Soil respiration induced by the D- and L-isomers of a range of amino acids

The rates of CO2 production from soils amended with the D-and L-isomers of alanine, glutamine, glutamic acid, histidine, methionine and tryptophan have been determined in two soils with similar microbial biomass contents but contrasting qCO(2) (basal respiration/biomass C) values and ergosterol (indicator of fungi) contents. The L-amino acids used include representatives of all the main structural groups and some D-amino acids rarely encountered in nature. The L-amino acid SIRs were always greater than the D-amino acid SIRs, and in most cases the ratio of L-amino acid-to-D-amino acid SIR (L-to-D ratio) was greater for the soil with lower qCO(2) and smaller fungal biomass. The enhancements of both D-amino acid SIR and L-amino acid SIR following conditioning of the soils with the amino acid substrate for 7d were equally susceptible to streptomycin and less susceptible to cycloheximide. These observations indicate that the difference between D-and L-amino acid SIRs was not due to gross differences in the proportions of bacteria and fungi in the soils. (C) 1997 Elsevier Science Ltd.