Carex and feather moss peats from northern Alberta were used as growth media for jack pine and slender wheatgrass and their microbiological properties assessed. Differences between the two peats were minimal. Following this assessment, mixtures of the two peats were used to amend oil sand tailings (55% peat, V/V) and either mineral fertilizers or sewage sludge were added at different rates. Slender wheatgrass and jack pine were grown in the greenhouse in the various growth media, and plant growth, microbial activity and mycorrhizal development were measured. Growth of both species was poor without the addition of either fertilizer or sewage. Fertilization up to the equivalent of 112:49:72 kg N:P:K ha-1 substantially increased shoot and root growth of slender wheatgrass. Shoot growth of jack pine reached a maximum at the 28:12:18 kg ha-1 rate, and pine roots weights were depressed when more than 56:24:36 kg ha-1 were applied. Maximum shoot growth of slender wheatgrass and jack pine occurred with the equivalent of 46 and 23 mT ha-1 sewage respectively. The addition of 92 mT ha-1 strongly depressed the growth of jack pine while slender wheatgrass was unaffected. In the absence of peat, slender wheatgrass was much more sensitive to sewage. VA mycorrhizal inoculum in the peat was sparse and high levels of fertilizer or the lowest level of sewage completely inhibited VA mycorrhizal infection. Ectomycorrhizal inoculum was abundant but infections were nil when more than 56:24:36 kg N:P:K ha-1 was applied and very strongly reduced when 23 mT ha-1 or more of sewage was applied. In the presence of slender wheatgrass, fertilizer increased microbial biomass but had no effect when no plants were present, i.e. fertilizer did not affect the decomposition of peat. Microbial activity (C02 efflux) was unaffected by the addition of fertilizer but decomposition of grass litter was reduced by high rates of fertilizer. When sewage was added to the growing medium, microbial activity and microbial biomass were increased and the decomposition potential was decreased. The presence of the fibrous rooted slender wheatgrass consistently inhibited the decay of grass litter as compared to unplanted systems.