dc.description.abstract |
The agricultural demand for inorganic phosphate fertilisers needs to be reduced whilst the dependence
on more sustainable sources of phosphorus, such as sewage sludge, is increased. However, the presence
of heavy metals in sewage sludge pose a threat to soil microorganisms and can inhibit the action of
phosphatase enzymes if present in high concentrations. The long-term impact of Zn and Cu on soil
microorganisms and phosphatase enzyme activity was investigated at four field sites from the Defra
‘Long-Term Sludge Experiment’ in order to determine the overall impact on organic phosphorus
mineralisation. Following the final applications of sludge in 1997, the total concentrations of Zn and
Cu at each site were comparable to the UK statutory limits for sludge amended soils. Almost 20 years
later 63-91 % of the applied metal loadings still remained and total concentrations of Zn and Cu were
found to be significantly higher in contaminated soils, in comparison to untreated soil, and soils
receiving uncontaminated sewage sludge. A significant correlation between exchangeable and total
metal concentration could still be seen in soils contaminated with Zn, whereas the solubility of Cu was
found to be very low (<1 %); though a significant percent remained bound to soil organic matter. No
long-term decrease in microbial biomass carbon (Cmic) could be detected in the contaminated soils at
any of the sites. However analysis of ergosterol showed an increase in the proportion of microbial
biomass carbon derived from fungi. Significant changes were also observed in the PLFA profiles of
microbial communities within contaminated soils, indicating the microbial community has adapted and
become tolerant to the heavy metal contamination. Combining results using meta-analysis indicated
that Zn and Cu caused an increase in fungal biomass carbon of approximately 25-35 % in comparison
to soil receiving uncontaminated sludge. Whereas overall decreases of 16 and 8 % were seen for Cmic
in soils contaminated with Zn and Cu, respectively, indicating a loss of bacterial biomass from the
contaminated soils. Hence, the current UK statutory limits may not be sufficient to prevent changes in
soil microbial community and a reduction in the limits set for sludge amended soils is recommended in
order to protect microbial diversity. No long-term decrease in the activity of phosphomonoesterase was
detectable in the contaminated soils. However combining the results using meta-analysis indicated
phosphomonoesterase activity per milligram of biomass carbon was greater in soils contaminated with
Cu. This may be an indication that Cu is inhibiting extracellular phosphatase enzyme activity within
the soil environment, though this remains undetermined. Orthophosphate, phosphomonoesters, and
pyrophosphate were the predominant forms of phosphorus within the applied sludge treatments; a broad
phosphodiester signal was also present. Overall, the greatest range of organic phosphorus forms was
seen in the uncontaminated sewage sludge. However, no difference in the range of organic phosphorus
compounds was observed between the untreated and sludge amended soils at the field sites investigated.
It was therefore concluded that the organic phosphorus content of sludge amended soils is returning to
that of untreated soil, with no long-term interference caused by the presence of Zn and Cu. |
en_UK |