Monitoring sewage sludge using heterotrophic nitrogen fixing microorganisms

Sewage sludge was studied using free-living N2-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 as 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 N2-fixing activities of the soils were unaffected by additions of sludge with elevated concentrations of Cd, Co, Pb, Zn and naphthalene, corresponding to the recommended rate and 2RR and 10RR. Addition of a sludge with low contents of heavy metals and organic pollutants reduced N2-fixation activities at all concentrations in the Gälbo soil. Additions of a sludge with an elevated content of Pb and PCBs reduced the N2-fixation when added at 10RR. The Ulleråker soil, which had higher contents of organic carbon and clay than the Gälbo soil, showed reduced N2-fixation when treated with these sludges at 10RR. Addition of a sludge with elevated concentrations of Ag, Cr, Hg and naphthalene reduced N2-fixation when applied at 10RR in both soils. Sludge with the highest concentration of Cu together with elevated concentrations of Ag, nonylphenol and PCBs reduced the N2-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 with elevated concentrations of nonylphenol and toluene, reduced N2-fixation when applied at 2RR in both soils. Additions of cow manure with high concentrations of ammonium and nonylphenol did not affect N2-fixation in the Gälbo soil at the recommended rate, but reduced it at all concentrations in the Ulleråker soil. Additions of pig manure, with elevated concentrations of ammonium, phenols and toluene, strongly reduced N2-fixation at all rates in both soils. The adverse effects of sludges or manures on potential N2-fixing activities could partly be explained by the increasing concentrations of ammonium present in soil. Above a certain concentration of added ammonium, which varied between the soils, N2-fixation was inhibited. In some sludge-soil combinations, however, adverse effects on N2-fixation were observed that could not be explained by elevated ammonium rates. In these combinations, elevated concentrations of Ag, Cu and nonylphenol were observed. Adverse effects of these substances on heterotrophic N2-fixation were also observed in pure culture studies where Azotobacter sp. isolated from the Gälbo soil were found to be sensitive to Ag, Cd, Co, Cu, Hg, Ni, Zn and nonylphenol at maximum concentrations present in the soils after additions of the sludges. Our findings indicate that particular elements or substances, such as Ag, Cu and nonylphenol present in the sludges or manures, are particularly toxic and deserve attention and that measurement of N2-fixation in sludge or manure treated soils is a useful method for predicting the biological quality of a particular organic slurry.