EFFECTS OF HEAVY METALS ON THE SIZE AND ACTIVITY OF THE SOIL MICROBIAL BIOMASS AFTER LONG-TERM TREATMENT WITH SEWAGE SLUDGE
Report No FR0469

S R Smith

Nov 1994

SUMMARY

I BENEFITS

To provide the scientific basis for maximum permissible concentrations of potentially toxic metals in agricultural soils receiving sewage sludge in relation to soil fertility.

II OBJECTIVES

To assess the potential long-term risk to soil fertility, and in particular to the soil microbial biomass, of current UK soil limits for heavy metals in sludge-treated agricultural land.

III REASONS

Data from a limited number of sludge-treated experimental sites indicate that detrimental effects of heavy metals on soil micro-organisms are apparent below the current UK statutory limit concentrations for potentially toxic metals in sludge-amended agr icultural land. Information is needed to assess the likely extent and significance of the effects of heavy metals in sewage sludge-treated soils in operational practice to determine whether the fertility of agricultural land is protected adequately by the regulations controlling the use of sewage sludge in agriculture.

IV CONCLUSIONS

The soil microbial biomass is considered a sensitive indicator of the effects of heavy metals on the fertility of sludge-treated agricultural land. The current regulations on copper (Cu) in sludge-amended agricultural soil are highly precautionary and avoid toxic effects on the soil microbial biomass. For example, no effect on the soil microbial biomass was detected at the maximum concentration of 460 mg Cu kg-1 measured in the soils examined. By contrast, zinc (Zn) and cadmium (Cd) disrupted the microbial biomass, but at soil concentrations exceeding the current limit value for Cd and the maximum amount of Zn permitted in soil at pH 6.0-7.0. However, the results suggested that Zn may have a disruptive effect on the soil microbial biomass at the maximum permissible concentration for Zn of 450 mg kg-1. in sludge-treated soil with pH >7.0. Soil pH conditions also had a significant effect on the size and carbon (C) metabolism of the microbial biomass. This is important and requires careful consideration for sludge-treated soils because different metal limits are allowed in the UK regulations according to banded ranges of soil pH.

V RECOMMENDATIONS

A structured investigation is need in respect of the risks to soil micro-organisms from Zn, and also Cd, in sludge-amended agricultural land. This should encompass soils of different pH value and include calcareous soils in particular. A survey of operational sludge-treated sites, low in organic matter content, would provide a useful and important baseline for comparison with experimental field trials.

VI RESUME OF CONTENTS

Soil microbial biomass C and respiration activity were measured in soil samples from 13 sites contaminated with heavy metals due to past-histories of treatment with sewage sludge, distillery waste (`pot-ale') or pig slurry. Microbial biomass content was measured by a fumigation-extraction method and the respiration rate of soil was determined from the efflux of carbon dioxide using an infra-red gas analyser. Biomass C-to-organic C ratios (Cmic:Corg) of soil and the metabolic quotient of the microbial biomass (qCO2) were estimated and used to assess the potential for stress-induced changes due to heavy metals at the statutory limit concentrations of metals in sewage sludge-treated agricultural land in the UK.

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