Synthesis of 15N-labelled microbial biomass in soil in situ and extraction of biomass N

F. Azam, R. L. Mulvaney, F. J. Stevenson

Research output: Contribution to journalArticlepeer-review


A Pakistani soil (Hafizabad silt loam) was incubated at 30°C with varying levels of 15N-labelled ammonium sulphate and glucose (C/N ratio of 30 at each addition rate) in order to generate different insitu levels of 15N-labelled microbial biomass. At a stage when all of the applied 15N 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 CHCl3 vapour for 24 h followed by extraction with 500 m M K2SO4. A correction is made for inorganic and organic N in 500 m M K2SO4 extracts of the unfumigated soil. Results obtained using this approach were compared with the amounts of immobilized 15N extracted by 500 m M K2SO4 containing different amounts of CHCl3. The extraction time varied from 0.5 to 4 h. The amount of N extracted ranged from 27 to 270 μg g-1, the minimum occurring at the lowest (67 μg g-1) and the maximum at the highest (333 μg g-1) N-addition rate. Extractability of biomass 15N ranged from 25% at the lowest N-addition rate to 65%a for the highest rate and increased consistently with an increase in the amount of 15N and glucose added. The amounts of both soil N and immobilized 15N extracted with 500 m M K2SO4 containing CHCl3 increased with an increase in extraction time and in concentration of CHCl3. The chloroform fumigation-extraction method gives low estimates for biomass N because some of the organic N in K2SO4 extracts of unfumigated soil is derived from biomass.

Original languageEnglish (US)
Pages (from-to)180-185
Number of pages6
JournalBiology and Fertility of Soils
Issue number2
StatePublished - Jan 1989


  • Extractability ratios
  • Microbial biomass
  • N immobilization-remineralization
  • Priming effect

ASJC Scopus subject areas

  • Microbiology
  • Agronomy and Crop Science
  • Soil Science


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