1. The ability of indigenous soil protozoa to survive and multiply when exposed to various concentrations of the fungicide fenpropimorph was investigated. The number of protozoan taxa in relation to biocide concentration was examined in enrichment cultures. The population dynamics of bacterivorous and fungivorous protozoa, hyphal forming units, and respiration activity were followed in soil microcosms amended with glucose and various concentrations 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 increasing from 0 to 60 mg L–1. Naked amoebae and ciliates were present at all fenpropimorph concentrations. The 50-mg soil portions initially contained 1·9 × 103 heterotrophic flagellates, 1·4 × 103 naked amoebae and about 5 ciliates. The presence of the two latter groups even at concentrations of 60 mg L–1 therefore suggests that they are more tolerant to fenpropimorph than the soil flagellates.
3. The addition of glucose had a strong stimulatory effect on soil respiration, which lasted for about 20 days. Soil respiration in microcosms amended with glucose and pesticide was of the same order as in systems with glucose only; however, soil respiration was significantly impeded in microcosm systems with a low pesticide content and stimulated in systems with a high pesticide content.
4. Bacterivorous protozoa (naked amoebae and heterotrophic flagellates) were 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 flagellates 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 harmful effects on the protozoan populations are probably not mediated through an 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 protozoa play an important role in nitrogen mineralization, the beneficial effect of the fungicide may be counteracted by detrimental side-effects on soil N-mineralization.