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 qCO2 (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 qCO2 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 7 d 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.