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Decomposition of Carbon-14-Labeled Proteins, Peptides, and Amino Acids; Free and Complexed with Humic Polymers¹

ABSTRACT

After 4 to 12 weeks incubation, from 71 to 95% of the ¹⁴C of labeled proteins, peptides, and amino acids applied to soil had evolved as ¹⁴CO₂. Losses over 90% occurred only from ¹⁴COOH groups of amino acids. The physical mixing of freeze-dried amino compounds with freeze-dried humic acid reduced decomposition of protein about 35% but exerted little effect on amino acids. More intimate association achieved by freeze-drying of amino solutions with humic acid solutions at pH 6.0 reduced decomposition of proteins by 60% but only slightly reduced decomposition of the amino acids. Linkage of the proteins, peptides and individual amino acids into model phenolase polymers reduced degradation by about 80 to 90%, but the amino acid portions of the polymer molecules were still a little more susceptible to biological degradation than were the aromatic portions. Greater quantities of individual amino acids linked into model phenolase polymers from hydroxybenzoic and cinnamic acids than of hydroxyphenols and toluenes; and the amino acids linked into the polymers from hydroxyphenolic acids were also less susceptible to biological degradation. Although all amino acid units of 2- and 3-unit peptides linked into model humic acid-type polymers were relatively stable to biological degradation in soil and in pure culture, the N-terminal unit which was linked to the phenolic moiety was a little more stable than the COOH-terminal amino acid unit.

¹ Contribution from Dept. of Soil Science and Agricultural Engineering, Univ. of California, Riverside and Institut für Biochemie des Bodens, Forschungsanstalt für Landwirtschaft, Braunschweig, Germany. The authors thank J. O. Ervin, Miss E. Pleiss, and Miss H. Lemke for technical laboratory assistance.

² Graduate Student and Professor of Soil Science, Univ. of Calif., Riverside, and Biochemist, Inst. für Biochemie des Bodens, Braunschweig, Germany, respectively.

Received for publication August 2, 1974. Accepted for publication November 5, 1974.

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