RELATIONSHIP BETWEEN MICROBIAL BIOMASS AND SUBSTRATE-INDUCED RESPIRATION IN SOILS AMENDED WITH D-ISOMER AND L-ISOMER OF AMINO-ACIDS

The rates of CO2 evolution induced in five different soils by addition of the D- and L-isomers of the amino acids alanine, glutamine and glutamic acid were measured. The D-amino acid substrate-induced respiration (SIR) rates were all less than the corresponding L-amino acid SIRs, although this difference was not significant for the alanine isomers in every soil. The D- and L-glutamine induced ammonification rate, measured in one of the soils, was also less for the D-isomer than the L-isomer. The difference between the D- and L-amino acid SIRs was greater for glutamine and glutamic acid than for alanine. There were significant relationships between all three L-amino acid SIRs and the glucose SIR, which indicated that L-amino acid SIRs may be used for estimating soil microbial biomass. The D-amino acid SIRs were not constant proportions of the L-amino acid SIRs across all five soils, and there was no significant correlation between either D-glutamine SIR or D-glutamic acid SIR and glucose SIR. The D-glutamine and D-glutamic acid SIRs apparently varied between soils independently of the total microbial biomass but the effects of streptomycin and cycloheximide on amino acid SIRs were inconclusive in establishing whether D-amino acid SIR could be attributed to a particular component of the soil microbial community. Conditioning of one of the soils with peptone, to increase microbial activity, led to an increase in both D- and L-amino acid SIRs. Both the D- and L-alanine SIRs, and the D- and L-glutamic acid SIRs increased by similar proportions following conditioning with peptone, but conditioning with peptone had different effects on D- and L-glutamine SIRs; D-glutamine SIR increased by a factor of 5.6 compared to 3.8 for L-glutamine SIR.