Transformation of 15N-labelled leguminous plant material in three contrasting soils

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

Research output: Contribution to journalArticlepeer-review


Two soils from Pakistan (Hafizabad silt loam and Khurrarianwala silt loam) and one from Illinois, USA (Drummer silty clay loam) were incubated with 15N-labelled soybean tops for up to 20 weeks at 30°C. Mineralization of soybean 15N was slightly more rapid in the Pakistani soils, and after 20 weeks of incubation, 50%, 53%, and 56% of the applied 15N was accounted for as (NH4++NO3-)-N in Drummer, Hafizabad, and Khurrarianwala soils, respectively. Potentially mineralizable N (determined by anaerobic incubation) varied between 1.5% and 10% of the applied 15N in the three soils at different stages of incubation; somewhat higher percentages were mineralizable in the Pakistani soils than in the Drummer soil. From 3.7% to 9% of the applied 15N was accounted for in the microbial biomass. From 10% to 32% of the applied N was recovered in the humic acid and fulvic acid fractions of the organic matter by sequential extraction with Na4P2O7 and NaOH; from 12% to 49% was recovered in the humin fraction. Of the three soils, Drummer soil contained more 15N as humic and fulvic acids. In all cases, the 15N was approximately equally distributed between the humic and fulvic acid fractions. A significant percentage of the humin 15N (52%-78%, equivalent to 8%-34% of the applied 15N) occurred in non-hydrolyzable (6 N HCl) forms. Of the hydrolyzable 15N, 42%-51% was accounted for as amino acid-N followed in order by NH3 (17%-30%), hydrolyzable unknown forms (20%-22%), and amino sugars (6%-2%). The recovery of applied 15N for the different incubation stages was 87±22%. Recovery was lowest with the Khurrarianwala soil, presumably because of NH3 volatilization losses caused by the high pH of this soil.

Original languageEnglish (US)
Pages (from-to)54-60
Number of pages7
JournalBiology and Fertility of Soils
Issue number1
StatePublished - Jul 1989


  • Biomass N
  • Humus fractions
  • Hydrolyzable N
  • Legume N
  • Mineralizable N
  • N-labelled material
  • N-mineralization

ASJC Scopus subject areas

  • Microbiology
  • Agronomy and Crop Science
  • Soil Science


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