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. In many temperate grassland soils, earthworms play key roles in linking .. Relationships between plant biomass data and earthworm activity. Observations on earthworm populations in the Carse of Stirling indicate that there are definite differences according to soil type both in total population numbers.
The soil ingested by earthworms undergoes chemical and microbial changes when it passes throught the gut. Organic matter is digested and both the pH and the microbial activity of the gut contents increase Edwards et al. Earthworms accelerate nitrogen mineralization from organic matter, but the effect depends on the species and their interaction with soil characteristics, organic matter location and soil biota Butenschoen et al.
The conservation of biodiversity is necessary to maintain the sustainable functioning of soil. InDarwin was one of the first scientists who noted that the topsoil consisted mostly of earthworm castings, thus highlighting the importance of earthworms in pedogenesis processes soil organo-mineral complex. For example, the earthworm population builds galleries and ingests large quantities of organic and mineral matter, thus modifying the porosity and aggregation of the soil.
This earthworm bioturbation may subsequently be reflected in soil profiles Zhang et al. This redistribution of OM depends on the earthworm ecological groups. Endogeic earthworms keep moving inside the soil to feed on soil organic matter SOM while anecic ones feed on plant litter and organic residues at the soil surface and tend to stay in the same burrow Lavelle et al. Epigeic species, which consume considerable amounts of raw OM have a broad range of enzymatic capacities, probably mainly originating from ingested microflora Curry et al.
As discussed by Lavellethe soil biogenic structure mixture of casts, burrows, OM, etc. Soil degradation is often associated with decreases in biodiversity and the abundances of earthworms and other invertebrate communities Lee et al.
However, there is a perceived lack of information to characterize adequately their functional role in soil ecosystem processes such as soil carbon sequestration and loss, decomposition of organic residues, and the maintenance of soil structure.
- Earthworms and Soil Pollution
Almost all OM in soil is directly or indirectly derived from plants via photosynthesis. Specifically, atmospheric carbon dioxide is transformed by reduction into simple and complex organic carbon OC compounds, which in combination with key nutrients enable the plant to function and grow. Soil organic matter SOM provides food and substrates for soil organisms, ranging from macroinvertebrates to heterotrophic bacteria Lavelle et al.
This is of great importance, given that the soil biota is increasingly recognized as playing a major role in soil functions. In cultivated soils, earthworm communities could play an important role in SOM dynamics through regulation of the mineralization and humification processes Lavelle et al.
On the basis of the results of current literature, it appears that there are some differences among studies regarding the effects of earthworms on soil organic carbon content. In the experiment of Desjardins et al. The observed losses of SOM in continuously cropped fields are often attributed to a rapid mineralization of SOM following cultivation. Earthworms caused a decrease in SOM and carbon mineralization by mobilizing recalcitrant forms of OM.
Earthworms enhance mineralization by fragmenting SOM and by mixing SOM, mineral particles and microorganisms, thus creating new contact surfaces between the SOM and microorganisms Parmelee et al. Since earthworms of different ecological groups prefer different food resources, they likely affect nutrient mineralization. Anecic earthworms incorporate litter material into the mineral soil thereby making it available for the soil food web Bossuyt et al.
Endogeic earthworm species, in contrasts, primarily consume soil and associated humified OM in the upper layer of the mineral soil. Calcium and N-mineralization increased with increasing earthworms density, as did microbial metabolic activity. In addition, Coq et al. The higher mineralization in casts might indicate a higher concentration of labile compounds soluble carbon, lignin, etc.
Earthworms have indirect effects on soil organic carbon as determinants of microbial activity. In addition, mucus production associated with water excretion in the earthwom gut is known to enhance the microbial activity Barois, However, microorganisms may constitute an important part of the diet of earthworms, which can feed on them selectively Moody et al. In most natural and managed ecosystems, up to half of the OC added to soil on an annual basis by plant detritus and root exudates is rapidly consumed by microorganisms, and released as carbon dioxide Hopkins et al.
The remainder of the added OM, together with organic compounds synthesized by soil organisms during decomposition and which is released mainly as detritus, persist in the soil for an extended period.
The importance of soil fauna in the decomposition of OM is well known. However, the complex interactions between earthworm and soil microorganisms are less understood. Consumption and humification 8Epigeic earthworm species may feed directly on microorganisms or litter material and inhabit the organic layer of soil.
They have been shown to strongly affect decomposition processes Sampedro et al. Generally, effects of earthworms on microbial biomass and activity depend on soil conditions Shaw et al. It appears that E.
