SYMBIOTIC N2 -FIXATION AND MICROBIAL ACTIVITY IN SOILS
CONTAMINATED WITH HEAVY METALS RESULTING FROM LONG-TERM SEWAGE SLUDGE APPLICATION
Report No FR0128
S R SMITH, J P OBBARD*, K H M KWAN* AND K C JONES* * University of Lancaster * Monitoring and Assessment Research Centre, University of London
To determine the effects of increasing soil metal concentration by sewage sludge application on soil microbial processes in relation to the fertility of soils used for agriculture.
To assess effects of metal contamination of soil by sewage sludge application on symbiotic N2-fixation by clover.
To determine effects of heavy metals and some soil physico-chemical characteristics on soil microbial activity.
To establish a strategy for further research to provide the basis for the long-term security of sludge recycling to agricultural land.
There is increasing concern that the application of sewage sludge to agricultural land, even within current maximum limits for soil metal concentrations, may have long-term deleterious effects on soil micro-organisms and soil fertility.
III RESUME OF CONTENTS
The presence of effective strains of rhizobia capable of symbiotic N2-fixation with white clover (Trifolium repens) in a range of metal contaminated soils was investigated. A number of historically sewage sludge-amended sites (including experimental, pasture grassland, and arable sites) were selected and compared with highly contaminated samples from abandoned heavy metal mines. Many sites had metal concentrations above the limits recently established by the Department of the Environment. Acetylene reduction activity (ARA) was used to screen the samples for effective N2-fixation. Effective ARA was found to be dependent on whether indigenous clover was present in the sward, and was affected by the relative and total concentration of the metals present. The influence of individual metals on ARA could not be determined conclusively because of the confounding effects of soil physico-chemical properties and the presence of other toxic elements.
However, Cd appeared to be particularly important in determining the presence of effective ARA in soils without indigenous clover. Zinc and Cu had greater effects on the size of the effective rhizobial population than Ni and Cr in an historical experimental site.
Microbial activity measured by soil ATP content, dehydrogenase activity and nitrogen mineralisation/nitrification potential was also studied in one highly contaminated historically-sludged arable site. Activity increased with increasing organic matter content, irrespective of the level of metal contamination. Therefore, beneficial effects of sludge application on soil physico-chemical characteristics were more significant than potential deleterious effects of increasing metal concentrations. The importance of using other historical sites to evaluate the impact of heavy metal accumulation in soil due to sewage sludge application is described. A complementary research strategy is also proposed.
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