A long-term chemical and spectroscopic study on a soil amended with Municipal Sewage Sludge

A long-term field experiment was carried out to evaluate the effect of a seven-year soil amendment with municipal sewage sludge on chemical fertility, soil enzyme activities and the changes in the composition of humic substances. The amended soil showed an increase in total and NaOH soluble organic C, available P and K. The heavy metals contained in the sludge did not affect soil phosphatases (alkaline phosphomonoesterase and phosphodiesterase), arylsulphatase or L-asparaginase activity, even if the concentration of total and available Cu, Ni and Zn increased in the arable layer of soil. The fulvic acids FT-IR spectra showed that amended soil contains a higher concentration of carboxylic and aliphatic groups and polysaccharides compared to unamended soil. The polysaccharide increase in the seven-year amended soil is attributable to the humification process since only a small amount of polysaccharides were present in the sludge. The humic acids FT-IR spectra showed that amended soil contained a higher concentration of aliphatic and N-containing groups and a lower concentration of polysaccharides than unamended soil. The humification parameters (i.e. humification ratio, humification index and degree of humification) highlighted the differences in the humification status between sludge humic-like substances and humus from soil, but were unable to show the differences between unamended and amended soils.