As amino acids form a readily bioavailable source of N in the soil, the decomposition kinetics and reactions of the three contrastingly charged amino acids, glutamate (-1), glycine (0) and lysine (+1) was studied in detail. Sorption of the amino acids to the soil's solid phase could be described by the Langmuir equation with the strength and amount of sorption following the series lysine>glycine>glutamate. Amino acid decomposition was hypothesized to be a purely biological process as CHC13 fumigation resulted in no observable mineralization. For all concentrations (0.01-10 mM) the biological utilization of the three substrates followed the series glutamate>glycine>lysine. Although the substrate utilization rate appeared to be non linear at high substrate concentrations, kinetic studies of initial usage indicted that uptake into microbial cells could be described by a combination of a saturatable 'high affinity' component and a linear 'low affinity' transport component. The K(m) for the saturatable component was in the region of 500-1000 μM whilst V(max) was in the region of 20-70 nmol g-1 soil h-1. In general, a greater proportion of the glutamate, glycine and lysine were used for new biomass production producing yields of 0.81, 0.79 and 0.68 μmol biomass-C μmol amino acid-C-1 respectively. Microbial yield appeared to be largely independent of substrate concentration. The presence of either glucose or citrate added at a 10-fold excess had little effect on either amino acid uptake or mineralization rate, while an excess of other amino acids significantly depressed the utilization of all three amino acids. Comparison of plant root and microbial amino acid transport kinetics and simple rhizosphere calculations indicated that the competition for amino acids between roots and soil microorganisms will be intense.