Population dynamics of the fast-growing sub-populations of Pseudomonas andtotal bacteria, and their protozoan grazers, revealed by fenpropimorph treatment

The population dynamics of indigenous soil bacteria and protozoa on decayin g barley roots were followed by using litter bags buried in laboratory-incu bated soil. The soil was either non-treated or treated with the fungicide f enpropimorph (in the formulation Corbel) at concentrations corresponding to the recommended and at 10 times field dose (1.3 and 13 mg kg(-1) dry wt.). Number of total bacteria and number of Pseudomonas were detected, using bo th traditional plating and short time incubations of 'early' colonies, to d etermine the fast-responding subpopulation of the culturable bacteria. The number of protozoa corresponding to the two subpopulations was followed. Th e results strongly indicate a predatory association between the protozoa an d bacteria. This was shown by a tight temporal association, and by a stimul ation of bacteria following predatory release when protozoa were inhibited by fenpropimorph. Thus, fenpropimorph disturbed population dynamics ir? con centrations, which can be reached in surface soils after distribution in th e field. (C) 2000 Elsevier Science Ltd. All rights reserved.