J

Soil dumps

Endoecie

Food storage chambers

Periecie

Fungus combs f

Endoecie

Outer soil capping

Outer soil capping

i mmm^zmmm&M*

FIGURE 7.8 Termite mounds. Diagrammatic representation of different types of concentrated nest systems. (a) Hodotermes mossambicu. (b) Macrotermes subhyalinus. (c) Nasutitermes exitiosus (from Lee and Wood, 1971).

Actions of Worms, with Observations of Their Habits," which called attention to the beneficial effects of earthworms. Since then, a vast literature has established the importance of earthworms as biological agents in soil formation, organic litter decomposition, and redistribution of organic matter in the soil (Hendrix, 1995; Edwards, 1998).

Earthworms are classified within the Phylum Annelida, Class Oligochaeta. Species within the Families Lumbricidae and Megascolecidae are ecologically the most important in North America, Europe, Australia, and Asia. Some of these species have been introduced worldwide by human activities and now dominate the earthworm fauna in many temperate areas. Any given locality may be inhabited by all native species, all exotic species, a combination of native and exotic species, or no earthworms at all. Relative abundance and species composition of local fauna depend greatly on soil, climate, vegetation, topography, land use history, and, especially, past invasions by exotic species.

Earthworm Distribution and Abundance

Earthworms occur worldwide in habitats where soil water and temperature are favorable for at least part of the year. They are most abundant in forests and grasslands of temperate and tropical regions and least so in arid and frigid environments, such as deserts, tundra, or polar regions. Earthworm densities in a variety of habitats worldwide range from <10 to >2000 individuals m~2, the highest values occurring in fertilized pastures and the lowest in acid or arid soils (coniferous or sclerophyll forests). Typical densities from temperate deciduous or tropical forests and certain arable systems range from <100 to over 400 individuals m~2, representing a range of from 4 to 16 g dry mass m~2. Intensive land management (especially soil tillage and application of toxic chemicals such as common soil and plant pesticides) often reduces the density of earthworms or may completely eliminate them. Conversely, degraded soils converted to conservation management often show increased earthworm densities after a suitable period of time (Curry et al., 1995; Edwards and Bohlen, 1996).

Biology and Ecology

Earthworms are often grouped into functional categories based on their morphology, behavior, and feeding ecology and their microhabitats within the soil (Lee, 1985; Lavelle, 1983). Epigeic and epi-endogeic species are often polyhumic, meaning they prefer organically enriched substrates and utilize plant litter on the soil surface and C-rich upper layers of mineral soil. Polyhumic endogeic species inhabit mineral soil with high organic matter content (>3%), such as the rhizosphere, while meso- and oligohumic endogeic species inhabit soil with moderate (1-3%) and low (<1%) organic matter contents, respectively. Anecic species exploit both the surface litter as a source of food and the mineral soil as a refuge. The familiar Lumbricus ter-restris is an example of an anecic species, constructing burrows and pulling leaf litter down into them. The American log worm (Bimastos parvus) exploits leaf litter and decaying logs with little involvement in the soil, making it an epigeic species. Epigeic species promote the breakdown and mineralization of surface litter, whereas anecic species incorporate organic matter deeper into the soil profile and facilitate aeration and water infiltration through their formation of burrows.

Influence on Soil Processes

Earthworms, as ecosystem engineers (Lavelle et al., 1998), have pronounced effects on soil structure as a consequence of their burrowing activities as well as their ingestion of soil and production of castings (Lavelle and Spain, 2001; van Vliet and Hendrix, 2003). Casts are produced after earthworms ingest mineral soil or particulate organic matter, mix and enrich them with organic secretions in the gut, and then deposit the material as a slurry lining their burrows or as discrete fecal pellets. Excretion of fecal pellets can occur within or upon the soil, depending on earthworm species. Turnover rates of soil through earthworm casting range from 40-70 t ha-1 y-1 in temperate grasslands (Bouché, 1983) to 500-1000 t ha-1 y-1 in tropical savannas (Lavelle et al., 1992).

