This laboratory study examines the impact of different application rates of silage effluent on the biomass and activity of microorganisms in a typical grassland soil (Denbigh series). Application rates ranged fi-om an equivalent of 0-5 l m(2) silage effluent, and amended soils were incubated at 25 degrees C for 32 days and periodically sampled (days 2, 4, 8, 16 and 32) for microbial biomass C, CO2 evolution and activities of various soil enzymes. After 2 and 4 days of incubation there was a significant trend of increasing microbial biomass C (P<0.001 and P<0.01, respectively), microbial respiration (CO2 evolution) (P<0.001), and dehydrogenase activity (P<0.01) with increasing rates of silage effluent application. Thereafter microbial biomass C, microbial respiration and dehydrogenase activity declined and were not significantly different from the unamended control soil. Specific respiration (respired C:biomass C) increased with increasing silage effluent application suggesting that microorganisms in amended soils may be physiologically stressed or that there is an increase in the proportion of effluent-derived bacteria relative to fungi in the amended soil. In a second experiment, there was a significant (r(2)=0.98; P<0.001) positive linear relationship between increasing rates of silage effluent application up to 30 l m(2) and microbial respiration. Dehydrogenase activity increased up to an equivalent application of 15 l m(2) and then declined, perhaps because high concentrations of certain chemical compounds with silage effluent have an adverse abiotic impact on the enzyme assay itself. The application of different rates of silage effluent had no effect on soil pH.