Effects of long-term sewage sludge amendment on the composition, structure and proton binding activity of soil fulvic acids

The objective of this research was the evaluation of long-term effects of amendment with sewage sludge (SL) on the compositional, structural, and functional characteristics of soil fulvic acids (FAs) with special emphasis on their acid-base properties. For this purpose, FAs were isolated from SL, soils amended with SL after 9 and 36 months from amendment, and the corresponding unamended soil. With respect to unamended soil FA, SL-FA was characterized by a prevalent aliphatic character, larger contents of C, H, N, and S, smaller contents of 0, carboxyl and phenolic OH, total acidity, and C/N and C/H ratios. The SL-amendment determined a number of modifications in soil FA, including an increase of aliphaticity and N, H and S contents, a decrease of 0 and acidic functional group contents and C/N ratios, and a slightly increase of proton affinity of the carboxylic-type and phenolic-type groups. Further, SL-application induced a decrease of heterogeneity of carboxylic-type groups, whereas the heterogeneity of phenolic-type groups increased substantially. These changes, in turn, influenced the acid-base buffering capacity of SL-amended soils, which is expected to have an important impact on the biological availability, mobilization, and transport of nutrients and heavy metal ions in SL-amended soils. However, these effects on SL-amended soil FAs become less and less apparent with a clear trend to approach the molecular properties of native soil FA.