Methodology for assessing respiration and cellular incorporation of radiolabeled substrates by soil microbial communities.

A method is described for determining biodegradation kinetics of both naturally occurring and xenobiotic compounds in surface and sub-surface soil samples. The method measures both respiration and uptake into cellular biomass of(14)C-labeled substrates. The estimation of biomass incorporation entailed removal of cells from soil particles by washing the soil with a polyvinyl-pyrrolidone/pyrophosphate solution and H2O2. After separation of the cells and the soil particles by centrifugation, the cells were trapped on membrane filters for liquid scintillation counting. Mass balances were easily obtained. The technique was used to measure metabolic activity in soil profiles, including unsaturated and saturated zones. First order rate constants (K1) were in the range of 10(-3)-10(-2) hour(-1) for amino acid metabolism and 10(-5)-10(-4) hour(-1) for m-cresol metabolism. Saturation kinetics were observed for amino acids and m-cresol. m-Cresol K1 values for uptake often exceeded those for respiration by greater than a factor of ten. Vmax values were low (amino acids, 10(1)-10(2) ng g(-1) hour(-1); m-cresol, 10(-1) ng g(-1) hour(-1)), whereas Km values were quite high (amino acids, 10(3)-10(4) ng g(-1); m-cresol 10(3)-10(5) ng g(-1)). Saturation was not observed in many horizons even at 10(5) ng g(-1) dry soil. Frequently, respiration obeyed saturation kinetics whereas uptake was first order. It is concluded that measuring only kinetics of respiration may lead to severe underestimations of biodegradation rates.