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organic detritus. The factors plus ecosystem processes acting over time lead to ecosystem properties (Coleman et al., 1983; Elliott, 1994).

The immediate result of faunal feeding activity is the production of fecal pellets, some of which can be identified as species- or group-specific (Kuhnelt, 1958; Jongerius, 1964; Zachariae, 1965; Rusek, 1975; FitzPatrick, 1984; Pawluk, 1987). For example, the fecal pellets of collembola and oribatid mites are surrounded by a chitin-rich layer called the "peritrophic membrane" (Krantz, 1978), which acts to retard the rate at which a fecal pellet disappears (Fig. 5.20). A comprehensive review (Bal, 1982) of soil fauna activities in soil refers to "zoological ripening" as faunal movement of organic matter and mineral materials in previously uncolonized soil. This soil maturation and development process has been of great significance in Dutch polder regions, and has been demonstrated in Canadian (Nielson and Hole, 1964) and New Zealand (Stockdill, 1966) soils as well. These processes are reviewed extensively in Brussaard and Kooistra (1993).

In addition to physical signs, there are chemical indicators of faunal presence and activity. For example, in certain cool, moist New Zealand tussock grassland soils, nearly 10% of the organic phosphorus is comprised of phosphonates (carbon-phosphorus [C-P] bonded) (Newman

FIGURE 5.20. Fecal pellets in the soil profile from the Horseshoe Bend agroecosystem site, Athens, Georgia, United States, at a depth of 5-10 cm (from Larry T. West, personal communication).

and Tate, 1980; Tate and Newman, 1982), as contrasted with the more prevalent phosphate esters. Phosphonates are produced by ciliates, and their subsequent rates of input and flow through the soil phosphorus cycle remain unknown (Stewart and McKercher, 1982).

Several authors have reviewed work on experimental pedogenesis (soil formation), examining roles of primary colonizing plants, including dissolution of rock minerals by lichens and fungi, as well as faunal impacts on mineral or soil movement, and organic matter transformation (Hallsworth and Crawford, 1965; Bal, 1982; Landeweert et al., 2001). Webb (1977) studied the effects of particle size and decompos-ability of macrofaunal and microfaunal fecal pellets. There are differing effects of comminution (breaking up) of leaf litter by large and small fauna and they play different roles in facilitating further leaf litter decomposition. Webb (1977) noted that fecal pellets of Narceus annularis (Diplopoda: Spirobolidae) had a lower surface-to-mass ratio, whereas those of microarthropods such as oribatids had a greater surface-to-volume ratio than the original leaf litter. This should lead to greater decomposition per unit time (Fig. 5.21) (see previous comments about the peritrophic membrane).

Physical interpretation of organic matter decomposition should be tempered with careful observation of life-history details, such as likelihood of localized aggregation of mite or collembolan fecal pellets that may decompose locally at a much slower rate than hypothesized from in vitro laboratory studies. Substrate quality plays an important role here. Dunger (1983) noted that macroarthropods ingest mineral soil along with litter material. Kilbertus and Vannier (1981) and Touchot et al. (1983) demonstrated ingestion of argillic (clay) material by Tomo-cerus and Folsomia sp. (collembola), a trait that was particularly evident when they ingested polyphenol-rich Quercus leaves. This detoxification process presumably led to greater decomposition of the leaf material, with enhanced bacterial growth in the pellets with clay particles versus those without the clay adsorbent material. The impact of Collembola is greatest in mor soils, which may have entire layers in the F or H horizon filled with collembolan fecal pellets (Pawluk, 1987).

Research over the last 8 to 10 years has shown a significant impact of root-associated organisms on nutrient dynamics of phosphorus and nitrogen in experimental microcosms. These studies are reviewed in Coleman et al. (1983) and Anderson et al. (1981a, b). The use of both microcosm and mesocosm (i.e., field enclosures larger than a square meter) in soil ecological studies has proliferated in recent years and greatly increased our understanding of biological effects on nutrient cycling in soils (Ingham, 1985; Ingham, 1986a, b; Ingham et al., 1989; Parmelee et al., 1990; Beare et al., 1992; Moore et al., 1996).

In the laboratory, groups of rhizosphere bacteria, fungi, and microbivorous nematodes were grown singly or in combination, all with

FIGURE 5.21. Graphical representation of physical conglomerate feces differentiation theory (from Webb, 1977). As particle size of litter (right to left) is reduced, surface area and decomposition increase until constituent particles are small enough to aggregate into more stable conglomerates (limit to free particle size reduction). Physical conglomerates increase in size as constituent particle size decreases, but arthropod pellets decrease in size because of the direct relationship of body size to degree of pulverization and pellet (conglomerate) size. Micropellets are therefore able to maintain a much smaller conglomerate size and break the limit to free particle size reduction (from Webb, 1977).

FIGURE 5.21. Graphical representation of physical conglomerate feces differentiation theory (from Webb, 1977). As particle size of litter (right to left) is reduced, surface area and decomposition increase until constituent particles are small enough to aggregate into more stable conglomerates (limit to free particle size reduction). Physical conglomerates increase in size as constituent particle size decreases, but arthropod pellets decrease in size because of the direct relationship of body size to degree of pulverization and pellet (conglomerate) size. Micropellets are therefore able to maintain a much smaller conglomerate size and break the limit to free particle size reduction (from Webb, 1977).

growing seedlings of the shortgrass prairie grass, Bouteloua gracilis (blue grama). In all treatments that had the root, microbe and microbial grazer (Pelodera sp. as bacterial-feeder), and Aphelenchus avenae as fungal-feeder, there was an enhanced shoot growth and dry-matter yield, when compared to the plant-alone control (Ingham et al., 1985).

Other work using mesofauna (nematodes) (Ingham et al., 1986a, b) and macrofauna (isopods) (Anderson et al., 1985) has shown significant enhancement of nutrient cycling (nitrogenous compounds) in field experimental situations. Thus an enhanced (20-50%) nutrient return (mineralization) occurs in the presence of the fauna, compared with experiments in which they are present in very low numbers, or completely absent (Anderson et al., 1983). This work was further amplified by simulation models of detrital food webs, which showed a significant (about 35%) contribution to mineralization of nitrogen by microfauna (amoebae and flagellates) and bacterial-feeding nematodes (Hunt et al., 1987; De Ruiter et al., 1993; Moore et al., 1996). More detailed studies using 13C and 15N tracers in microcosms with varying degrees of organic matter accumulation ("hot spots") and microbes alone, microbes and protozoa, or nematodes and microbes with both faunal groups in combination (Bonkowski et al., 2000) have revealed similar patterns to earlier microcosm studies (e.g., Ingham et al., 1985). Rye grass seedlings significantly increased in dry matter and nitrogen content, with protozoa and nematodes and protozoa present (Bonkowski et al., 2000). Interestingly, the pattern of decomposition of labeled litter closely followed the nitrogen dynamics, with protozoa, and nematodes and protozoa, showing significantly more 13CO2-C respiration between weeks 2 and 4 of the 6-week experiment. This study was conducted con-comitantly with a detailed analysis of microbial community composition. Griffiths et al. (1999) found significant selectivity of protozoa for species of soil bacteria, with a definite preference shown for several Gram-positive species.

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