While in the earthworm gut, casts are colonized by microbes that begin to break down soil organic matter. As casts are deposited in the soil, microbial colonization and activity continue until readily decomposable compounds are depleted. Mechanisms of cast stabilization include organic bonding of particles by polymers secreted by earthworms and microbes, mechanical stabilization by plant fibers and fungal hyphae, and stabilization due to wetting and drying cycles and age-hardening effects (Tomlin et al., 1995). Mineralization of organic matter in earthworm casts and burrow linings produces zones of nutrient enrichment compared to bulk soil. These zones are referred to as the "drilosphere" and are often sites of enhanced activity of plant roots and other soil biota (Lavelle et al., 1998). Plant growth-promoting substances have also been suggested as constituents of earthworm casts. Many earthworm castings from commercial vermicomposting operations are sold commercially as soil amendments to improve soil physical properties and enhance plant growth (Edwards, 1998).

Earthworm burrowing in soil creates macropores of various sizes, depths, and orientations, depending on species and soil type. Burrows range from about 1 to >10 mm in diameter and constitute among the largest of soil pores. Continuous macropores resulting from earthworm burrowing may enhance water infiltration by functioning as by-pass flow pathways through soils. These pores may or may not be important in solute transport, depending on soil water content, the nature of the solute, and chemical exchange properties of the burrow linings (Edwards and Shipitalo, 1998).

Earthworm Effects on Ecosystems

Despite the many beneficial effects of earthworms on soil processes, some aspects of earthworm activities may be undesirable (Lavelle et al., 1998, Parmelee et al., 1998). Detrimental effects include: (1) removing and burying of surface residues that would otherwise protect soil surfaces from erosion; (2) producing fresh casts that increase erosion and surface sealing; (3) increasing compaction of surface soils by decreasing soil organic matter, particularly for some tropical species; (4) riddling irrigation ditches, making them leaky; (5) increasing losses of soil N through leaching and denitrification; and (6) increasing soil C loss through enhanced microbial respiration. Earthworms may transmit pathogens, either as passive carriers or as intermediate hosts, raising concerns that some earthworm species could be a vector for the spread of certain plant and animal diseases. The net result of positive and negative effects of earthworms, or any other soil biota, determines whether they have detrimental impacts on ecosystems (Lavelle et al., 1998). An effect, such as mixing of O and A horizons, may be considered beneficial in one setting (e.g., urban gardens) and detrimental in another (e.g., native forests). Edwards (1998) provides a review of the potential benefits of earthworms in agriculture, waste management, and land remediation.

FORMICIDAE (ANTS)

Formicidae, the ants, are probably the most significant family of soil insects, due to the very large influence they have on soil structure. Ants are numerous, diverse, and widely distributed from arctic to tropical ecosystems. Ant communities contain many species, even in desert areas (Whitford, 2000), and local species diversity is especially large in tropical areas. Populations of ants are equally numerous. About one-third of the animal biomass of the Amazonian rain forest is composed entirely of ants and termites, with each hectare containing in excess of 8 million ants and 1 million termites (Holldobler and Wilson, 1990). Furthermore, ants are social insects, living in colonies with several castes.

Ants have a large impact on their ecosystems. They are major predators of small invertebrates. Their activities reduce the abundance of other predators such as spiders and carabid beetles (Wilson, 1987). Ants are ecosystem engineers, moving large volumes of soil, as much as earthworms do (Holldobler and Wilson, 1990). Ant influences on soil structure are particularly important in deserts, where earthworm densities are low. Given the large diversity of ants, identification to species is problematic for any but the taxonomist skilled in ants. Wheeler and Wheeler (1990) offer keys to subfamilies and genera of the Nearctic ant fauna.

TERMITIDAE (TERMITES)

Along with earthworms and ants, termites are the third major earth-moving group of invertebrates. Termites are social insects with a well-developed caste system. Through their ability to digest wood they have become economic pests of major importance in some regions of the world (Lee and Wood, 1971; Bignell and Eggleton, 2000). Termites are highly successful, constituting up to 75% of the insect biomass and 10% of all terrestrial animal biomass in the tropics (Wilson, 1992; Bignell, 2000). While termites are mainly tropical in distribution, they occur in temperate zones as well. Termites have been called the tropical analogs of earthworms, since they reach a large abundance in the tropics and process large amounts of litter. Termites in the primitive families, such as Kalotermitidae, possess

The EXTERNAL RUMEN: Incorporation of feces into internal mound materials and fresh constructions, with stimulation of microbial activity and further processing of soil organic matter

P3, with aerobic periphery and anaerobic core

Mandibles with crushing molar plates

Anterior P1 with peristaltic musculature

Enteric valve and armature

P4b with peristaltic and antiperistaltic musculature

FOOD = rich soil mound material

Mandibles with crushing molar plates

Anterior P1 with peristaltic musculature

Enteric valve and armature

FOOD = rich soil mound material

Labile end-products Labile end-products

Selection of soil fractions rich in silt and clay

Non-enzymatic secretion, or microbial lysis?

