MICROBIAL EFFECTS OF HEAVY METALS IN SEWAGE SLUDGE-AMENDED AGRICULTURAL SOILS - EFFECTS ON MICROBIAL ACTIVITY AND DECOMPOSITION PROCESSES
Report No FR0275

J P Obbard*, K C Jones* and S R Smith * Inst Env & Biol Sciences, Lancaster University

March 1992

SUMMARY

I OBJECTIVES

To assess the impact of heavy metals from sewage sludge applications to land on soil microbial activity and organic matter decomposition processes.

II REASONS

Limited evidence has indicated that there may be long-term adverse effects of sludge-borne heavy metals on soil fertility. Soil micro-organisms perform a crucial role in the carbon cycle in breaking down organic matter, and it is important that these pr ocesses are not adversely impacted.

III CONCLUSIONS

Heavy metals were seen to have measurable effects on both the potential activity of the microbial biomass and initial decomposition rates in sludge-amended soils in the long-term. Whilst these effects were mostly observed in soils with concentrations gr eater than current soil limit values, further studies are required of the complex interaction to identify individual critical soil concentrations of heavy metals so as to provide the robust scientific basis for soil limits in the future.

IV RESUME

This report describes a series of soil microbial investigations focusing on the carbon cycling in soil. The methods employed were soil respiration (CO₂ evolution), dehydrogenase (enzyme) activity, and cotton strip assay (a measure of cellulose degradation). The soils used in these studies were collected from historically sludged sites, historic sludge experimental sites and operational sites. Soil metal concentrations ranged from background to well in excess of current maximum limit values.

Respiratory activity of the soils was robust and not affected by increased concentrations of metals in soil. This implies that essential microbially mediated processes involved in the degradation of organic matter and turnover of soil organic carbon are maintained in the long-term in metal contaminated sludge treated soil. However, dehydrogenase activity was affected to some extent at maximum permissible levels of metals, but the relevance of this in relation to soil fertility requires further investigation. Degradation of cellulose in cotton strips was initially reduced during incubation in metal contaminated soil although the effect was short-lived and not important in the long-term.

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