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, organi c C and heavy metal content: in the first the microbial substrate util ization efficiency after growth on glucose was determined, and in the second the microbial maintenance requirement during C limitation. In t he first experiment growth characteristics and changes in chloroform-l abile total C(C-tot) and ninhydrin-N were determined as well as the fa te of the added glucose C(C-glu) over 12 d. In the second experiment t he microbial biomass was labelled by additions of C-14-glucose in extr emely small amounts so that the biomass was still subject to C limitat ion. In the first experiment there was an exponential increase in the rate of CO2 production upon glucose addition, indicating exponential m icrobial growth. Microbial growth characteristics derived from the res piration curves showed differences between soils. but no consistent re sponse to low pH or heavy metal stress. The C-glu 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)(2 )SO4- and the sewage sludge-amended soils than in the Ca(NO3)(2)-ferti lized and FYM-amended soils. Changes in biomass C and N were contradic tory so that we could not conclude whether or not microorganisms subje cted to stress were characterized by a lower substrate utilization eff iciency. The C-tot specific respiration rate during conditions of C-li mitation in the second experiment was higher in the (NH4)(2)SO4-fertil ized and in the sewage sludge-amended soil compared to that in the Ca( NO3)(2)-fertilized and FYM-amended soils, indicating a higher maintena nce requirement in the former soils. Respiration of C-glu reflected di fferences in the specific respiration rate. Loss of microbially-incorp orated C-glu was larger in the (NH4)(2)SO4- and the sewage sludge-amen ded soils. The results indicate that higher maintenance requirements m ay contribute to the lower biomass C-to-soil organic C ratio that char acterized the acid-and the heavy metal-contaminated soil. (C) 1998 Els evier Science Ltd. All rights reserved.