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 a vailable carbon affect microbial activity and the percentage of the so il microbial biomass that is active. Total and active microbial biomas s were determined by the fumigation-incubation method (FI) and the sub strate-induced respiration method (SIR), respectively. The ratio bioma ss-SIR: biomass-FI, indicating the percentage of active microbial biom ass, and the ratio C mineralization:biomass-FI, indicating the activit y of the microbial biomass, were both higher in soils with a coarse te xture than in soils with a fine texture. The ratio C mineralization:bi omass-SIR was not affected by soil texture. The ratios biomass-SIR: bi omass-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 mineraliza tion: biomass-SIR was not affected by soil depth. During 25 weeks incu bation, the amount of biomass-SIR as well as the C mineralization rate decreased significantly. The amount of biomass-FI, however, did not c hange significantly during incubation. Assuming that the amount of ava ilable C decreases both with soil depth and during incubation, the obs ervations indicate that smaller amounts of available carbon coincide w ith a reduced activity of the biomass and a lower percentage of active biomass. The close similarity between C mineralization rate and amoun t of biomass-SIR shows that biomass-SIR is a good indicator of microbi al activity, whereas the FI method is not suitable for this purpose.