Decomposition and Mineralization

An extensive literature has addressed the effects of detritivores on decomposition and mineralization rates (Coleman et al. 2004). Generally, the effect of arthropods on the decay rate of litter can be calculated by subtracting the decay rate when arthropods are excluded from the decay rate when arthropods are present (see Table 14.1). Detritivores affect decomposition and mineralization processes, including fluxes of carbon as CO2 or CH4, by fragmenting litter and by affecting rates of microbial catabolism of organic molecules. The magnitude of these effects depends on the degree to which feeding increases the surface area of litter and inoculates or reduces microbial biomass.

1. Comminution Large comminuters are responsible for the fragmentation of large detrital materials into finer particles that can be processed by fine comminuters and saprophytic microorganisms. Cuffney et al. (1990) and J. Wallace et al. (1991) reported that 70% reduction in abundance of shredders from a small headwater stream in North Carolina, United States, reduced leaf litter decay rates by 25-28% and export of fine particulate organic matter by 56%. As a result, unprocessed leaf litter accumulated (J. Wallace et al. 1995). Wise and Schaefer (1994) found that excluding macroarthropods and earthworms from leaf litter of selected plant species in a beech forest reduced decay rates 36-50% for all litter types except fresh beech litter. When all detritivores were excluded, comparable reduction in decay rate was 36-93%, indicating the prominent role of large com-minuters in decomposition. Tian et al. (1995) manipulated abundances of millipedes and earthworms in tropical agricultural ecosystems. They found that millipedes alone significantly accounted for 10-65% of total decay over a 10-week period. Earthworms did not affect decay significantly by themselves, but earthworms and millipedes combined significantly accounted for 11-72% of total decay. Haimi and Huhta (1990) demonstrated that earthworms significantly increased mass loss of litter by 13-41%. N. Anderson et al. (1984) noted that aquatic xylophagous tipulid larvae fragmented >90% of decayed red alder, Alnus rubra, wood in a 1-year period.

Termites have received considerable attention because of their substantial ecological and economic importance in forest, grassland, and desert ecosystems. Based on laboratory feeding rates, K. E. Lee and Butler (1977) estimated wood consumption by termites in dry sclerophyll forest in South Australia. They reported that wood consumption by termites was equivalent to about 25% of annual woody litter increment and 5% of total annual litterfall. Based on termite exclusion plots,Whitford et al. (1982) reported that termites consumed up to 40% of surficial leaf litter in a warm desert ecosystem in the southwestern United States (Fig. 14.3). Overall, termites in this ecosystem consumed at least 50% of estimated annual litterfall (K. Johnson and Whitford 1975, Silva et al. 1985). N. M. Collins (1981) reported that termites in tropical savannas in West Africa consumed 60% of annual wood fall and 3% of annual leaf fall (24% of total litter production), but fire removed 0.2% of annual wood fall and 49% of annual leaf fall (31% of total litter production). In that study, fungus-feeding Macrotermitinae were responsible for 95% of the litter removed by termites. Termites apparently consume virtually all litter in tropical savannas in East Africa (J. Jones 1989,1990).Termites consume a lower proportion of annual litter inputs in more mesic ecosystems. N. M. Collins (1983) reported that termites consumed about 16% of annual litter production in a Malaysian rainforest receiving 2000 mm precipitation year-1 and 1-3% of annual litter production in a Malaysian rainforest receiving 5000 mm precipitation year-1.

Accumulation of dung from domestic mammalian grazers has become a serious problem in many arid and semi-arid ecosystems. Termites removed as much as 100% of cattle dung over 3 months in Kenya (Coe 1977), 80-85% over 5-9 months in tropical pastures in Costa Rica (Herrick and Lal 1996), and 47% over 4 months in the Chihuahuan Desert in the southwestern United States (Whitford et al. 1982). In the absence of termites, dung would require 25-30 years to disappear (Whitford 1986). Dung beetles (Scarabaeidae) and earthworms also

Earthworm Gut Mineralization

Rate of gallery carton deposition (top) and mass loss (bottom) of creosote bush, Larrea tridentata, and fluff grass, Erioneuron pulchellum, foliage when subterranean termites were present (black symbols) or absent (white symbols) in experimental plots in southern New Mexico. Litter (10 g) was placed in aluminum mesh cylinders on the soil surface on August, 15,1979. Vertical lines represent standard errors. From Whitford et al. (1982) with permission from Springer-Verlag. Please see extended permission list pg 573.

are important consumers of dung in many tropical and subtropical ecosystems (e.g., Coe 1977, Holter 1979, Kohlmann 1991).

