RELATIONSHIP BETWEEN THE AMOUNT AND THE ACTIVITY OF THE MICROBIAL BIOMASS IN DUTCH GRASSLAND SOILS - COMPARISON OF THE FUMIGATION INCUBATION METHOD AND THE SUBSTRATE-INDUCED RESPIRATION METHOD

The aim of this study was to test whether soil texture and amount of available carbon affect microbial activity and the percentage of the soil microbial biomass that is active. Total and active microbial biomass were determined by the fumigation-incubation method (FI) and the substrate-induced respiration method (SIR), respectively. The ratio biomass-SIR: biomass-FI, indicating the percentage of active microbial biomass, and the ratio C mineralization:biomass-FI, indicating the activity of the microbial biomass, were both higher in soils with a coarse texture than in soils with a fine texture. The ratio C mineralization:biomass-SIR was not affected by soil texture. The ratios biomass-SIR: biomass-FI and C mineralization: biomass-FI were higher in the upper 10-cm soil layer than in the 10-25 cm layer, while the ratio C mineralization: biomass-SIR was not affected by soil depth. During 25 weeks incubation, the amount of biomass-SIR as well as the C mineralization rate decreased significantly. The amount of biomass-FI, however, did not change significantly during incubation. Assuming that the amount of available C decreases both with soil depth and during incubation, the observations indicate that smaller amounts of available carbon coincide with a reduced activity of the biomass and a lower percentage of active biomass. The close similarity between C mineralization rate and amount of biomass-SIR shows that biomass-SIR is a good indicator of microbial activity, whereas the FI method is not suitable for this purpose.