Earthworms and Soil Pollution

By Ngai Ieng Chao: September 2009

Supervisors: Dr Nick Voulvoulis and Dr Martin Head

Introduction

The overarching aim of this study is to investigate the links between soil, earthworms and soil pollution.

Soil is a pivotal component of the environment, and it supports diverse and numerous life forms among which earthworms are considered to be representative organisms.

Soil is also a medium for the spread of pollutants and plays a critical role in determining the toxicity of pollutants. Soil dwellers such as earthworms are inevitably subject to the toxicity effects of pollutants. On the other hand earthworms also have an impact on soil properties, structure and pollution. All these form the complex links between soil, earthworms and pollution.

Objectives

  1. Develop a sampling strategy to have a better understanding of the links between soil pollution and earthworms.
  2. Investigate the relationship between soil properties and soil pollution.
  3. Explore the impacts of different soil properties on earthworm population, distribution, species and diversity, and ecological groups.
  4. Further investigate the effect of soil pollution upon earthworm population, distribution, species and ecological groups.

Method

There are many studies investigating the relationships between soil, earthworms and pollution. There is evidence that earthworm population, distribution and species are not only influenced by soil properties such as temperature, moisture, pH, compaction and texture etc. but also contaminant levels in the soil. However, most of the studies were carried out under laboratory and controlled conditions, and only a few were under field conditions. Even fewer field studies focused on the relationship between earthworms and toxicity levels of contaminants.

The soil and earthworm survey was launched by OPAL, which developed a Field Guide for the public to identify earthworm species ‘easily’. This survey enables a vast database of soil properties and earthworm population and species distribution across England to be compiled, and also forms a basis of this study.

The first stage of this study was to review past studies on the links between soil, earthworms and soil pollution. The outcome of this stage set the theoretical background and hypothesis for this study, and shed light on the next stage. The second stage involved field sampling to investigate these potential links in the field. The last stage of this study was data analysis; ordination was applied to produce a meaningful summary of the patterns underlying the multivariate data in this study, and investigate the relationships between environmental variables and data of species and earthworm ecological groups

Results and conclusion

The significant relationships found in this study are listed below:

  • Higher mercury concentrations in soils with the presence of unusual objects
     
  • Presence of plant roots in the soil with greater earthworm populations in soils containing plant roots
     
  • Earthworm species L. rubellus and pH value, with populations high in pH 6.0


The findings suggest that the main sources of mercury pollution in the sampling soils may come from foreign objects. The study also strengthens the findings in the literature that plant roots in the soils are positively correlated to earthworm population. However, the analysis on the earthworm species in this study seems to be far less convincing due to the low number of adult earthworms encountered in the sampling.

This study also found that earthworm species A. caliginosa, L. terrestris and A. longa in the field can still survive in soils with unfavourable conditions. Earthworms also existed in field soils with higher mercy concentrations than the lethal levels reported in the literature. Thus, this study further demonstrated the differences in the relationships between soil, earthworms and soil pollution under confounding field conditions and controlled laboratory conditions.

It is also noted that there are some limitations in this study. The number and frequency of earthworms encountered in the field were affected by the temperature and weather which may result in bias in the data. The method of hand-sorting earthworms may also lead to underestimation of earthworm populations. Furthermore, the descriptive questions in the OPAL Soil and Earthworm Survey inevitably result in some subjectivity in the data.

The links between soil, earthworms and soil pollution studied in this work has implications on earthworms to serve as bio-indicators of soil condition and pollution in future regulation and policy. Future studies should try to exclude the limitations above and include more quantitative data. As the links between soil, earthworms and pollution are not very clear yet, further studies are still needed.

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