Decomposition of sewage sludge in soil, (3)

Activated sludge from a sewage disposal plant was labeled with 15N in the laboratory. The labeled sludge (dewatered cake) was then incubated under aerobic conditions for 30 days with four kinds of cultivated soils: volcanic ash soil, red-yellow soil, paddy soil, and sandy dune soil. The nitrogen mineralization of the soil organic matter was remarkable only in the case of the paddy soil. In this soil, a priming effect caused by the addition of the sludge was observed. After 30 days of incubation, the 15N organic fractions remaining in the red-yellow soil were determined. It was found that the amino acid and the unidentified fraction in the hydrolyzable nitrogen forms of the sludge contributed mainly to the amount of nitrogen mineralized. Finally, using sludges which originated from the same organic matter source, the influence of a generally-used inorganic coagulator (lime and ferric chloride) of sludge on the nitrogen cycle was clarified. This coagulator caused a reduction in the amount of inorganic nitrogen mineralized in the soil and also accelerated the nitrification process. (author)