Encapsulation of protein in humic acid from a histosol as an explanation for the occurrence of organic nitrogen in soil and sediment

Recent work suggests that nitrogen in humic acids exists primarily as amide functional groups that mirror those in protein. However, the mode for the existence of such labile materials as protein still remains unclear. With the combined applications of NMR spectroscopy, tetramethylammonium hydroxide (TMAH) thermochemolysis, pyrolysis/GC/MS, and elemental analysis, we propose that the survival of proteins in humic acids is carried out by encapsulation into hydrophobic domains of humic acids. To test this hypothesis, we simulated encapsulation of 15N-labeled protein extracts into humic acids and demonstrated that complete hydrolysis of the protein is prevented by the encapsulating humic acid. Results from this study constitute evidence to support the encapsulation mechanism involved in the formation of refractory organic nitrogen during sediment diagenesis.