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Chemical Distribution of Residual Fertilizer Nitrogen in Soil as Revealed by Nitrogen-15 Studies¹

ABSTRACT

Chemical distribution patterns were obtained for the residual N in field plots previously amended with ¹⁵N-labeled urea and oxamide. From 25 to 40% of the fertilizer N was present in the soil (0 to 25 cm) after the first growing season, about half of which still remained after 5 years. Essentially all of the fertilizer-derived N (97.0%) occurred in organic combination; only a small fraction (3.0%) was accounted for in inorganic forms, chiefly as fixed NH₄⁺. In comparison to the native humus N, higher percentages of the fertilizer N left after the first growing season occurred as amino acids (52.0 vs. 33.7%) and amino sugars (8.2 vs. 7.5%); lower percentages occurred in acid-insoluble forms (9.0 vs. 15.2%), as acid-hydrolyzable organic NH₃ (9.0 vs. 17.0%), and as unidentified acid-soluble N (8.8 vs. 20.3%). Considerable humification occurred during the subsequent 4 years with relocation of amino acids N (and possibly amino sugar-N) to more resistant humus forms. The findings suggest that fertilizer N, once incorporated into soil organic matter, becomes increasingly stable with time and is not readily mineralized or subject to leaching.

Key Words: stable isotope • N-fractions • amino sugars • amino-acids • humification

¹ Contribution from the Department of Agronomy, University of Illinois, Urbana, and the Illinois Agr. Exp. Sta. This work was done in cooperation with the Division of Agricultural Development, Tennessee Valley Authority, and the U. S. Atomic Energy Commission. It is part of the Ph.D. thesis by the senior author. Appreciation is expressed to Dr. E. G. Perkins and Mr. E. Mayhood for assistance with the isotopic analyses through an NIH Biomedical Facilities Grant.

² Research Assistant and Professors of Soil Chemistry, respectively. The senior author is now Assistant Professor, Department of Agriculture, Langston University, Langston, Okla. 73050.

Received for publication March 28, 1972.