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Changes in the Humic Acid Fraction of Soil Resulting from Sludge Application¹

ABSTRACT

Humic acids (HA) extracted from sludge, soil, and sludge-amended soil were characterized by elemental and functional group analyses and visible and infrared spectroscopy. Humic acid extracted from a Chalmers soil appeared to be similar to an average HA (Schnitzer, 1978). Humic acid extracted from Chicago sludge, as compared to the average HA and Chalmers HA, had a lower C/H ratio, increased N content, decreased total acidity and carboxyl content, and a lower absorptivity. With respect to the average HA and Chalmers HA, the sludge HA appeared to be a more aliphatic, N-enriched polymer with decreased amounts of carboxyls and phenolic hydroxyls. Proteinaceous materials associated with the sludge HA appeared to be the major source of N enrichment. In general, HA's extracted from the sludge-amended soil had increased N contents, lower C/H ratios, lower values of total acidity and phenolic hydroxyl, somewhat lower values of CO₂H groups, and lower absorptivities as compared to HA from soil not receiving sludge amendment. Sludge organic matter was apparently recovered in HA extracted from soil 2 and 14 months after sludge application. The association of sludge-borne proteinaceous and aliphatic materials with the sludge and sludge-amended soil HA fractions was indicated by increased amide and C-H stretch infrared absorptions as well as by the increased N contents and decreased C/H ratios. The observation of proteinaceous materials associated with HA extracted from sludge-amended soil as long as 14 months after application suggested these proteinaceous materials were resistant to decomposition, possibly as a result of their incorporation into the HA structure. Fatty acids may be associated with HA's extracted from sludge and sludge-amended soil through esterification to phenolic hydroxyls.

¹ Contribution of the Purdue Agric. Exp. Stn., Journal Paper no. 8087. Research was supported in part by the U.S. Environmental Protection Agency (Grant R804547-2-1).

² Graduate Research Assistant, and Professors, respectively, Dep. of Agronomy, Purdue Univ., W. Lafayette, IN 47907. The senior author is now Assistant Professor, Dep. of Crop & Soil Sci., Michigan State Univ., East Lansing, MI 48824.

Received for publication May 27, 1980. Accepted for publication August 8, 1980.