Relatively few studies have provided estimates of wood consumption by bark-and wood-boring insects, despite their recognized importance to wood decomposition. Zhong and Schowalter (1989) reported that bark beetles consumed 0.1-7.6% of inner bark and wood-boring beetles consumed an additional

0.05-2.3% during the first year of decomposition, depending on conifer tree species.Ambrosia beetles consumed 0-0.2% of the sapwood during the first year. Schowalter et al. (1998) found that virtually the entire inner bark of oak logs was consumed by beetles during the first 2 years of decomposition, facilitating separation of the outer bark and exposing the sapwood surface to generalized saprophytic microorganisms. Edmonds and Eglitis (1989) used exclusion techniques to demonstrate that, over a 10-year period, bark beetles and wood-borers increased decay rates of large Douglas-fir logs (42 cm diameter at breast height) by 12% and of small logs (26 cm diameter at breast height) by 70%.

Payne (1965) explored the effects of carrion feeders on carrion decay during the summer in South Carolina, United States. He placed baby pig carcasses under replicated treatment cages, open at the bottom, that either permitted or restricted access to insects. Carcasses were weighed at intervals. Carcasses exposed to insects lost 90% of their mass in 6 days, whereas carcasses protected from insects lost only 30% of their mass in this period, followed by a gradual loss of mass, with 20% mass remaining in mummified pigs after 100 days.

Not all studies indicate significant effects of litter fragmentation by macro-arthropods. Setala et al. (1996) reported that manipulation of microarthropods, mesoarthropods, and macroarthropods in litter baskets resulted in slower decay rates in the presence of macroarthropods. Most litter in baskets with macroarthropods (millipedes and earthworms) was converted into large fecal pellets that decayed slowly.

A number of studies have demonstrated that microarthropods are responsible for up to 80% of the total decay rate, depending on litter quality and ecosystem (see Table 14.1, Fig. 14.4) (Coleman et al. 2004, González and Seastedt 2001, Heneghan et al. 1999, Seastedt 1984). Seastedt (1984) suggested that an apparent, but insignificant, inverse relationship between decay rate as a result of microarthropods and total decay rate indicated a greater contribution of arthropods to decomposition of recalcitrant litter fractions compared to more labile fractions.Tian et al. (1995) subsequently reported that millipedes and earthworms contributed more to the decomposition of plant residues with high C : N, lignin, and polyphenol contents than to high-quality plant residues.

2. Microbial Respiration

Microbial decomposers are responsible for about 95% of total heterotrophic respiration in soil. Arthropods generally increase microbial respiration rates and carbon flux but may reduce respiration rates if they overgraze microbial resources (Huhta et al. 1991, Seastedt 1984). Several studies have documented increased microbial respiration as a result of increased arthropod access to detrital substrate and stimulation of microbial production.

Litter fragmentation greatly increases the surface area exposed for microbial colonization. Zhong and Schowalter (1989) reported that ambrosia beetle densities averaged 300 m-2 bark surface in Douglas-fir and western hemlock, Tsuga heterophylla, logs, and their galleries extended 9-14 cm in 4-9 cm thick sapwood, indicating that considerable sapwood volume was made accessible to microbes colonizing gallery walls. The entire sapwood volume of these logs was colonized

Mesofauna Leaf Grazers

Decomposition rate of blue grama grass in litterbags treated to permit decomposition by abiotic factors alone, abiotic factors + microbes, and abiotic factors + microbes + mesofauna (microarthropods). Decomposition in the abiotic treatment was insignificant after the first month; decomposition showed a 2-month time lag in the treatment including mesofauna. From Vossbrinck et al. (1979) with permission from the Ecological Society of America.

by various fungi within the first year after logs were cut (Schowalter et al. 1992). Mixing of organic material and microbes during passage through detritivore guts ensures infusion of consumed litter with decomposers and may alter litter quality in ways that stimulate microbial production (Maraun and Scheu 1996). Gut mixing is especially important for species such as termites and other wood-borers that require microbial digestion of cellulose and lignin into labile carbohydrates (Breznak and Brune 1994).

