Impact of earthworms on trace element solubility in contaminated mine soils amended with green waste compost

Effect of Biochar on Micronutrient Availability and Uptake Into Leafy Greens in Two Urban Tropical Soils With Contrasting Soil pH

Biochars have been proposed as a novel biotechnology to increase crop yields in acidic soils due to a liming effect. However, the application of biochar to soils with a neutral soil pH is less likely to improve yield. A rise in pH typically increases the availability of macronutrients (e.g., PO43-, NO3-) but biochar is known to immobilize some elements due to a pH increase and adsorption on the biochar surface. Therefore, biochar application may reduce the uptake of important micronutrients (e.g., Cu, Fe, and Zn) into the edible portions of food crops. Before recommending indiscriminate biochar application to tropical soils, an understanding of the potentially negative impacts of biochar application to contrasting soil types should be fully appreciated to prevent unintended consequences. Our aim was to determine the impact of biochar amendment to an acidic soil and a neutral soil on micronutrient availability and uptake into leafy greens. We produced biochars from 3 different organic feedstock materials (corn cobs, rice husk and teak sawdust) and applied these in pot experiments to an acidic tropical soil (pH 4.5) and a neutral tropical soil (pH 6.9) collected from urban farms in Tamale and Kumasi, respectively, in Ghana. We grew leafy greens (Amaranthus, Corchorus, and Lettuce) and measured their growth and the uptake of Cu, Fe, and Zn, alongside supporting measurements of soil pH and micronutrient availability in the soil. We also measured water soluble Cu, Fe, and Zn in the soils amended with biochars pyrolyzed at different temperatures. The corn cobs biochar increased soil pH and considerably increased plant growth in the acidic soil from Tamale. In the neutral soil from Kumasi we found that, while corn cob biochar increased soil pH, rice husk biochar decreased soil pH. Furthermore, corn cob biochar considerably reduced plant growth in the neutral soil. The concentration of micronutrients in the edible portions of leafy greens was not greatly affected by biochar application, but the total uptake (i.e., concentration multiplied by biomass) of micronutrients into leaves was generally increased by biochar application in the acidic (Tamale) soil and application of the corn cob biochar generally decreased total uptake of micronutrients in the neutral (Kumasi) soil. Our results highlight the need for site-specific information on biochar feedstock and soil pH prior to recommending biochar application to tropical urban soils so that the benefits can be optimized and unintended consequences can be prevented.

Field observations to establish the impact of fluvial flooding on potentially toxic element (PTE) mobility in floodplain soils

Inundation of river water during flooding deposits contaminated sediments onto floodplain topsoil. Historically, floodplains were considered an important sink for potentially toxic elements (PTEs). With increasing flood frequency and duration, due to climate change and land use change, it is important to understand the impact that further flooding may have on this legacy contamination. In this study a field-based approach was taken, extracting soil pore waters by centrifugation of soils sampled on multiple occasions from multiple locations across a floodplain site, which lies adjacent to the River Loddon in southeast England. Flooding generally decreased pore water PTE concentrations and significantly lower pore water concentrations of Cd, Cu, and Cr were found post-flood compared to pre-flood. The dominant process responsible for this observation was precipitation with sulphides resulting in PTE removal from the pore water post-flood. The changes in pH were found to be associated with the decreased pore water concentration of Cu, which suggests the pH rise may have aided adsorption mechanisms or precipitation with phosphates. The impact of flooding on the release and retention of PTEs in floodplain soils is the net effect of several key processes occurring concurrently. It is important to understand the dominant processes that drive mobility of individual PTEs on specific floodplains so that site-specific predictions can determine the impact of future floods on the environmental fate of legacy contaminants.

