CHARACTERISTICS OF THE SOIL MICROBIAL BIOMASS IN SOILS FROM A LONG-TERM FIELD EXPERIMENT WITH DIFFERENT LEVELS OF C-INPUT

Soil samples were taken from a 40 year old field experiment and were c hosen so as to obtain soils that mainly differed in the amount rather than quality of past C input. The microbial community of these soils w as characterized in terms of its qCO(2), the SIR-to-biomass C ratio an d its growth and substrate utilization characteristics using glucose a s substrate. The microbial substrate utilisation efficiency was also s tudied in relation to the rate of substrate addition. The amount of mi crobial biomass was closely related to the soil C concentration. The C -mic-to-C-org ratio was, however, not constant but increased with the soil C concentration. Except for the fallow soil, the characteristics of the soil microbial biomass studied differed little between the soil s. The microbial community in the fallow soil mainly contrasted from t hat in the other soils by a lower SIR-to-biomass C ratio and a higher qCO(2). It is concluded that differences in the C-mic-to-C-org ratio b etween the soils was mainly due to differences in the amount of past C input resulting in differences in the quality of soil organic matter, rather than due to intrinsic differences in the microbial efficiency of substrate utilization. The microbial substrate utilization efficien cy measured as the ratio of respired-to-biomass incorporated glucose C decreased with the rate of glucose application. At the same rate of a pplication the efficiency was lower in soils with a smaller native bio mass than soils with a larger biomass. Compared at a rate of glucose C application of approximately 2x the amount of native biomass C there were only small differences in the microbial substrate utilization bet ween the soils that were not related to the amount of native biomass C . (C) 1998 Elsevier Science B.V.