Nutrient availability is a major constraint to plant production and carbon storage in arctic ecosystems, but there are few studies coupling processes in the decomposer and microbial food web and the implications these processes have on the control of nutrient mineralization. We studied the relationship between microbial biomass and the abundance of microbivore and the role of grazing on nutrient turnover after annual addition of carbon (sucrose), fertilizer (NPK), fungicide (benomyl) and bactericides (streptomycin and penicillin) to two dwarf shrub communities, a low and a high altitude heath. After four years of repeated additions, we measured microbial biomass by fumigation-extraction and phospholipid fatty acid (PLFA) analysis, the fungal to bacterial biomass ratio by PLFA analysis and estimated the numbers of protozoa and nematodes, assigned into feeding groups. The fungal to bacterial ratio of nematode feeding groups was around 0.2, indicating a bacterial-based food web in both communities. The size of the microbial biomass did not change after the additions, except when the amount of available carbon was increased (low altitude heath) or when addition of carbon was combined with fertilizer (high altitude heath). In contrast, fertilizer but not carbon increased the number of microbivores. This suggests that the amount of available carbon and not grazing pressure controls the size of the microbial biomass, Furthermore, it suggests that the food quality, e.g. nutrient content of the micro-organisms, had a larger effect on the microbivore than the size of the microbial biomass. The addition of bactericides and fungicide did not significantly change the fungal to bacterial biomass ratio of the micro-organisms. We could not detect any effects of the bactericides. In contrast, the fungicide strongly decreased nematode density, least in the fungal feeders, probably due to increased abundance of the insensitive Aphelenchoides ssp. (C) 2000 Elsevier Science Ltd. All rights reserved.