Many arthropods directly transport and inoculate saprophytic microorganisms into organic residues. For example, Schowalter et al. (1992) documented transport of a large number of fungal genera by wood-boring insects. Some of these fungi are mutualists that colonize wood in advance of insects and degrade cellulose into labile carbohydrates that subsequently are used by insects (Bridges and Perry 1985, French and Roeper 1972, Morgan 1968). Others may be acquired accidentally by insects during feeding or movement through colonized material (Schowalter et al. 1992). Behan and Hill (1978) documented transmission of fungal spores by oribatid mites.

Fungivorous and bacteriophagous arthropods stimulate microbial activity by maximizing microbial production. As discussed for herbivore effects on plants in Chapter 12, low to moderate levels of grazing often stimulate productivity of the microflora by alleviating competition, altering microbial species composition, and gouging new detrital surfaces for microbial colonization. Microarthropods also can stimulate microbial respiration by preying on bacteriophagous and mycophagous nematodes (Seastedt 1984, Setala et al. 1996). Higher levels of grazing may depress microbial biomass and reduce respiration rates (Huhta et al. 1991, Seastedt 1984).

Seastedt (1984) suggested a way to evaluate the importance of three pathways of microbial enhancement by arthropods, based on the tendency of microbes to immobilize nitrogen in detritus until C : N ratio approaches 10-20 : 1. Where arthropods affect decomposition primarily through comminution, nitrogen content of litter should be similar with or without fauna. Alternatively, where arthropods stimulate microbial growth and respiration rates, the C : N ratio of litter with fauna should be less than the ratio without fauna. Finally, where arthropods graze microbial tissues as fast as they are produced, C : N ratio of litter should be constant, and mass should decrease.

Seasonal variation in arthropod effects on microbial production and biomass may explain variable results and conclusions from earlier studies. Maraun and Scheu (1996) reported that fragmentation and digestion of beech leaf litter by the millipede, Glomeris marginata, increased microbial biomass and respiration in February and May but reduced microbial biomass and respiration in August and November. They concluded that millipede feeding generally increased nutrient (nitrogen and phosphorus) availability but that these nutrients were only used for microbial growth when carbon resources were adequate, as occurred early in the year. Depletion of carbon resources relative to nutrient availability in detritus limited microbial growth later in the year.

Although CO2 is the major product of litter decomposition, incomplete oxidation of organic compounds occurs in some ecosystems, resulting in evolution of other trace gases, especially methane (Khalil et al. 1990). P. Zimmerman et al. (1982) first suggested that termites could contribute up to 35% of global emissions of methane. A number of arthropod species, including most tropical representatives of millipedes, cockroaches, termites, and scarab beetles, are important hosts for methanogenic bacteria and are relatively important sources of biogenic global methane emissions (Hackstein and Stumm 1994).

Termites have received the greatest attention as sources of methane because their relatively sealed colonies are warm and humid, with low oxygen concentrations that favor fermentation processes and emission of methane or acetate (Brauman et al. 1992, Wheeler et al. 1996). Thirty of 36 temperate and tropical termite species assayed by Brauman et al. (1992), Hackstein and Stumm (1994), and Wheeler et al. (1996) produced methane, acetate, or both. Generally, aceto-genic bacteria outproduce methanogenic bacteria in wood- and grass-feeding termites, but methanogenic bacteria are much more important in fungus-growing and soil-feeding termites (Brauman et al. 1992).

P. Zimmerman et al. (1982) suggested that tropical deforestation and conversion to pasture and agricultural land could increase the biomass and methane emissions of fungus-growing and soil-feeding termites, but Martius et al. (1996) concluded that methane emissions from termites in deforested areas in Amazonia would not contribute significantly to global methane fluxes. Khalil et al. (1990), Martius et al. (1993), and Sanderson (1996) calculated CO2 and methane fluxes based on global distribution of termite biomass and concluded that termites contribute ca 2% of the total global flux of CO2 (3500 tg year-1) and 4-5% of the global flux of methane (<20 tg year-1) (Fig. 14.5). However, emissions of CO2 by termites are 25-50% of annual emissions from fossil fuel com-

Geographic Distribution Termites

I Geographic distribution of emissions of methane (top) and carbon dioxide (bottom) by termites. Units are 106 kg yr-1. From Sanderson (1996) courtesy of the American Geophysical Union.

