The impact of the fungicide fenpropimorph (Corbel (R)) on bacterivorous and fungivorous protozoa in soil

1. The ability of indigenous soil protozoa to survive and multiply when exp osed to various concentrations of the fungicide fenpropimorph was investiga ted. The number of protozoan taxa in relation to biocide concentration was examined in enrichment cultures. The population dynamics of bacterivorous a nd fungivorous protozoa, hyphal forming units, and respiration activity wer e followed in soil microcosms amended with glucose and various concentratio ns of fenpropimorph. 2. The average number of flagellate taxa detected in 50-mg portions of air- dried soil declined from 12 to zero with fenpropimorph concentrations incre asing from 0 to 60 mg L-1. Naked amoebae and ciliates were present at all f enpropimorph concentrations. The 50-mg soil portions initially contained 1. 9 x 10(3) heterotrophic flagellates, 1.4 x 10(3) naked amoebae and about 5 ciliates. The presence of the two latter groups even at concentrations of 6 0 mg L-1 therefore suggests that they are more tolerant to fenpropimorph th an the soil flagellates. 3. The addition of glucose had a strong stimulatory effect on soil respirat ion, which lasted for about 20 days. Soil respiration in microcosms amended with glucose and pesticide was of the same order as in systems with glucos e only, however, soil respiration was significantly impeded in microcosm sy stems with a low pesticide content and stimulated in systems with a high pe sticide content. 4. Bacterivorous protozoa (naked amoebae and heterotrophic flagellates) wer e affected at all tested concentrations (0.74-750 mg L-1) of fenpropimorph. High concentrations, 6.6-750 mg L-1, impeded growth of the total protozoan population directly. Lower concentrations only affected heterotrophic flag ellates significantly, possibly through a change in the microbial food web leading to increased competition from and/or predation by ciliates. 5. A low concentration of fenpropimorph (20 mg L-1) had only little effect on hyphal forming units and fungivorous protozoa. Hence, the observed harmf ul effects on the protozoan populations are probably not mediated through a n effect on the fungal populations. 6. Bacterivorous protozoa were affected by fenpropimorph at concentrations lower than those expected in soil after normal field application. Since pro tozoa play an important role in nitrogen mineralization, the beneficial eff ect of the fungicide may be counteracted by detrimental side-effects on soi l N-mineralization.