Ecotoxicological impact of arsenic on earthworms and collembolans as affected by attributes of a highly weathered tropical soil

Abstract

High levels of heavy metals in soils may impose serious impacts on terrestrial organisms. In Brazil, the prevention values for evaluating the ecological risk of these elements are based only on soil chemical analyses and/or on data from ecotoxicological assays performed in soils of temperate regions. However, the attributes of the Brazilian highly-weathered tropical soils can influence the availability of heavy metals for soil fauna, resulting in different toxic values. To provide more accurate ecotoxicological risk values for arsenic (As) in tropical soils, we assessed the impacts of sodium arsenate (NaHAsO·7HO) on the reproduction of earthworms (Eisenia andrei) and collembolans (Folsomia candida), as well as on As bioaccumulation and growth (weight loss) of E. andrei in a tropical artificial soil (TAS) and in an Oxisol. In TAS, As doses reduced the reproduction of the species and promoted weight loss of earthworms. On the other hand, the reproductions of the species as well as the earthworm growth were not altered by As in the Oxisol. The effective concentrations that reduce the reproduction of E. andrei and F. candida by 50 % (EC) obtained in TAS (22.7 and 26.1 mg of As kg of dry soil, respectively) were lower than those in the Oxisol (>135 mg kg, for both species). Although there was As bioaccumulation in earthworms in both soils, the internal concentrations in the earthworms were much higher in the oligochaetes exposed to arsenic in TAS. All these differences were attributed to the higher availability of As in the TAS, compared to the Oxisol, which increased the exposure of the species to the metal. The lower availability in the Oxisol was related to higher contents of type 1:1 silicate minerals and Fe and Al oxides and hydroxides, which strongly bind to As. These results highlight the importance of using tropical soils of humid regions to derive the Brazilian ecological risk prevention values for heavy metals, since the toxicity values are specific for these soils.

Arsenic toxicity to earthworms in soils of historical As mining sites: an assessment based on various endpoints and chemical extractions

“…In general, it was found that total As concentration in soil was not the crucial factor that determined the response of earthworms. Similar results were also reported by other researchers (Alves et al, 2018 ; Romero-Freire et al, 2015 ) who conducted the experiments on spiked soils and emphasized the importance of factors that determined arsenic solubility in soils, such as pH, soil texture and organic matter content. The effects of pH seem to be crucial mainly because soil pH strongly affects the solubility of As, while its direct impact on earthworms in not that important because E. fetida itself tolerates a broad range of soil pH from 4.0 to above 7.0 (Edwards & Arancon, 2022 ).…”

“…The latter effect should be probably explained by the fact that the weight was only determined for those individuals that survived, which may have had a particular individual resistance to arsenic. A similar interpretation was provided by Alves et al ( 2018 ) who also reported a significant trend to increasing earthworm weights at the highest As concentrations in soils. They explained it by a higher tolerance of the biggest earthworms to arsenic exposure, or by the fact that those individuals that survived benefitted from reduced density in soil and higher food availability that enabled them to grow bigger.…”

Biochemical response of earthworm, Eisenia fetida to heavy metals toxicity

“…All tubs were covered with gunny bags and the moisture content was maintained constant between 40-45% by sprinkling water. In order to prevent the likelihood of pre-exposure to contaminants, healthy adult earthworms (400-500 mg) from third generation were used as test organisms (Alves et al, 2018). The heavy metals used in this experiment were sodium arsenate (AR/ACS) (Na 2 HAsO 4 7H 2 O) and potassium dichromate (GR) (K 2 Cr 2 O 7 ) purchased from Hi-Media.…”