Substrate induced respiration in soil amended with different amino acid isomers

The substrate induced respiration (SIR) by the soil microbial community was determined using glucose, l-alanine, d-alanine, l-glutamine and d-glutamine as the substrate in order to compare the utilization of l- and d-isomers of alanine and glutamine by the microbial community in a podzolic soil. l-alanine and l-glutamine resulted in greater SIR than d-alanine and d-glutamine, respectively. Pre-incubation of the soil for 7 days with any of the amino acid isomers prior to SIR determination led to an increase in the SIR when the same amino acid isomer was used as the substrate. Both d-alanine SIR and d-glutamine SIR were enhanced to a greater extent when pre-incubated with l-glutamine, d-alanine or d-glutamine, than when pre-incubated with l-alanine. Incorporation of streptomycin into the soil at the pre-incubation stage led to almost complete reduction in the enhancement of both d-alanine SIR and d-glutamine SIR resulting from pre-incubation with d-alanine and d-glutamine, respectively, whereas incorporation of cycloheximide led to no such reduction in enhancements. This indicates that the enhancement of d-amino acid SIR was because of bacteria rather than fungi.