Characteristics of the soil microbial biomass in soils from a long-term field experiment with different levels of C input

Abstract

Soil samples were taken from a 40 year old field experiment and were chosen so as to obtain soils that mainly differed in the amount rather than quality of past C input. The microbial community of these soils was characterized in terms of its qCO 2, the SIR-to-biomass C ratio and its growth and substrate utilization characteristics using glucose as substrate. The microbial substrate utilisation efficiency was also studied in relation to the rate of substrate addition. The amount of microbial 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 soils. The microbial community in the fallow soil mainly contrasted from that 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 between 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 efficiency measured as the ratio of respired-to-biomass incorporated glucose C decreased with the rate of glucose application. At the same rate of application the efficiency was lower in soils with a smaller native biomass than soils with a larger biomass. Compared at a rate of glucose C application of approximately 2× the amount of native biomass C there were only small differences in the microbial substrate utilization between the soils that were not related to the amount of native biomass C.