Summary
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 mM K2SO4. A correction is made for inorganic and organic N in 500 mM K2SO4 extracts of the unfumigated soil. Results obtained using this approach were compared with the amounts of immobilized 15N extracted by 500 mM 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 mM 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.
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Azam, F., Mulvaney, R.L. & Stevenson, F.J. Synthesis of 15N-labelled microbial biomass in soil in situ and extraction of biomass N. Biol Fert Soils 7, 180–185 (1989). https://doi.org/10.1007/BF00292580
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DOI: https://doi.org/10.1007/BF00292580