Distribution Of Protozoa In Soil Profiles

Although protozoa are distributed principally in the upper few centimeters of a soil profile, they are also found at depth, over 200 m deep in groundwater environments (Sinclair and Ghiorse, 1989). Small (2-3 ^m cell size) microflagellates decreased 10-fold in numbers during movement through 1 m in a sandy matrix under a trickling-filter facility (in dilute sewage), compared to a 10-fold reduction in bacterial transport over a 10-m distance (Harvey et al., 1995).

rotifera

These small fauna are often found only when a significant proportion of water films exist in soils. While they may not be listed in major compendia of soil biota (Dindal, 1990), they are a genuine, albeit secondary, component of the soil fauna (Wallwork, 1976). Rotifers exist in bagged leaf litter and in even more extreme environments, such as the soils of the Antarctic Dry Valleys (Treonis et al., 1999).

More than 90% of soil rotifers are in the Order Bdelloidea, or worm-like rotifers. In these creeping forms, the suctorial rostral cilia and the adhesive disc are employed for locomotion (Donner, 1966). Rotifers, like tardigrades and nema-todes, can enter a desiccated resistant state (anhyrobiosis) at any stage in their life cycle, in response to environmental stress. When the stress is removed, they rehydrate and become active. Additional life history features of interest include the construction of shells from a body secretion, which may have particles of debris and/or fecal material adhering to it. Some rotifers will use the empty shells of Testacea, the thecate amoebae, to survive. The Bdelloidea are vortex feeders, creating currents of water that conduct food particles, such as unicellular algae or bacteria, to the mouth for ingestion. The importance of these organisms is largely unknown, although they may reach numbers exceeding 105 m~2 in moist, organic soils (Wallwork, 1970). Rotifers are extracted from soil samples and enumerated using methods similar to those used for nematodes (see the following).

nematoda

The Phylum Nematoda contains nematodes or roundworms, which are among the most numerous and diverse of the multicellular organisms found in any ecosystem. It has been estimated that four of every five animals on earth are nematodes (Bongers and Ferris, 1999). As with the protozoa, rotifers, and tardigrades, nematodes live in water films or water-filled pore spaces in soils. Nematodes have a very early phylogenetic origin among the Eukarya (Blaxter et al., 1998), but as with other invertebrate groups, the fossil record is fragmentary. Nematodes are most closely related to the Rotifers, Gastrotrichs, and Nematophora. They are considered to be triploblastic pseudocoelomates (three body layers: ectoderm, mesoderm, and endoderm).

The overall body shape is cylindrical, tapering at the ends (Fig. 7.4). Nematode body plans are characterized by a "tube within a tube" (alimentary tract/the body wall). They have a complete digestive system or an alimentary tract, consisting of a stoma or stylet, pharynx (or esophagus), and intestine and rectum, which opens externally at the anus. The reproductive structures are complex, and sexes are generally dimorphic. Some species are parthenogenetic, producing only females. Nematodes are highly diverse but can be identified to order or family by examining specific morphological characteristics under high magnification (>100X) using compound microscopes.

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