Info

max r = rapid growth strategy k = slow growth strategy

Modified from Humphreys, 1979; Hunt et al., 1987; Payne, 1970; de Ruiter et al., 1993.

up the food chain can be calculated. There is indeed energy to spare for such elaborate food chains. Using the maximal values for production efficiency, such as that for bacteria of 70% (Payne, 1970) and 80% for soil amoebae and flagellates (Humphreys, 1979), and moving on to protozoan-consuming nematodes (Fig. 6.3) (which doubtless occurs in

FIGURE 6.3. Calculations of annual carbon flows along a bacteriophagic food chain assemblage, exhibiting considerable omnivory. Average standing crops are indicated. Flows via protozoan feeding are estimated as probably two to three times greater than via nematodes in that ecosystem. Proportions may be reversed in lower-pH forested systems, and more flows via fungi (not shown) (from Hunt et al., 1987, reprinted with permission).

FIGURE 6.3. Calculations of annual carbon flows along a bacteriophagic food chain assemblage, exhibiting considerable omnivory. Average standing crops are indicated. Flows via protozoan feeding are estimated as probably two to three times greater than via nematodes in that ecosystem. Proportions may be reversed in lower-pH forested systems, and more flows via fungi (not shown) (from Hunt et al., 1987, reprinted with permission).

certain "hot spots," e.g., at the zone of elongation of a growing root), then there is an adequate amount of carbon available for passage through the four- and five-membered detrital food chains of interest. Considerable omnivory is prevalent in these soil systems (DeAngelis, 1992). The protozoa and nematode feeding pathway highlighted in Figure 6.3 (Hunt et al., 1987) accounted for 37% of the total nitrogen mineralization and some 82% of the total mineralization resulting from soil fauna. Similar percentages were obtained for a wide range of agroecosystems in the United States and Europe (de Ruiter et al., 1993; Moore and de Ruiter, 2000).

The relative contributions of the soil fauna to microbial turnover and nutrient mineralization are directly related to the demographics of the soil biota (Coleman et al., 1983, 1993), as noted for average standing crops and energetic parameters and turnover times per year for microorganisms, micro-, meso-, and macro-fauna in a grassland and a no-tillage agroecosystem (Coleman et al., 1993) (Table 6.2). Thus the protozoa, and naked amoebae in particular, turn over 10 or more times per season, and consume several times their mass of living microbial tissues. The microbes and several other faunal groups have much lower turnover rates, on average. Although the amoebae are considered to be primarily bacterial feeders, there are important instances when other amoebal species will feed on protoplasm in fungal hyphae, or even on the fungal spores themselves (Chakraborty and Warcup, 1983; Chakraborty et al., 1983). When considered in combination with the information in Table 6.1 on the range of assimilation and production efficiencies, the impacts of these small organisms are very marked. It should be noted that extensive studies in Sweden on arable lands (Andren et al., 1990) have reached similar conclusions. The increasing miniaturization of sensors, so that one can carry out microcalorimetry (Battley, 1987) at localized microsites, will enable us to measure direct energetic transformations more readily in situ.

The practical implications of soil food webs in agroecosystems have been of interest to researchers in several countries, notably the Netherlands, Sweden, and the United States. In a major synthesis of several research papers (some of which are cited earlier in this chapter), Bloem et al. (1997) calculated the impact of microbivorous invertebrate fauna in agroecosystems. Using a combination of experimental results and simulation modeling runs, they calculated that in fields that had greater additions of organic matter, including manure, average nitrogen mineralization was 30% higher than in fields that did not have such organic matter additions. This reflected the activities of protozoa and nematodes, which were 64% and 22% higher numbers, respectively, in the fields with organic additions. Nitrogen mineralization was performed mainly by the bacteria, which dominated in these fields, but

TABLE 6.2. Average Standing Crop and Energetic Parameters for Microorganisms, Mesofauna, and Earthworms in a Lucerne Ley and Georgia No-Tillage Agroecosystem"

Naked Microbivorous amoebae Flagellates Ciliates Bacteria Fungi nematodes Collembola Mites Enchytraeids Earthworms

Typical size in soil

Mode of living

Biomass (kgdw ha-1)

% active

Estimated turnover times, season-1

30 Mm

In water films on surfaces

10 Mm

80 Mm

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