Can the low microbial biomass C-to-organic C ratio in an acid and a metal contaminated soil be explained by differences in the substrate utilization efficiency and maintenance requirements?

Two experiments were carried out on four soils of differing pH, organic C and heavy metal content; in the first the microbial substrate utilization efficiency after growth on glucose was determined, and in the second the microbial maintenance requirement during C limitation. In the first experiment growth characteristics and changes in chloroform-labile total C(Ctot) and ninhydrin-N were determined as well as the fate of the added glucose C(gluC) over 12 d. In the second experiment themicrobial biomass was labelled by additions of 14C-glucose in extremely small amounts so that thebiomass was still subject to C limitation. In the first experiment there was an exponential increase in the rate of CO2 production upon glucose addition, indicating exponential microbial growth. Microbial growth characteristics derived from the respiration curves showed differences between soils, but noconsistent response to low pH or heavy metal stress. The gluC data did not give a valid estimate of the substrate utilization efficiency, but indicated a lower extractability of the glucose-derived biomass C in the (NH4)2SO4- and the sewage sludge-amended soils than in the Ca(NO3)2-fertilized and FYM-amended soils. Changes in biomass C and N were contradictory so that we could not conclude whether or not microorganisms subjected to stress were characterized by a lower substrate utilization efficiency. The totC specific respiration rate during conditions of C-limitation in the second experiment was higher in the (NH4)2SO4-fertilized and in the sewage sludge-amended soil compared to that in the Ca(NO3)2-fertilized and FYM-amended soils, indicating a higher maintenance requirement in the former soils. Respiration of gluC reflected differences in the specific respiration rate. Loss of microbially-incorporated gluC was larger in the (NH4)2SO4- and the sewage sludge-amended soils. The results indicate that higher maintenance requirements may contribute to the lower biomass C-to-soil organic C ratio that characterized the acid- and the heavy metal-contaminated soil.