I Geographic distribution of emissions of methane (top) and carbon dioxide (bottom) by termites. Units are 106 kg yr-1. From Sanderson (1996) courtesy of the American Geophysical Union.

bustion (Khalil et al. 1990). Contributions to atmospheric composition by this ancient insect group may have been more substantial prior to anthropogenic production of CO2, methane, and other trace gases.

3. Mineralization

Measurements of changes in elemental concentrations represent net mineralization rates. Net mineralization includes loss of elements as a result of mineralization and accumulation by microflora of elements entering as microparticulates, precipitation, and leachate or transferred (e.g., via hyphae) from other organic material (Schowalter et al. 1998, Seastedt 1984). Although microbial biomass usually is a negligible component of litter mass, microbes often represent a large proportion of the total nutrient content of decomposing detritus and significantly affect the nutrient content of the litter-microbial complex (e.g., Seastedt 1984). Arthropods affect net mineralization in two measurable ways: through mass loss and assimilation of consumed nutrients and through effects on nutrient content of the litter-microbe system. Seastedt (1984) proposed the following equation to indicate the relative effect of arthropods on mineralization:

Y = [% mass^% massx) x (concentration/concentration) (14.2)

where Y is the relative arthropod effect, % massi is the percentage of initial mass remaining that has been accessible to arthropods, % massx is the percentage of initial mass remaining that has been unavailable to arthropods, and concentra-tioni and concentration are the respective concentrations of a given element. Net immobilization of an element is indicated by Y > 1, and net loss is indicated by Y < 1. Temporal changes in nutrient content depend on the structural position of the element within organic molecules, microbial use of the element, and the form and amounts of the element entering the detritus from other sources.

Nitrogen generally is considered to be the element most likely to limit growth of plants and animals, and its release from decomposing litter often is correlated with plant productivity (Vitousek 1982). As noted earlier in this chapter, sapro-phytic microbes usually immobilize nitrogen until sufficient carbon has been respired to make carbon or some other element more limiting than nitrogen (Maraun and Scheu 1996, Schowalter et al. 1998, Seastedt 1984). Thereafter, the amount of nitrogen released should equal the amount of carbon oxidized. Microbes have considerable capacity to absorb nitrogen from precipitation, canopy leachate, and animal excrement (see Fig. 12.14) (Lovett and Ruesink 1995, Seastedt and Crossley 1983, Stadler and Müller 1996), permitting nitrogen mineralization and immobilization even at high C : N ratios. Generally, exclusion of microarthropods decreases the concentration of nitrogen in litter, but the absolute amounts of nitrogen in litter are decreased or unaffected by micro-arthropod feeding activities (Seastedt 1984).

Yokoyama et al. (1991) compared nitrogen transformations among cattle dung (balls) colonized by dung beetles, Onthophagus lenzii; uncolonized dung; and residual dung remaining after beetle departure. They reported that dung beetles reduced ammonia volatilization from dung 50% by reducing pH and ammonium concentration in dung (through mixing of dung and soil). However, dung beetles increased denitrification 2-3-fold by increasing the rate of nitrate formation. Dung beetles also increased nitrogen fixation 2-10-fold, perhaps by reducing inorganic nitrogen concentrations in a substrate of easily decomposable organic matter.

Phosphorus concentrations often show initial decline as a result of leaching but subsequently reach an asymptote determined by microbial biomass (Schowalter and Sabin 1991, Schowalter et al. 1998, Seastedt 1984).

Microarthropods can increase or decrease rates of phosphorus mineralization, presumably as a result of their effect on microbial biomass (Seastedt 1984).

Calcium dynamics are highly variable. This element often is bound in organic acids (e.g., calcium oxalate) as well as in elemental and inorganic forms in detritus. Some fungi accumulate high concentrations of this element (Cromack et al. 1975,1977, Schowalter et al. 1998), and some litter arthropods, especially millipedes and oribatid mites, have highly calcified exoskeletons (Norton and Behan-Pelletier 1991, Reichle et al. 1969). Nevertheless, calcium content in arthropod tissues is low compared to annual inputs in litter. No consistent arthropod effects on calcium mineralization have been apparent (Seastedt 1984).

