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

Abstract

AbstractHumic 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.