Populations of predatory protozoa in field soils after 5 years of elemental s fertilizer application

The effect of 5 yr of repeated application of elemental S (S°) fertilizer on predatory protozoa in soil was investigated. Protozoa that feed on the bacteria Arthrobacter globiformis and Enterobacter aerogenes or the fungi Fusarium solani and Neurospora crassa were enumerated by most probable number (MPN) methods. The application of S° fertilizer reduced the microbial biomass and its activity in soil. Soils treated with 44kg S° ha⁻¹ yr⁻¹ for 5 yr exhibited a 30–71% decline in MPN of protozoa feeding on bacteria and more than a 84% decline in the population of mycophagous amoebae. This decline in protozoa populations parallelled changes in microbial biomass, especially in the case of mycophagous amoebae and fungal biomass. The adverse effect of repeated S° applications on microbial biomass and predatory protozoa was long lasting. Since nutrient transformations (e.g. mineralization) in soil are influenced by microbial interactions, our results suggest reduced nutrient turnover via microbial predation in S° treated soils.