Potassium and sodium are highly soluble elements, and their initial losses (via leaching) from decomposing litter invariably exceed mass losses (Schowalter and Sabin 1991, Schowalter et al. 1998, Seastedt 1984). Amounts of these elements entering the litter in precipitation or throughfall approach or exceed amounts entering as litterfall. In addition, these elements are not bound in organic molecules, so their supply in elemental form is adequate to meet the needs of microflora. Arthropods have been shown to affect mineralization of 134Cs or 137Cs, used as analogs of potassium (Crossley and Witkamp 1964, Witkamp and Crossley 1966), but not mineralization of potassium (Seastedt 1984). Sodium content often increases in decomposing litter, especially decomposing wood (Cromack et al. 1977, Schowalter et al. 1998). Sollins et al. (1987) suggested that this increase represented accumulation of arthropod tissues and products, which usually contain relatively high concentrations of sodium (e.g., Reichle et al. 1969). However, Schowalter et al. (1998) reported increased concentrations of sodium during early stages of wood decomposition, prior to sufficient accumulation of arthropod tissues. They suggested that increased sodium concentrations in wood reflected accumulation by decay fungi, which contained high concentrations of sodium in fruiting structures. Fungi and bacteria have no known physiological requirement for sodium (Cromack et al. 1977). Accumulation of sodium, and other limiting nutrients, in decomposing wood may represent a mechanism for attracting sodium-limited animals that transport fungi to new wood resources.

Sulfur accumulation in decomposing wood or forest and grassland soils (Schowalter et al. 1998, Stanko-Golden et al. 1994, Strickland and Fitzgerald 1986) reflects both physical adsorption of sulfate and biogenic formation of sulfonates by bacteria (Autry and Fitzgerald 1993). Although arthropods have no demonstrated role in these processes, arthropod feeding on bacterial groups responsible for sulfur mobilization or immobilization should influence sulfur dynamics. Because sulfur flux plays a major role in soil acidification and cation leaching, factors affecting sulfur immobilization require further investigation.

The generally insignificant effects of arthropods on net mineralization rates, compared to their substantial effects on mass loss, can be attributed to the compensatory effects of arthropods on microbial biomass. The stimulation by arthropods of microbial respiration and immobilization of nutrients results in loss of litter mass, especially carbon flux through respiration, but not of the standing crops of other elements within litter (Seastedt 1984). Other aspects of fragmentation also may contribute to nutrient retention, rather than loss. Aquatic com-minuters generally fragment detritus into finer particles more amenable to downstream transport (J. Wallace and Webster 1996). However, some filterfeeders concentrate fine detrital material into larger fecal pellets that are more likely to remain in the aquatic ecosystem (e.g., Wotton et al. 1998). Some shredders deposit feces in burrows, thereby incorporating the nutrients into the substrate (R. Wagner 1991). Furthermore, Seastedt (2000) noted that most studies of terrestrial detritivore effects have been relatively short term. Accumulating data (e.g., Setala et al. 1996) suggest that mixing of recalcitrant organic matter and mineral soil in the guts of some arthropods may produce stable soil aggregates that reduce the decay rate of organic material.

B. Soil Structure, Fertility, and Infiltration

Fossorial arthropods alter soil structure by redistributing soil and organic material and increasing soil porosity (J.Anderson 1988). Porosity determines the depth to which air and water penetrate the substrate. A variety of substrate-nesting vertebrates, colonial arthropods, and detritivorous arthropods and earthworms affect substrate structure, organic matter content, and infiltration in terrestrial and aquatic systems.

Defecation by a larval caddisfly, Sericostoma personatum, increases subsurface organic content in a stream ecosystem by 75-185% (R. Wagner 1991). The cad-disfly feeds on detritus on the surface of the streambed at night and burrows into the streambed during the day, trapping organic matter in burrows.

Ants and termites are particularly important soil engineers. Colonies of these insects often occur at high densities and introduce cavities into large volumes of substrate. Eldridge (1993) reported that densities of funnel ant, Aphaenogaster barbigula, nest entrances could reach 37 m-2, equivalent to 9% of the surface area over portions of the eastern Australian landscape. Nests of leaf-cutting ants, Atta vollenweideri, reach depths of >3 m in pastures in western Paraguay (Jonkman 1978). Moser (1963) partially excavated a leaf-cutting ant, Atta texana, nest in central Louisiana, United States. He found 93 fungus-garden chambers, 12 dormancy chambers, and 5 detritus chambers (for disposal of depleted foliage substrate) in a volume measuring 12 x 17 m on the surface by at least 4 m deep (the bottom of the colony could not be reached). Whitford et al. (1976) excavated nests of desert harvester ants, Pogonomyrmex spp., in New Mexico, United States, and mapped the 3-dimensional structure of interconnected chambers radiating from a central tunnel (Fig. 14.6).They reported colony densities of 21-23 ha-1 at 4 sites. Each colony consisted of 12-15 interconnected galleries (each about 0.035 m3) within a 1.1 m3 volume (1.5 m diameter x 2 m deep) of soil, equivalent to about 10 m3 ha-1 cavity space (Fig. 14.6). These colonies frequently penetrated the calcified hardpan (caliche) layer 1.7-1.8 m below the surface.