Impact of Eisenia fetida earthworms and biochar on potentially toxic element mobility and health of a contaminated soil

This study aimed to evaluate the influence of Eisenia fetida (Savigny), added to an acidic soil contaminated with potentially toxic elements (PTEs; As, Sb, Cd, Pb, Zn) and amended with a softwood-derived biochar (2 and 5% w/w), on the mobility of PTEs and soil health (i.e. nutrient availability, enzyme activity and soil basal respiration). The PTEs bioaccumulation by E. fetida and the acute ecotoxicity effects of the amended soils were also evaluated. The interaction between earthworms and biochar led to a significant increase in soil pH, organic matter, dissolved organic carbon content, cation exchange capacity, and exchangeable Ca compared to the untreated soil. Moreover, the water-soluble and readily exchangeable PTE fraction decreased (with the exception of Sb) between 1.2- and 3.0-fold in the presence of biochar and earthworms. Earthworms, biochar, and their combination, led to a reduction of phosphomonoesterase activity which in soils amended with biochar and earthworms decreased between 2.2- and 2.5-fold with respect to the untreated soil. On the other hand, biochar and earthworms also enhanced soil basal respiration and protease activity. Although the survival rate and the weight loss of E. fetida did not change significantly with the addition of 2% biochar, adding the highest biochar percentage (5%) resulted in a survival rate that was ~2-fold lower and a weight loss that was 2.5-fold higher than the other treatments. The PTE bioaccumulation factors for E. fetida, which were less than 1 for all elements (except Cd), followed the order Cd > As>Zn > Cu > Pb > Sb and were further decreased by biochar addition. Overall, these results highlight that E. fetida and biochar, especially at 2% rate, could be used for the restoration of soil functionality in PTE-polluted environments, reducing at the same time the environmental risks posed by PTEs, at least in the short time.

Impacts of earthworm species on soil acidification, Al fractions, and base cation release in a subtropical soil from China

Soil-exchangeable aluminum (Al) has toxic effects on living organisms in acidic soils. Earthworm presence and activity can alter soil pH, which has a significant influence on Al toxicity. However, the effects of earthworms on soil Al toxicity and fractions are still largely unknown. This laboratory study focused on the effects of three earthworm species (endogeics Pontoscolex corethrurus and Amynthas robustus, anecis Amynthas aspergillum) on soil acidification, Al fraction distribution, and base cation release. Three native earthworm species and a soil (latosolic red soil) collected from a botanical garden in South China were incubated under laboratory conditions. After 40 days of incubation, six Al fractions in soil, namely exchangeable (AlEx), weakly organically bound (AlOrw), organically bound (AlOr), amorphous (AlAmo), Al occluded in crystalline iron oxides (AlOxi), and amorphous aluminosilicate and gibbsite (AlAag) fractions, were extracted using a sequential procedure. Soil pH; organic carbon; total nitrogen; total Al (AlTotal); exchangeable K, Na, Ca, Mg contents; and CEC were determined as well. Compared to control soil, pH values increased by 0.79, 0.41, and 0.57 units in casts in the presence of P. corethrurus, A. robustus, and A. aspergillum, and 0.70, 0.32, and 0.50 units in non-ingested soil, respectively. Compared to control soil, the 61.7%, 30.7%, and 36.1% of AlEx contents in casts and 68.5%, 25.9%, and 39.0% of AlEx in non-ingested soil significantly decreased with the addition of P. corethrurus, A. robustus, and A. aspergillum, respectively. Moreover, compared to control soil, the 78.7%, 37.7%, and 40.1% of exchangeable Ca2+ and 12.3%, 24.7%, and 26.8% of exchangeable Mg2+ contents in casts significantly increased with the presence of P. corethrurus, A. robustus, and A. aspergillum, respectively. Soil treated with P. corethrurus had higher soil pH values, exchangeable Ca2+ contents, and lower AlEx than those with A. robustus and A. aspergillum. Results of principal component analyses showed that P. corethrurus, A. robustus, and A. aspergillum casts and non-ingested soil differ for soil pH, Al fractions, and exchangeable base cations release. These results indicate that earthworms, especially P. corethrurus, can reduce soil Al toxicity, increase soil pH, and affect the release of exchangeable base cations.

