MICROBIAL EFFECTS OF HEAVY METALS IN SEWAGE SLUDGE AMENDED SOILS - EFFECTS ON SYMBIOTIC NITROGEN FIXATION
Report No FR0308

J P Obbard, K C Jones and S R Smith

Oct 1992

SUMMARY

I BENEFITS

To provide an increased understanding of the effects of heavy metals in sewage sludge-treated soils.

To determine whether the fertility of soils is protected by current environmental standards when sludge is used in agriculture.

II OBJECTIVES

To assess effects of metal contamination of soil by sewage sludge application on symbiotic N₂ fixation by clover, peas and beans in relation to current statutory soil metal limit concentrations.

To determine the nature and extent of the effects of sludge application to agricultural land on free-living Rhizobium sp of agronomically important legume crops.

III REASONS

There is increasing concern that the application of sewage sludge to agricultural land, even within current maximum soil limit concentrations for metals, may have long-term deleterious effects on soil micro-organisms and soil fertility.

IV CONCLUSIONS

Where white clover (Trifolium repens) was indigenous to field swards, nitrogenase activity was present in host plant nodules irrespective of soil metal concentration and viable populations of Rhizobium leguminosarum bv trifolii effective in N₂ fixation were also present in the soil. The apparent loss of effectiveness in N₂ fixation of Rhizobium reported for isolates from historically sludged soil at Woburn is not a common phenomenon; effective strains were isolated from many soils even with metal levels exceeding CEC statutory soil limits. Zinc, Ni and Cu may be important in determining the size of effective populations in sludged soil and also rates of N₂ fixation by clover in mixed swards, although effects were modulated by soil metal concentration and soil chemical properties. Some effects were measured with metal concentrations below CEC limits, but in light textured soils of very low pH value. Large concentrations of metals could eliminate effective cells from sludged soil, or populations were small and not detected by the N-free agar test. Heavy metals therefore had a quantitative effect under certain conditions on soil rhizobial populations, and not a genetic effect, but the established practice of seed coating with effective strains may provide a management technique to circumvent any problems due to small soil populations. Nodulation of peas and beans was reduced or completely absent from most contaminated soils. The rate of N₂ fixation by plants grown in contaminated soil was only slightly reduced in soils with an excess of CEC limits from Zn, Cu and Ni and would probably not be important at the statutory limits. No effect on yield was recorded.

V RECOMMENDATIONS

More research using soils contaminated with specific elements is needed to elucidate which metals and at what concentrations effects on Rhizobium and N₂-fixation may occur. In the short-term this is only possible by selecting historic operationally sludge-treated soils with known metal concentration profiles and using experimental sites where controlled additions of sludges have been made in the past. In the long-term (5-10 years +) new field trials on a range of soil types would also be of benefit.

VI RESUME OF CONTENTS

Published information on the effects of metals in sludge-treated soils on R. leguminosarum bv. trifolii is reviewed. Metal contaminated soils, which had received past applications of sludge on an operational basis were screened for the presence of effective Rhizobium. Two experimental sludge-amended sites at Luddington and Braunschweig were studied and the population dynamics of Rhizobium in these soils were assessed. Effects of metals in sludge-amended soils on N₂ fixation by white clover, peas and beans were also determined using 15-N techniques. Results are discussed in relation to the CEC statutory soil metal limit concentrations where sewage sludge is applied to agricultural land.

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