Comparison of substrate induced respiration, selective inhibition and biovolume measurements of microbial biomass and its community structure in unamended, ryegrass-amended, fumigated and pesticide-treated soils

Two UK grassland soils, one from Rothamsted (24% clay) and the other from Woburn (8% clay) were incubated at 25°C, unamended or amended with ryegrass followed by fumigation 20 d later followed by a further 20 d incubation. Other portions of the Rothamsted grassland soil were treated separately with a fungicide (Captan), a bacteriocide (Bronopol), or a herbicide (Dinoseb). The substrate-induced respiration (SIR) method coupled with use of antibiotics (selective inhibition) and biovolume measurements by direct microscopy were used to comparatively measure total microbial biomass and the proportions of fungal and bacterial biomass in these two treated soils. Both methods gave similar estimates of total microbial biomass and the proportions of bacteria and fungi in the two soils. The different treatments did not significantly change the proportions of bacteria and fungi in the soil microbial biomass. It was concluded that both SIR and biovolume measurements are equally valid in measuring total biomass as are selective inhibition and biovolume measurements in measuring the proportions of fungi and bacteria in soils which are either unamended or undergoing rapid changes in metabolism due to substrate amendment, fumigation or biocidal treatments.