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 cyclo-heximide. These observation 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.