A Pakistani soil (Hafizabad silt loam) was incubated at 30°C with varying levels of15N-labelled ammonium sulphate and glucose (C/N ratio of 30 at each addition rate) in order to generate different insitu levels of15N-labelled microbial biomass. At a stage when all of the applied15N was in organic forms, as biomass and products, the soil samples were analysed for biomass N by the chloroform (CHCl3) fumigation-extraction method, which involves exposure of the soil to CHCl3vapour for 24 h followed by extraction with 500 mMK2SO4. A correction is made for inorganic and organic N in 500 mMK2SO4extracts of the unfumigated soil. Results obtained using this approach were compared with the amounts of immobilized15N extracted by 500 mMK2SO4containing different amounts of CHCl3. The extraction time varied from 0.5 to 4 h
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