MONITORING SEWAGE-SLUDGE USING HETEROTROPHIC NITROGEN-FIXING MICROORGANISMS

Sewage sludge was studied using free-living N-2-fixing bacteria in two types of soil amended with six types of municipal sewage sludges and cow and pig manures, respectively. Sludge and manure treatments were a s follows: no addition, Swedish recommended rates of 5 t dry wt ha(-1) , twice the standard rate of addition (2RR), and 10 times the standard rate (10RR). The N-2-fixing activities of the soils were unaffected b y additions of sludge with elevated concentrations of Cd, Co, Pb, Zn a nd naphthalene, corresponding to the recommended rate and 2RR and 10RR . Addition of a sludge with low contents of heavy metals and organic p ollutants reduced N-2-fixation activities at all concentrations in the Galbo soil. Additions of a sludge with an elevated content of Pb and PCBs reduced the N-2-fixation when added at 10RR. The Ulleraker soil, which had higher contents of organic carbon and clay than the Galbo so il, showed reduced N-2-fixation when treated with these sludges at 10R R. Addition of a sludge with elevated concentrations of Ag, Cr, Hg and naphthalene reduced N-2-fixation when applied at 10RR in both soils. Sludge with the highest concentration of Cu together with elevated con centrations of Ag, nonylphenol and PCBs reduced the N-2-fixation when applied at the standard rate and 10RR in both soils. The sludge with a moderate concentration of all studied heavy metals except Ag, but wit h elevated concentrations of nonylphenol and toluene, reduced N-2-fixa tion when applied at 2RR in both soils. Additions of cow manure with h igh concentrations of ammonium and nonylphenol did not affect N-2-fixa tion in the Galbo soil at the recommended rate, but reduced it at all concentrations in the Ulleraker soil. Additions of pig manure, with el evated concentrations of ammonium, phenols and toluene, strongly reduc ed N-2-fixation at all rates in both soils. The adverse effects of slu dges or manures on potential N-2-fixing activities could partly be exp lained by the increasing concentrations of ammonium present in soil. A bove a certain concentration of added ammonium, which varied between t he soils, N-2-fixation was inhibited. In some sludge-soil combinations , however, adverse effects on N-2-fixation were observed that could no t be explained by elevated ammonium rates. In these combinations, elev ated concentrations of Ag, Cu and nonylphenol were observed. Adverse e ffects of these substances on heterotrophic N-2-fixation were also obs erved in pure culture studies where Azotobacter sp. isolated from the Galbo soil were found to be sensitive to Ag, Cd, Co, Cu, Hg, Ni, Zn an d nonylphenol at maximum concentrations present in the soils after add itions of the sludges. Our findings indicate that particular elements or substances. such as Ag, Cu and nonylphenol present in the sludges o r manures, are particularly toxic and deserve attention and that measu rement of N-2-fixation in sludge or manure treated soils is a useful m ethod for predicting the biological quality of a particular organic sl urry. (C) 1997 Elsevier Science Ltd.