Cadmium inhibition of microorganisms was found to be bacterial and chemical species dependent. Escherichia coli inhibition was a function of the cadmium ion concentration irregardless of the presence of citrate, a chelator for cadmium that it could not metabolize. Whereas with a Pseudomonas sp. able to metabolize citrate, cadmium inhibition was a function of both the cadmium ion and the presence of citrate. With no citrate, inhibition of this organism occurred only at relatively high cadmium ion concentrations; when citrate was added to the same cadmium-containing growth medium inhibition was observed at a 1000 times lower cadmium ion concentration. This observation is contrary to the classical understanding where a chelate reduces the toxic form of a metal allowing increased growth of the organism. The species of cadmium also differentially inhibited the Douglas fir litter respiration and nitrogen fixing community activities.