Effect of metal-rich sludge amendments on the soil microbial community

Abstract

The effects of heavy-metal-containing sewage sludge on the soil microbial community were studied in two agricultural soils of different textures, which had been contaminated separately with three predominantly single metals (Cu, Zn, and Ni) at two different levels more than 20 years ago. We compared three community-based microbiological measurements, namely, phospholipid fatty acid (PLFA) analysis to reveal changes in species composition, the Biolog system to indicate metabolic fingerprints of microbial communities, and the thymidine incorporation technique to measure bacterial community tolerance. In the Luddington soil, bacterial community tolerance increased in all metal treatments compared to an unpolluted-sludge-treated control soil. Community tolerance to specific metals increased the most when the same metal was added to the soil; for example, tolerance to Cu increased most in Cu-polluted treatments. A dose-response effect was also evident. There were also indications of cotolerance to metals whose concentration had not been elevated by the sludge treatment. The PLFA pattern changed in all metal treatments, but the interpretation was complicated by the soil moisture content, which also affected the results. The Biolog measurements indicated similar effects of metals and moisture to the PLFA measurements, but due to high variation between replicates, no significant differences compared to the uncontaminated control were found. In the Lee Valley soil, significant increases in community tolerance were found for the high levels of Cu and Zn, while the PLFA pattern was significantly altered for the soils with high levels of Cu, Ni, and Zn. No effects on the Biolog measurements were found in this soil.

FIG. 1

Relationship between Zn concentrations in soil (aqua regia extracted) and bacterial community tolerance (IC₅₀) for the Luddington soil. A high value indicates a high community tolerance. IC₅₀s are expressed as log metal concentrations in the TdR incorporation bioassay.

FIG. 2

(A) Score plot from the principal-components analysis with PLFA data from the Luddington soil. Each value is the mean ± SE of four replicate determinations. US, uncontaminated sludge; NS, no sludge; LCu and HCu, low- and high-Cu-amended sludge, respectively; LNi and HNi, low- and high-Ni-amended sludge, respectively; LZn and HZn, low- and high-Zn-amended sludge, respectively. (B) Loading values for individual PLFAs from the principal-components analysis of the Luddington soil.

FIG. 3

Relationship between the soil moisture content and the scores for the first principal component of the PLFA data from the Luddington soil. For abbreviations of the treatments, see the legend to Fig. 2A.

FIG. 4

Score plot from the principal-components analysis with the Biolog data after 96 h of incubation for the Luddington soil. Note that axes 1 and 2 have been transposed to facilitate comparison with the PLFA data in Fig. 2A. Each value is the mean ± SE of four replicate determinations. For abbreviations of the treatments, see the legend to Fig. 2A.