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Decomposition in Soil of the Humic Acid-type Polymers (Melanins) of Eurotium echinulatum, Aspergillus glaucus Sp. and Other Fungi¹

ABSTRACT

Studies were conducted to compare the decomposition rates in soil of ¹⁴C-labeled humic acid-type polymers (melanins) of Eurotium echinulatum and Aspergillus glaucus sp. with those of other fungal melanins, soil and peat humic acids, fungal cells, nondialysable culture medium polymers not precipitated by acid, and other organic residues. Carbon and N contents, COOH groups, and total acidity were also determined for the fungal products. After 12 weeks incubation at 22°C the percentages of the added ¹⁴C activity evolved as ¹⁴CO₂ from the acid-precipitable fungal melanins ranged from 4–6% for E. echinulatum, 4–13% for A. glaucus, 6–8% for Hendersonula toruloidea and 6 to 50% for Stachybotrys atra. The fungal polymers were almost as resistant to biodegradation as soil or peat humic acids. The fungal cells and culture medium polymers not precipitated by acid lost from 26–58% of the applied C, which compared with losses of 50–75% for hyaline fungal cells, corn stalks, and wheat straw. The N source and growth conditions did not influence biodegradability. The C contents of the melanins ranged from 51–60%, the N from 1.4–6.8%, COOH groups from 81–385 meq/100g, and total acidity from 375–876 meq/100 g.

¹ Contribution from Dep. of Soil and Environ. Sci., Univ. of Calif., Riverside, CA 92521 and Instituto de Microbiologia, Universidade Federal Do Rio de Janeiro, Brazil.

² Visiting Professor of Microbio. and Professor of Soil Science, respectively. The authors thank J. O. Ervin for technical laboratory assistance.

Received for publication March 9, 1978. Accepted for publication June 9, 1978.

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