The infusion of large soil volumes with galleries and tunnels greatly alters soil structure and chemistry. Termite and ant nests usually represent sites of concentrated organic matter and nutrients (J. Anderson 1988, Culver and Beattie 1983,

Earthworm Gut Mineralization

Vertical structure of a harvester ant, Pogonomyrmex rugosus, nest in southern New Mexico. From Whitford et al. (1976) with permission of Birkhauser Verlag.

Vertical structure of a harvester ant, Pogonomyrmex rugosus, nest in southern New Mexico. From Whitford et al. (1976) with permission of Birkhauser Verlag.

Herzog et al. 1976, Holdo and McDowell 2004, J. Jones 1990, Lesica and Konnowski 1998, Mahaney et al. 1999, Salick et al. 1983, D. Wagner 1997, D. Wagner et al. 1997). Nests may have concentrations of macronutrients 2-3 times higher than surrounding soil (Fig. 14.7). J. Jones (1990) and Salick et al. (1983) noted that soils outside termite nest zones become relatively depleted of organic matter and nutrients. L. Parker et al. (1982) reported that experimental exclusion of termites for 4 years increased soil nitrogen concentration 11%. Ant nests also have been found to have higher rates of microbial activity and carbon and nitrogen mineralization than do surrounding soils (Dauber and Wolters 2000, Lenoir et al. 2001).

MAGNESIUM (ppm)

PHOSPHATE (ppm)

Grnd

POTASSIUM (ppm)

Grnd

SODIUM (ppm)

Grnd

Grnd Hum Ant

Concentrations of major nutrients from bog soil (Grnd), hummocks (Hum), and Formica nests (Ant) in bogs in Montana, United States. Vertical bars represent 1 standard error. Means with different letters are significantly different at P < 0.05. From Lesica and Kannowski (1998) with permission from American Midland Naturalist. Please see extended permission list pg 573.

Nest pH often differs from surrounding soil. Mahaney et al. (1999) found significantly higher pH in termite mounds than in surrounding soils. Jonkman (1978) noted that soil within leaf-cutter ant, Atta spp., nests tended to have higher pH than did soil outside the nest. However, D.Wagner et al. (1997) measured significantly lower pH (6.1) in nests of harvester ants, Pogonomyrmex barbatus, than in reference soil (6.4). Lenoir et al. (2001) reported that Formica rufa nests had higher pH than did surrounding soil at one site and lower pH than did surrounding soil at a second site in Sweden. Ant mounds in Germany did not differ from surrounding soils (Dauber and Wolters 2000).

Termites and ants also transport large amounts of soil from lower horizons to the surface and above for construction of nests (Fig. 14.8), gallery tunnels, and "carton" (the soil deposited around litter material by termites for protection and to retain moisture during feeding above ground; Fig. 14.9) (Whitford 1986). Whitford et al. (1982) reported that termites brought 10-27 g m-2 of fine-textured

Pictures Termite Castles

Termite castle in northern Australian woodland. Dimensions are approximately 3 m height and 1.5 m diameter.

soil material (35% coarse sand; 45% medium fine sand; and 21% very fine sand, clay, and silt) to the surface and deposited 6-20 g of soil carton per gram of litter removed (see Fig. 14.3). Herrick and Lal (1996) found that termites deposited an average of 2.0 g of soil at the surface for every gram of dung removed. Mahaney et al. (1999) reported that the termite mound soil contained significantly more (20%) clay than did surrounding soils.

