Several investigators have noted the obvious parallel between the protozoan-microbe interaction in water films in soil, and on root surfaces, and in open-water aquatic systems (Stout, 1963; Coleman, 1976; Clarholm, 1994; Coleman, 1994a). The so-called "microbial loop" Pomeroy (1974) has proven to be a powerful conceptual tool; rapidly feeding protozoa may consume several standing crops of bacteria in soil (see Chapter 6) every year (Clarholm, 1985; Coleman, 1994b).
Darbyshire and Greaves (1967) noted that this tendency is particularly marked in the rhizosphere, which provides a ready food source for microbial prey. This was demonstrated impressively for protozoa in arable fields (Cutler et al, 1923), and more recently for bacteria, naked amoebae, and flagellates in the humus layer of a pine forest in Sweden after a rain (Clarholm, 1994). Bacteria and flagellates began increasing immediately after a rainfall event and rose to a peak in 2-3 days; naked amoebae rose more slowly and peaked at days 4-5, and then tracked the bacterial decrease downward, as did the flagellates (Fig. 4.6).
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