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 W oburn (8% clay) were incubated at 25 degrees C, unamended or amended with r yegrass followed by fumigation 20 d later followed by a further 20 d incuba tion. Other portions of the Rothamsted grassland soil were treated separate ly with a fungicide (Captan), a bacteriocide (Bronopol), or a herbicide (Di noseb). The substrate-induced respiration (SIR) method coupled with use of antibiotics (selective inhibition) and biovolume measurements by direct mic roscopy were used to comparatively measure total microbial biomass and the proportions of fungal and bacterial biomass in these two treated soils. Bot h methods gave similar estimates of total microbial biomass and the proport ions of bacteria and fungi in the two soils. The different treatments did n ot significantly change the proportions of bacteria and fungi in the soil m icrobial biomass. It was concluded that both SIR and biovolume measurements are equally valid in measuring total biomass as are selective inhibition a nd biovolume measurements in measuring the proportions of fungi and bacteri a in soils which are either unamended or undergoing rapid changes in metabo lism due to substrate amendment, fumigation or biocidal treatments. (C) 199 9 Elsevier Science Ltd. All rights reserved.