Biology and Fertility of Soils

, Volume 7, Issue 1, pp 32–38 | Cite as

Quantification and potential availability of non-symbiotically fixed 15N in soil

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


Non-symbiotic N2 fixation was studied under laboratory conditions in two soils from Pakistan (Hafizabad silt loam and Khurrarianwala silt loam) and one from Illinois, USA (Drummer silty clay loam) incubated in a 15N-enriched atmosphere. N2 fixation was greatest with the Drummer soil (18–122 μg g−1 soil, depending upon the soil treatment) and lowest with the Khurrarianwala soil (4–81 μg g−1 soil). Fixation was increased by the addition of glucose, a close correlation being observed between the amount of glucose added and the amount of N2 fixed in the three soils (r = 0.96). Efficiency of N2 fixation varied with soil type and treatment and was greatest in the presence of added inorganic P. Application of Mo apparently had a negative effect on the amount and efficiency of N2 fixation in all the soils. The percentage of non-symbiotically fixed 15N in potentially mineralizable form (NH 4 + -N released in soil after a 15-day incubation period under anaerobic conditions) was low (2%–18%, depending upon the soil treatment), although most of the fixed N (up to 90%) was recovered as forms hydrolysable with 6N HCl. Recovery in hydrolysable forms was much greater for the fixed N than for the native soil N, indicating that the former was more available for uptake by plants.

Key words

Hydrolysable N Mineralizable N N2-fixation Priming effect Plant available N 


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Copyright information

© Springer-Verlag 1988

Authors and Affiliations

  • F. Azam
    • 1
  • R. L. Mulvaney
    • 1
  • F. J. Stevenson
    • 1
  1. 1.Department of AgronomyUniversity of IllinoisUrbanaUSA

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