A variety of vertebrate species in Africa have been observed to selectively ingest termite mound soil. Mahaney et al. (1999) suggested that the higher clay content of termite mounds, along with higher pH and nutrient concentrations, could mitigate gastrointestinal ailments and explain termite soil consumption by chimpanzees. Termite mound soils, as well as surrounding soils, had high concentrations of metahalloysite, used pharmaceutically, and other clay minerals that showed mean binding capacities of 74-95% for 4 tested alkaloids. Chimpanzees could bind most of the dietary toxins present in 1-10 g of leaves by eating 100 mg of termite mound soil.

Chimpanzees Eating Leavs

Termite gallery carton on stems of dead creosote bush. Soil particles are cemented together to provide protection and moisture control during termite feeding on detrital material.

Termite gallery carton on stems of dead creosote bush. Soil particles are cemented together to provide protection and moisture control during termite feeding on detrital material.

A number of studies have demonstrated effects of soil animals on soil moisture (Fig. 14.10). Litter reduction or removal increases soil temperature and evaporation and reduces infiltration of water. Burrowing and redistribution of soil and litter increase soil porosity, water infiltration, and stability of soil aggregates that control water- and nutrient-holding capacity.

Ant and termite nests have particularly important effects on soil moisture because of the large substrate surface areas and volumes affected. D. Wagner (1997) reported that soil near ant nests had higher moisture content than did more distant soil. Elkins et al. (1986) compared runoff and water infiltration in plots with termites present or excluded during the previous 4 years in New Mexico, United States. Plots with <10% plant cover had higher infiltration rates when termites were present (88 mm hour-1) than when termites were absent (51 mm hour-1); runoff volumes were twice as high in the termite-free plots with low plant cover (40 mm) as in untreated plots (20 mm). Infiltration and runoff

Termites Soil Plant Nutrient Content

FIG. 14.10

Effects of soil invertebrates on soil water balance. From J. Anderson (1988) with permission from Elsevier Science.

FIG. 14.10

Effects of soil invertebrates on soil water balance. From J. Anderson (1988) with permission from Elsevier Science.

volumes did not differ between shrub-dominated plots (higher vegetation cover) with or without termites.

Eldridge (1993,1994) measured effects of funnel ants and subterranean harvester termites, Drepanotermes spp., on infiltration of water in semi-arid eastern Australia. He found that infiltration rates in soils with ant nest entrances were 4-10-fold higher (1030-1380 mm hour-1) than in soils without nest entrances (120-340 mm-hour-1). Infiltration rate was correlated positively with nest entrance diameter. However, infiltration rate on the subcircular pavements covering the surface over termite nests was an order of magnitude lower than in the annular zone surrounding the pavement or in interpavement soils (Fig. 14.11). The cemented surface of the pavement redistributed water and nutrients from the pavement to the surrounding annular zone. Ant and termite control of infiltration creates wetter microsites in moisture-limited environments.

C. Primary Production and Vegetation Dynamics

Through control of decomposition, mineralization, and pedogenesis, detritivo-rous and fossorial arthropods have the capacity to control nutrient availability for, and perhaps uptake by, plants (Crossley 1977). In particular, release of nitrogen from decaying organic matter often is correlated with plant productivity (Vitousek 1982). However, relatively few studies have measured the effect of detritivores and burrowers on plant growth or vegetation dynamics.

C. Edwards and Lofty (1978) compared seedling emergence and shoot and root growth of barley between pots of intact, sterilized soil (from fields in which seed had been either drilled into the soil or planted during ploughing) with

I Effect of termite colony structure on infiltration of water under ponded conditions (yellow) and under tension (brown). Vertical lines indicate 1 standard error of the mean. From Eldridge (1994) with permission from Gustav Fischer Verlag.

microarthropods or earthworms absent or reintroduced. Percent seedling emergence, plant height, and root weight were higher in ploughed soil and direct-drilled soil with animals, compared to sterile direct-drilled soil, suggesting important effects of soil animals on mineralization, soil porosity, and infiltration.

R. Ingham et al. (1985) inoculated microcosms of blue grama grass, Bouteloua gracilis, in sandy loam soil, low in inorganic nitrogen, inoculated with bacteria or fungi; half of each microflora treatment was inoculated with microbivorous nematodes. Plants growing in soil with bacteria and bacteriophagous nematodes grew faster and acquired more nitrogen initially than did plants in soil with bacteria only. Addition of mycophagous nematodes did not increase plant growth. These differences in plant growth resulted from greater nitrogen mineralization by bacteria (compared to fungi), excretion of NH4+-N by bacteriophagous (but not mycophagous) nematodes, and rapid uptake of available nitrogen by plants. Mycophagous nematodes did not increase plant growth or nitrogen uptake over fungi alone because these nematodes excreted less NH4+-N, and the fungus alone mineralized sufficient nitrogen for plant growth.

