meter) in surface layers of deciduous forest soils in the southern Appalachian Mountains of North Carolina. Although enchytraeid densities are typically highest in acid soils with high organic content, Didden (1995) found no statistical relationship between average density and annual precipitation, annual temperature, or soil pH over a broad range of data; local variability may be at least as great as variation on a wider scale. Enchy-traeid densities show both spatial and seasonal variations. Vertical distributions of enchytraeids in soil are related to organic matter hori-

FIGURE 4.37. Two enchytraeid worms, indicated by arrows, tunneling through a pine needle (fecal pellets have been deposited on the outside) in the F1 layer (modified from Ponge, 1991).

zonation; up to 90% of populations may occur in the upper layers in forest and no-tillage agricultural soils, but densities may be higher in the Ah horizon of grasslands (Davidson et al, 2002). Seasonal trends in enchytraeid population densities appear to be associated with moisture and temperature regimes (van Vliet, 2000).

Enchytraeids have been shown to have significant effects on organic matter dynamics in soil and on soil physical structure. Litter decomposition and nutrient mineralization are influenced primarily by interactions with soil microbial communities. Enchytraeid feeding on fungi and bacteria can increase microbial metabolic activity and turnover, accelerate release of nutrients from microbial biomass, and change species composition of the microbial community through selective grazing. However, Wolters (1988) found that enchytraeids decreased mineralization rates by reducing microbial populations and possibly by occluding organic substrates in their feces. Thus the influence of enchytraeids on soil organic matter dynamics is the net result of both enhancement and

TABLE 4.8. Enchytraeid Abundances (annual average number/m2) in Different Ecosystems and Locations

Ecosystem

Location

Forest

Douglas fir

Pinus radiata 50 stems/ha Pacific silver fir, mature stand Pinus radiata 200 stems/ha Spruce

Rhododendron-Oak 1160 m altitude Rhododendron-Oak 750 m altitude Pine

Pinus radiata 100 stems/ha Scots pine forest Deciduous forest Spruce

Pacific silver fir, young stand Pinus radiata 0 stems/ha Spruce Spruce

Arable land

Sugarbeet

Winterwheat

NT corn-clover

CT corn-clover

Potato field

Barley, no N

Rye field

Barley, 120kgN

Rice/wheat/barley (organic)

Rice/wheat/barley (conven.)

Moor

Juncus peat Nardus Blanket bog Fen

Grassland

Grassland soil Lucerne ley Grassland 10 sheep/ha Grassland 30 sheep/ha

Wales

New Zealand WA, United States New Zealand Norway

NC, United States NC, United States Norway New Zealand Sweden

United Kingdom South Finland WA, United States New Zealand South Finland North Finland

The Netherlands

The Netherlands

GA, United States

GA, United States

Poland

Sweden

Poland

Sweden

Japan

Japan

United Kingdom United Kingdom United Kingdom Canada

Sweden Sweden

Australia (NSW) Australia (NSW)

134,300 64,002 49,400 39,270 34,700 32,630 26,811 22,900 21,391 16,200 14,590 13,400 11,400 10,647 8200 4000

30,000 19,437 16,830 15,270 13,200 10,000 9800 8100 4940 525

145,000 71,000 40,000 5600

24,000 9900 6000 2300

Modified from van Vliet, 2000.

inhibition of microbial activity, depending on soil texture and population densities of the animals (Wolters, 1988; van Vliet, 2000).

Enchytraeids affect soil structure by producing fecal pellets which, depending on the animal size distribution, may enhance aggregate sta-

bility in the 600-1000 Mm fraction (Didden, 1990). In forest floors, these pellets are composed mainly of fine humus particles, but in mineral soils, organic matter and mineral particles may be mixed into fecal pellets with a loamy structure (Kasprzak, 1982). Davidson et al. (2002) estimated that enchytraeid fecal pellets constituted nearly 30% of the volume of the Ah horizon in a Scottish grassland soil (Fig. 4.38). Encapsulation or occlusion of organic matter into these structures may reduce decomposition rates. Burrowing activities of enchytraeids have not been well studied, but there is evidence that soil porosity and pore continuity can increase in proportion to enchytraeid body size (50-200 Mm diameter) (Rusek, 1985; Didden, 1990). Van Vliet et al. (1993, 1997) observed that enchytraeids in small microcosms increased soil porosity and hydraulic conductivity, depending on the distribution of organic matter and enchytraeid population densities.

Enchytraeids are typically sampled in the field using cylindrical soil cores of 5-7.5cm diameter; large numbers of replicates may be needed for a sufficient sampling due to the clustered distribution of enchytraeid populations (van Vliet, 2000). Extractions are often done with a wet-funnel technique (O'Connor, 1955), similar to the Baerman funnel extraction used for nematodes. In this case, soil cores are submerged in water on the funnel and exposed for several hours to a heat and light source from above; enchytraeids move downward and are collected in the water below (see van Vliet, 2000, for a comparison of modifications of this technique).

FIGURE 4.38. Thin section micrographs of fecal pellets in a grassland soil. (a) Derived from enchytraeids (scale bar = 0.5 mm); (b) Derived from earthworms (scale bar = 1.0 mm) (from Davidson et al., 2002).
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