The effects of earthworms on microorganisms depend on the kind of food source and availability and the species of earthworms involved Flegel et al. They concluded that nutrient-enrichment processes but not gut associated processes were responsible for the increased microbial biomass and activity reported in the presence of L.
Meanwhile, endogeic earthworms can transport fresh organic detritus from the soil surface into burrows while mixing it with mineral soil. In the case of tropical endogeic species, it has been demonstrated that the addition of water and readily assimilable intestinal mucus to the ingested soil rapidly stimulates microbial activity. In the second half of the earthworm gut, the mucus will have been almost entirely metabolized and the microorganisms start to degrade the SOM into assimilable OM.
This form of OM is then used by both the worms and the microorganisms. Furthermore, the interactions between earthworms and microorganisms occur at several spatial scales in the drilosphere Brown et al. Lavelle completed the meaning of drilosphere by including earthworm communities, the digestive tract content, and all microbial and invertebrate populations.
Aporrectodea caliginosa earthworms consume a mixture of soil and OM, often choosing to feed in patches of soil that are relatively rich in OM, or in microsites since they are enriched with bacteria and fungi Wolter et al. The mixture of soluble OC, in the form of low-molecular-weight mucus with ingested OM, together with the moist conditions and neutral pH in the foregut, promoted the development of a microbial community that could digest cellulose and other substances that earthworms typically cannot digest.
Essentially, the earthworm gut can act like a bioreactor where microbial activity and biomass are increased due to favorable conditions, with readily available C, from mucus, and water. Hence, earthworm casts EC may contain large amounts of OM that has not been assimilated, but that has been modified both physically and chemically during passage through the earthworm gut.
The EC are usually rich in ammonium-nitrogen and partially digested OM, providing a good substrate for growth of microorganisms. It has been established that there are larger populations of fungi, bacteria, and actinomycetes Shaw et al.
The microorganisms entering the worm guts consume these nitrogenous compounds in mucus Zhang et al. The biological decomposition of OM is mediated by a variety of biochemical processes in which enzymes play a key role Garcia et al. The major constituents of OM, like cellulose, hemicellulose, lignin, and proteins, are degraded by specific enzymes. Earthworms fragment the substrate in the process of feeding and thereby increase the surface area for further microbial colonization.
The enhanced microbial activity accelerates the decomposition process leading to humification, thus oxidizing unstable OM into more stable forms. Humification processes are accelerated and enhanced not only by the fragmentation and size reduction of the OM, but also by the greatly increased microbial activities within the intestines of the earthworms and by the aeration and turnover of the OM through earthworm movement and feeding.
Nutrient inputs, mineralization 13Earthworms are known to be important regulators of major soil processes and functions such as soil structure, OM decomposition, nutrient cycling, microbial decomposition and activity, and plant production. This was caused by enhanced mineralization of N forms, both of a 15N-labelled residue and that of the soil organic matter.
Earthworms can impact plant growth by promoting N-availability Li et al. Several factors may contribute to the mineral weathering mediated by earthworms, such as low pH and a bacteria-rich microenvironment in the gut of earthworms. However, the presence of earthworms may have an effect on the production of greenhouse gases such as nitrous oxide N2O.
Research by Rizhiya et al. The formation and production of N2O in soils is determined by microbial processes: The earthworm gut provides ideal conditions for N2O producing microorganisms by providing abundant substrate, an anaerobic environment, suitable pH and a high moisture content Horn et al.
Impacts of earthworms on soil components and dynamics. A review | Université de Liège
The powerful mechanical grinding action of the gut is caused by the peristaltic actions used to move food along the gut, and the action of ligands originating from earthworms and their gut microorganisms Carpenter et al. Earthworm guts are, consequently, enriched in microorganisms, with concentrations much higher than in the surrounding environment Carpenter et al. High numbers of other organisms that are capable of producing N2O i.
Production of N2O by nitrate-dissimilating bacteria is favored in systems that contain high levels of organic carbon, like the rumen or the gastrointestinal tracts of animals.
Many endospore-forming bacilli that are abundant in soil Felske et al. Indeed, Dash et al. In the gut of earthworms, it is possible that the mucus secreted from the gut epithelium provides an energy source that stimulates biological N-fixation Lee, Through interactions of earthworms with the microbial community and by processing OM, earthworms can increase the system flux of CO2 gaseous C loss. These same interactions, coupled with earthworm excretion, can also lead to increased availability of N.
Nutrient dynamics 17Earthworms are important decomposers contributing to nutrient cycling processes involving nitrogen Lavelle et al. They ingest organic matter with relatively wide C: N ratios and convert it to earthworm tissues of lower C: N ratios Syers et al.
Earthworms | NRCS Soils
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. 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. 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.
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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.
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. 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.