Browsing by Author "Charlton, Alex"
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Item Open Access Long-term Impact of sewage sludge application on rhizobium leguminosarum biovar trifolii: an evaluation using meta-analysis(American Society of Agronomy; Crop Science Society of America; Soil Science Society of America, 2016-07-07) Charlton, Alex; Sakrabani, Ruben; McGrath, Steve P.; Colin D. Campbell, Colin D.The Long-Term Sludge Experiment (LTSE) began in 1994 at nine UK field sites as part of continuing research into the effects of sludge-borne heavy metals on soil fertility. The long-term effects of Zn, Cu, and Cd on the most probable numbers of cells (MPN) of Rhizobium leguminosarum biovar trifolii were monitored for 8 yr in sludge-amended soils. To assess the statutory limits set by the UK Sludge (Use in Agriculture) Regulations, the experimental data were reviewed using statistical methods of meta-analysis. Previous LTSE studies have focused predominantly on statistical significance rather than effect size, whereas meta-analysis focuses on the magnitude and direction of an effect, i.e., the practical significance rather than its statistical significance. Results showed Zn to be the most toxic element causing an overall significant decrease in Rhizobium MPN of −26.6% during the LTSE. The effect of Cu showed no significant effect on Rhizobium MPN at concentrations below the UK limits, although a −5% decrease in Rhizobium MPN was observed in soils where total Cu ranged from 100 to <135 mg kg−1. Overall, there was nothing to indicate that Cd had a significant effect on Rhizobium MPN below the current UK statutory limit. In summary, the UK statutory limit for Zn appears to be insufficient for protecting Rhizobium from Zn toxicity effects.Item Open Access Long-term impact of sewage sludge application on soil microbial biomass: An evaluation using meta-analysis(Elsevier, 2016-08-04) Charlton, Alex; Sakrabani, Ruben; Tyrrel, Sean F.; Rivas Casado, Monica; McGrath, Steve P.; Crooks, Bill; Cooper, Pat; Campbell, Colin D.The Long-Term Sludge Experiments (LTSE) began in 1994 as part of continuing research into the effects of sludge-borne heavy metals on soil fertility. The long-term effects of Zn, Cu, and Cd on soil microbial biomass carbon (Cmic) were monitored for 8 years (1997-2005) in sludge amended soils at nine UK field sites. To assess the statutory limits set by the UK Sludge (Use in Agriculture) Regulations the experimental data has been reviewed using the statistical methods of meta-analysis. Previous LTSE studies have focused predominantly on statistical significance rather than effect size, whereas meta-analysis focuses on the magnitude and direction of an effect, i.e. the practical significance, rather than its statistical significance. The results presented here show that significant decreases in Cmic have occurred in soils where the total concentrations of Zn and Cu fall below the current UK statutory limits. For soils receiving sewage sludge predominantly contaminated with Zn, decreases of approximately 7–11% were observed at concentrations below the UK statutory limit. The effect of Zn appeared to increase over time, with increasingly greater decreases in Cmic observed over a period of 8 years. This may be due to an interactive effect between Zn and confounding Cu contamination which has augmented the bioavailability of these metals over time. Similar decreases (7–12%) in Cmic were observed in soils receiving sewage sludge predominantly contaminated with Cu; however, Cmic appeared to show of recovery after a period of 6 years. Application of sewage sludge predominantly contaminated with Cd appeared to have no effect on Cmic at concentrations below the current UK statutory limit.Item Open Access Long-term impacts of zinc and copper on microbial biomass, phosphatase enzyme activities, and the mineralisation of organic phosphorus in sludge amended soils(Cranfield University, 2015-06) Charlton, Alex; Sakrabani, Ruben; Tyrrel, SeanThe 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.