# A Systematic Review on Earthworms in Soil Bioremediation
## 3 Results
### Combined Vermierendiation for Inorganic Pollution
In particular, the strong influence of earthworms was observable in trials with Cr contamination where treatment with plant alone showed no germination, while the combination of plant + amendment + earthworm enhanced the plant growth and seed germination. It was observed that the _E. fetida_ bioaccumulation factor was higher when vermiremediation and phytoremediation were jointly applied; also, the metals' accumulation efficiency of the plant increased in the combined treatments. Additionally, [PERSON] et al. [132] explored the interactive effects of _E. fetida_ and CD on the uranium (U) phytoextraction efficiency of two plant species (_Brassica juncea_ and _Helianthus annuus_). The combined application of the amendment and _E. fetida_ resulted in a major and significant increase of 53% in sunflower shoot biomass. Statistically, earthworms have increased the U solubility, thus making it more bioavailable for plant uptake and accumulation. Indeed, the total metal accumulation increased by 68.5% for Indian mustard and 85% for sunflowers compared to treatments with plants alone. Soil microorganisms are used in bioremediation because they bind, immobilise, oxidise, transform and volatilise inorganic contaminants [133]. There is a symbiotic relationship between soil microorganisms and earthworms, which, thanks to their burrowing activities, mucus and cast production, positively impact soil microbial properties [134; 135]. The gut microbiota represents a unique ecological niche [136], and it is the origin of the restoration capacity of earthworms [137]. Consequently, several authors have recently investigated the effectiveness of bioaugmentation combined with vermiremediation. [PERSON] et al. [138] investigated the remediation capacity of _E. fetida_ and fungal bio-accelerators (_Trichoderma harzianum, viride_ and _Phanerochaete chrysosporium_), alone or in combination; the substrate was composed of sewage sludge and rice straw in different percentages, contaminated with Cd, Ni and Pb. As can be seen from Figure 4, the use of _E. fetida_, individually or in combination, gave promising results; the highest percentage reductions for all analysed contaminants were found in treatment where the sewage sludge and rice straw ratio was 1:1 (S3 + E + M) where (Cd 72%, Ni 67%, Pb 62%). Furthermore, authors found a primary role of earthworms in increasing the metals' available fraction compared to the concentrations measured in the treatments with microorganisms alone. [PERSON] et al. [139] evaluated the influence of _Bacillus megatherium_ on the remediation process of earthworms in Cd removal from artificially contaminated soils. In this study, the combined treatment (S + E + M) had a Cd content at the end of the experiment statistically lower than the soil treated with earthworms alone (S + E) (Figure 4), contradicting what was observed by [PERSON] et al. [138]. Microorganisms can enhance the metals' bioavailability in soil by altering pH, oxidation/reduction reactions and chelator ions [140]. The use of _B. megatherium_ probably stimulated the accumulation of Cd in earthworms, making it more assimilable as a food source and leading to a 23.6% increase in tissue content. From the selection of research articles for this review, only one work mixed the three main bioremediation techniques, i.e., phytoremediation, vermiremediation and bioaugmentation, on soil contaminated with PTEs. [PERSON] et al. [141] investigated the separate and combined effects of _Rhizophagus irregularis_ (arbuscular mycorrhizal fungi, AMF) and _E. fetida_ on the Cd restoration ability of _Solanum nigrum_. The positive effect of earthworms was influenced by the different levels of Cd contamination (15, 30, 60, 120 mg kg\({}^{-1}\)), as _E. fetida_ improved plant growth by enhancing phosphorus absorption at high Cd concentrations compared to low ones. The shoot biomass yield was greater in the presence of both AMF and _E. fetida_, maybe because earthworm activities mitigate metal damage to these microorganisms.