Biodegradation kinetics and sorption reactions of three differently charged amino acids in soil and their effects on plant organic nitrogen availability

As amino acids form a readily bioavailable source of N in the soil, the dec omposition kinetics and reactions of the three contrastingly charged amino acids, glutamate (-1), glycine (0) and lysine (+1) was studied in detail. S orption 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 th e series lysine > glycine > glutamate. Amino acid decomposition was hypothe sized to be a purely biological process as CHCl3 fumigation resulted in no observable mineralization. For all concentrations (0.01-10 mM) the biologic al utilization of the three substrates followed the series glutamate > glyc ine > lysine, Although the substrate utilization rate appeared to be non li near at high substrate concentrations, kinetic studies of initial usage ind icted that uptake into microbial cells could be described by a combination of a saturatable 'high affinity' component and a linear 'low affinity' tran sport component. Thr K-m for the saturatable component was in the region of 500-1000 mu M 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 we re used for new biomass production producing yields of 0.81, 0.79 and 0.68 mu mol biomass-C mu mol amino acid-C-1 respectively. Microbial yield appear ed to be largely independent of substrate concentration. The presence of ei ther glucose or citrate added at a 10-fold excess had little effect on eith er amino acid uptake or mineralization rate, while an excess of other amino acids significantly depressed the utilization of all three amino acids. Co mparison of plant root and microbial amino acid transport kinetics and simp le rhizosphere calculations indicated that the competition for amino acids between roots and soil microorganisms will be intense. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.