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
S. Dahlin et E. Witter, 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, Soil biology & biochemistry, 30(5), 1998, pp. 633-641
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.