In a unique, definitive study, Setala and Huhta (1991) created laboratory microcosms with birch seedlings, Betula pendula, planted in partially sterilized soil reinoculated with soil microorganisms only or with soil microorganisms and a diverse soil fauna. During 2 growing periods the presence of soil fauna increased birch leaf, stem, and root biomass by 70%, 53%, and 38%, respectively, and increased foliar nitrogen and phosphorus contents 3-fold and 1.5-fold, respectively, compared to controls with microorganisms only (Fig. 14.12).

In addition to direct effects on nutrient availability, soil arthropods can influence plant growth indirectly by affecting mycorrhizal fungi. Grazing on mycorrhizal fungi by fungivorous arthropods could inhibit plant growth by interfering with nutrient uptake. Conversely, many fungivorous arthropods disperse mycorrhizal spores or hyphae to new hosts. Rabatin and Stinner (1988) reported that 28-97% of soil animals contained mycorrhizal spores or hyphae in their guts.

Soil animals also influence vegetation dynamics. Several studies have demonstrated that ant and termite mounds usually support distinct plant communities (Garrettson et al. 1998, Guo 1998, Holdo and McDowell 2004, King 1977a). Guo (1998) reported that the diversity of annual and perennial plants was highest on ant mounds and under shrubs, compared to kangaroo rat mound, half-shrub, and open-area microsites; biomass of these plants was highest under shrubs, followed

Week 10

Total 4-tr-tr

Week 10

Week 45

40 o

FIG. 14.12

Biomass production (left of break in horizontal axis) and nitrogen accumulation (right of break in horizontal axis) of birch, Betula pendula, seedlings. Bars above the horizontal axis are stems (orange) and leaves (yellow); bars below the horizontal axis are roots in humus (blue) and roots in mineral soil (yellow). C, fauna removed; F, refaunated. Vertical lines represent 1 standard deviation for all data (except nitrogen at week 45, where vertical lines represent minimum and maximum values). (For C versus F, *, P < 0.05; ***, P < 0.001). Stem nitrogen was not measured week 10. From Setala and Huhta (1991) with permission from the Ecological Society of America.

by kangaroo rat mounds and ant mounds, indicating that ant mounds are important determinants of vegetation structure.

Jonkman (1978) reported that abandoned nests of leaf-cutter ants, Atta vol-lenweideri, served as sites of accelerated succession in Paraguayan pastures. Collapse of the nest chamber formed a depression that held water and facilitated development of woody vegetation. At high nest densities, these oases coalesced, greatly increasing forest area. Brenner and Silva (1995) found that active nests of Atta laevigata were more frequently associated with groves of trees, and the size of nests increased with grove size and the abundance of forest tree species in Venezuelan savanna, suggesting that active nests both facilitated and were facilitated by formation of groves.

L. Parker et al. (1982) demonstrated that termite exclusion significantly reduced biomass of four annual plant species and significantly increased biomass of one annual plant species. They observed an overall trend toward increased biomass of annual plants in plots with termites excluded. These results likely reflected increased nitrogen availability in termite exclusion plots, compared to plots with unmanipulated termite abundance.

Lesica and Kannowski (1998) reported that wood ants, Formica podzolica, were responsible for mound formation in peat bogs in Montana, United States. The mounds provided elevated habitat that was warmer, was better aerated, and had higher nutrient content than did surrounding peat surfaces (see Fig. 14.7). Although active mounds supported only a few species of grasses, abandoned nests supported shrubs and plant species that could not grow in the saturated peat surface. The ants foraged primarily on honeydew from aphids tended on shrubs, indicating a positive feedback relationship.

The high nutrient concentrations of termite and ant nests are incorporated by plants growing on the nests and become available to higher trophic levels. Holdo and McDowell (2004) reported that trees growing on termite mounds had higher concentrations of all nutrients tested, except sodium and crude protein, than were trees from the surrounding woodland matrix in Zimbabwe. Trees on mounds also were subjected to more intense feeding by elephants. Termite and ant nests thereby affect food availability and feeding patterns for herbivores, providing indirect positive feedback for herbivore effects on litter quality and availability for detritivores.

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