K+ secretion; filament propagation and mixing with ingested soil

Alkaline hydrolysis at pH up to >12.0

Fermentation in core, and mineralization at periphery of lumen

Neutralization of alkaline pH and aerobic processing of soil organic matter?

FIGURE 7.9 Hypothesis of gut organization and sequential processing in soil-feeding Cubitermes-clade termites. The model emphasizes the role of filamentous prokaryotes, the extremely high pH reached in the P1, and the existence of both aerobic and anaerobic zones within the hindgut. Major uncertainties have question marks. Not to scale (from Brauman et al., 2000).

a gut flora of protozoans, which enables them to digest cellulose. Their normal food is wood that has come into contact with soil. Most species of termites construct runways of soil and some are builders of spectacular mounds. Members of the phylogenetically advanced family Termitidae do not have protozoan symbionts, but possess a formidable array of microbial symbionts (bacteria and fungi) that enable them to process and digest the humified organic matter in tropical soils (Breznak, 1984; Bignell, 1984; Pearce, 1997). A generalized sequence of events in a typical Termitinae soil-feeder gut is illustrated in Fig. 7.9 (Brauman et al., 2000).

Three nutritional categories include wood-feeding species, plant- and humusfeeding species, and fungus growers. The last group lacks intestinal symbionts and depends upon cultured fungus for nutrition. Termites have an abundance of unique microbes living in their guts. One recent study of bacterial microbiota in the gut of the wood-feeding termite Reticulitermes speratus found 268 phylotypes of bacteria (16S rRNA genes, amplified by PCR), including 100 clostridial, 61 spirochaetal, and 31 Bacteroides-related phylotypes (Hongoh et al., 2003). More than 90% of the phylotypes were found for the first time, but we do not know if they are active and participating in wood decay. Other phylotypes were mono-phyletic clusters with sequences recovered from the gut of other termite species. Cellulose digestion in termites, which was once considered to be solely due to the activities of fungi and protists and occasionally bacteria, has now been demonstrated to be endogenous to termites. Endogenous cellulose-degrading enzymes occur in the midguts of two species of higher termites in the genus Nasutitermes and in the Macrotermitinae (which cultivate basidiomycete fungi in elaborately constructed gardens) as well (Bignell, 2000).

In contrast to the C-degradation situation, only prokaryotes are capable of producing nitrogenase to fix N2. This process occurs in the organic-matter rich, microaerophilic milieu of termite guts. Some termite genera have bacteria that fix relatively small amounts of N, but others, including Mastotermes and Nasutitermes, fix from 0.7 to >21 ^g N g-1 fresh wt day-1. This equals 20-61 ^g N per colony per day, which would double the N content if N2 fixation was the sole source of N and the rate per termite remained constant (N content of termites assumed to be 11% on a dry weight basis) (Breznak, 2000). For an extensive exposition of the role of termites in the dynamics of soil organic matter and nutrient cycling in ecosystems worldwide, refer to Bignell and Eggleton (2000).

summary

Soil fauna may be considered as very efficient means to assist microbes in colonizing and extending their reach into the horizons of soils worldwide. Their roles as colonizers, comminutors, and engineers within soils have been emphasized, but new technologies and global environmental issues are yielding new questions about how soil fauna contribute to the long-term sustainability of soils. The demand for taxonomic specialists for all groups of soil biota is increasing as we currently recognize that molecular information alone is insufficient for many studies. Stable isotope technologies are revealing that some soil faunal species hitherto thought to be within one trophic group are not, thus leading to research about the structure and resilience of soil food webs. Information on the biogeography of soil fauna, their latitudinal gradient patterns, their relationship to aboveground hot spots and to land management strategies, as well as their taxonomic status and natural history, will be critical for understanding how microbes and soil fauna will interact and respond to multiple global changes (Wall et al., 2001). For example, soil invertebrates can be invasive species, which, depending on the species, can affect soil carbon sequestration, soil fertility, and plant and animal health and result in economic and ecosystem change. For further reading on the roles of fauna in soil processes, see Coleman et al. (2004) and Wall (2004).

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