Earthworm activities weaken the immobilizing effect of biochar as amendment for metal polluted soils

The effect of earthworms on the immobilization of heavy metals by biochar was investigated using 4-week soil pot experiments. Our results showed that both sludge and rice husk biochars applied to Cd, Pb and Zn contaminated soils significantly reduced the bioavailability of those heavy metals in soils, and their concentrations in soil pore water. The Cd and Pb concentrations in pakchoi shoots were decreased by 10.8%-11.7% and 21.5%-26.5%, respectively, in biochar treatments alone. Biochar and earthworm treatments, alone or in combination, can significantly increase pakchoi growth. However, compared with biochar-treated alone, applying earthworms into pots with biochar treatments significantly reduced soil pH by 0.1-0.19 units, increased the Cd, Pb and Zn concentrations in soil pore water, their bioavailability and total uptake in shoots. Additionally, earthworms weaken the immobilization effect of biochar on heavy metals. The results of principal component analysis and correlation analysis showed that earthworm treatment was the main cause of soil pH reduction, and soil pH was significantly negatively correlated with the bioavailable Cd, Pb and Zn concentrations in the soil. Furthermore, the effect of the earthworm digestive system (casts) on bioavailable Cd, Pb and Zn concentrations could not explain their changes in the soil. In addition, the application of biochar had no significant effect on the survival and heavy metal enrichment of earthworm. Therefore, the effect of earthworms must be considered in the process of Cd, Pb and Zn contaminated farmland soil remediation using biochar.

Influence of earthworm bioturbation on metals phytoavailability and human gastric bioaccessibility

At the global scale, urban agriculture is increasingly developing in cities due to demographic growth and sustainable food concerns. But, urban soils are frequently polluted with metals. In urban gardens, organic matter is also commonly added both to valorize organic household waste and to promote biophysicochemical fertility. As earthworms promote the decomposition and the recycling of soil organic matter, they can also influence the biogeochemical cycle of metals in urban polluted soils. In order to produce safe vegetables in urban areas, it is crucial to highlight the mechanisms involved in complex soil-earthworm-plant ecosystems. An experiment was set up to examine these relationships using lettuce cultivated in controlled conditions with RHIZOtest® devices. Thanks to the RHIZOtest® devices, metal transfer and bioaccessibility were for the first time compared for urban polluted soil without (1-urban soil polluted with Pb, Cd, Cu, and Zn: essential or toxic metals currently found in environment, SNB) and with bioturbation (2-this metal-polluted soil subjected to earthworm bioturbation, SB) and earthworm casts (3-earthworm casts produced in this polluted soil and naturally enriched in organic matter and microorganisms, T). Metal concentration, phytoavailability, and human gastric bioaccessibility were determined in the different samples. Results showed that earthworm bioturbation increased the phytoavailability of all the metals. For the experimental condition SB, the phytoavailability of metals was increased up to 75% compared to SNB. In addition, surprisingly, metal phytoavailability was always superior in SB compared to earthworm casts (T). Moreover, earthworms led to an increase in Zn gastric bioaccessibility up to 10% in the soils in the same way as for phytoavailability, meaning Zn bioaccessibility in SB > T > SNB, whereas it remained unchanged in the lettuces. These data are important to promote sustainable agriculture activities in urban areas; actually, databases concerning different experimental conditions are needed to develop decision support tools.

Effects of red earthworms (Eisenia fetida) on leachability of lead minerals in soil

Contamination of soils by metals and metalloids is an important environmental problem in many residential and industrial sites around the world. Lead is a common contaminant, which enters the soil through mining, industrial activities and waste disposal. A range of technologies can be used to remediate soil lead, however most remediation technologies adversely affect the environment and particularly soil biota. We have assessed the efficacy of vermiremediation (the use of earthworms for remediation) to reduce water extractable lead concentrations in soil. Earthworms were introduced to a sandy soil spiked with the common lead minerals cotunnite (PbCl₂), cerussite (PbCO₃), massicot (PbO) or galena (PbS) at 1000 mg (Pb) kg^-1. Lead concentrations in pore water extracted during the experiment were not significantly different in contaminated soil with and without worms. However, concentrations of lead in water from a deionised water extraction (washing) of contaminated soil were significantly lower in soil with earthworms than in soil without. Earthworms accumulated on average (±1 standard deviation) 276 ± 118, 235 ± 66, 241 ± 58 and 40 ± 30 mg kg^-1 (dry weight of earthworms) of lead in their bodies, in PbCl₂, PbCO₃, PbO and PbS-dosed soils, respectively. During the experiment, earthworms lost weight in all contaminated soils, except those containing PbS.

