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Metabolism of Labeled Organic Nitrogen in Soil: Regulation by Inorganic Nitrogen

Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546

Charles W. Rice

Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506

Eldor A. Paul

Dep. of Crop and Soil Sciences, Michigan State Univ., E. Lansing, MI 48824

* Corresponding author.

ABSTRACT

Regulation of organic N metabolism by inorganic N availability was investigated in short-term laboratory incubations of soil. A ¹⁴C-, ¹⁵N-labeled organic N substrate was produced by growing Pseudomonas stutzeri in labeled media and isolating a cytoplasmic fraction. This was added to soils that had been preincubated with glucose or glucose plus NH⁺₄ to induce conditions of N deficiency or sufficiency. Regulation by inorganic N was indicated by stimulated proteolytic enzyme activity and greater initial rates of cytoplasmic ¹⁴C mineralization in N deficient soils. However, effects of N deficiency on ¹⁴C mineralization persisted for no more than 24 h. Preinduced N deficiency significantly decreased the extent of ¹⁵N mineralized from cytoplasmic N. Mineralization of ¹⁴C from leucine added to soil was similarly affected by N availability, yet ¹⁴C-glutamate mineralization was apparently unaffected. In another experiment labeled cytoplasm was added simultaneously with or without a larger quantity of glucose. The glucose caused virtually complete assimilation of ¹⁵N but had no effect on apparent assimilation of ¹⁴C. Thus, there was no relationship between ¹⁵N assimilation and ¹⁴C assimilation, suggesting that the C and N contained in organic N are processed separately by soil microbes. Inorganic N availability may have short-term effects on metabolism of C in organic N but long-lasting effects appear to be minimal.

formerly Dep. of Crop and Soil Sciences, Michigan State Univ. The investigation reported in this paper (no. 88-3-117) is in connection with a project of the Univ. of Kentucky Agric. Exp. Stn. and is published with the approval of the director. The project was initiated while M.S. Smith and E.A. Paul were at the Dep. of Plant and Soil Biology, Univ. of California, Berkeley, and was supported in part by NSF grant B.S.R. 8306181.

Received for publication August 8, 1988.

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