Amelioration of iron mine soils with biosolids: Effects on plant tissue metal content and earthworms

The achievement of environmentally sound and economically feasible disposal strategies for biosolids is a major issue in the wastewater treatment industry around the world, including Swaziland. Currently, an iron ore mine site, which is located within a wildlife sanctuary, is being considered as a suitable place where controlled disposal of biosolids may be practiced. Therefore, this study was conducted to investigate the effects of urban biosolids on iron mine soils with regard to plant metal content and ecotoxicological effects on earthworms. This was done through chemical analysis of plants grown in biosolid-amended mine soil. Earthworm behaviour, reproduction and bioaccumulation tests were also conducted on biosolid-amended mine soil. According to the results obtained, the use of biosolids led to creation of soil conditions that were generally favourable to earthworms. However, plants were found to have accumulated Zn up to 346 mg kg^-1 (in shoots) and 462 mg kg^-1 (in roots). This was more than double the normal Zn content of plants. It was concluded that while biosolids can be beneficial to mine soils and earthworms, they can also lead to elevated metal content in plant tissues, which might be a concern to plant-dependant wildlife species. Nonetheless, it was not possible to satisfactorily estimate risks to forage quality since animal feeding tests with hyperaccumulator plants have not been reported. Quite possibly, there may be no cause for alarm since the uptake of metals from soil is greater in plants grown in pots in the greenhouse than from the same soil in the field since pot studies fail to mimic field conditions where the soil is heterogeneous and where the root system possesses a complex topology. It was thought that further field trials might assist in arriving at more satisfactory conclusions.

Effects of coastal managed retreat on mercury biogeochemistry

We investigated the impact of managed retreat on mercury (Hg) biogeochemistry at a site subject to diffuse contamination with Hg. We collected sediment cores from an area of land behind a dyke one year before and one year after it was intentionally breached. These sediments were compared to those of an adjacent mudflat and a salt marsh. The concentration of total mercury (THg) in the sediment doubled after the dyke was breached due to the deposition of fresh sediment that had a smaller particle size, and higher pH. The concentration of methylmercury (MeHg) was 27% lower in the sediments after the dyke was breached. We conclude that coastal flooding during managed retreat of coastal flood defences at this site has not increased the risk of Hg methylation or bioavailability during the first year. As the sediment becomes vegetated, increased activity of Hg-methylating bacteria may accelerate Hg-methylation rate.

Increased lead availability and enzyme activities in root-adhering soil of Lantana camara during phytoextraction in the presence of earthworms

Earthworms are known to increase availability of heavy metals in soils and also play an important role in maintaining the structure and quality of soil. The introduction of earthworms into soils contaminated with metals in the presence of a potential hyperaccumulator has been suggested as an aid for phytoremediation processes. The present study was conducted to evaluate: (i) the effects of earthworms on lead availability in artificially contaminated soil at 500 and 1000 mg kg(-1) Pb in the presence of Lantana camara, a hyperaccumulator, (ii) the effects of earthworms and lead on soil properties such as pH, cation exchange capacity (CEC), organic matter (OM), total and available N, P and K and (iii) soil enzyme activities. Earthworms increased the bioavailable Pb in root-adhering soil by a factor of 2 to 3 in the contaminated soils at concentrations of 500 to 1000 mg Pb kg(-1), respectively. In lead contaminated soils, the presence of earthworms led to a significant decrease in soil pH by about 0.2 but increased CEC by 17% and OM by more than 30%. Earthworm activities also increased the activities of N-acetylglucosamidase, β-glucosidase, cellulase, xylanase, alkaline and acid phosphatase, urease and fluorescein diacetate assay (FDA). These results indicate that the ecological context for phytoremediation should be broadened by considering plant-soil-earthworm interactions as they influence both plant health and absorption of heavy metals. They also showed that the enzyme activities monitored could serve as useful proxies for phytoremediation capability and, more generally, for soil quality as a whole.