Effects of heavy metal-mediated intraspecific variation in leaf litter on the feeding preferences of stream detritivores

Contribution of ingestive/dietary uptake to bioaccumulation of organics in worms

Ingestive uptake is critical for understanding the accumulation and trophic transfer of chemicals and synthesized particles in general. This study explored the contribution of ingestion in the bioaccumulation of chemicals focusing on worms. Novel theory and equations were developed to derive fractional ingestive contribution, fs, from a broad range of dietary uptake and accumulation studies, and to build a small dataset of fs (n = 43) from relevant toxicokinetic and bioaccumulation measurements. Worm fs could be fitted to log KOW-based sigmoidal models with small errors (RSE < 0.15, RMSE<0.15). The basis and limitations of the applied fs equations were elaborated. These included the assumption that aqueous-based and dietary-based elimination rate constants (kTw and kTS) may be statistically equivalent, as demonstrated using fish and worm data. Bioaccumulation and toxicokinetic parameters obtained at under-exposed conditions can also result in non-sensical, negative fs. The developed fs theory suggested a novel way to model bioaccumulation in the presence of aqueous and solid sources, and the potential to consolidate bioaccumulation data in their variant forms and definitions for assessment, modeling, and benchmarking purposes. While the presented fs-log KOW dependence remained to be explored in other species, the importance of ingestive uptake for high-log KOW chemicals questioned the validity of characterizing and regulating bioaccumulation potential of hydrophobic organics - for which dietary uptake matters - using aqueous-only bioconcentration factor (BCF). This question, along with other less important ones, is yet to be explored in future works.

Soil Cd increased the leaf litter Cd remains of Solanum nigrum and Solanum lycopersicum

Plant litter decomposition is a natural pathway of heavy metal cycling in soil ecosystems, but the dynamics of heavy metal release during litter decomposition are relatively poorly understood. The purpose of this study was to investigate the effects of species, soil fauna and soil Cd addition on litter decomposition and Cd release dynamics. Therefore, we selected two plants, Solanum nigrum and S. lycopersicum with large differences in Cd accumulation capacity. First, they were enriched with Cd during the growing period and leaf litter was harvested after 6 months of pretreatment. Then, the decomposition of leaf litter was conducted with or without soil Cd and Eisenia fetida through lab pot tests. Our results showed that leaf litter Cd led to a significant decrease in litter decomposition rate (K value), with a maximum decrease of 32.1% in S. nigrum and 30.1% in S. lycopersicum. We observed that the presence of E. fetida significantly increased K value, but the effect was similar in the +leaf Cd treatment and the -leaf Cd treatment, both for S. nigrum and S. lycopersicum. Interestingly, the litter Cd concentration did not decrease during decomposition, but showed an increasing trend, especially for S. nigrum in the +soil Cd treatment. Moreover, the litter Cd remains was higher in the +soil Cd treatment compared to the -soil Cd treatment for both S. nigrum and S. lycopersicum, no matter whether with or without E. fetida. This result suggests that the Cd may be transferred from soil to litter, thus increasing the litter Cd remains. Overall, our study shows that leaf litter Cd slowed down the carbon cycling in ecosystems. In addition, the release of litter Cd has a lag, and the litter has a certain adsorption capacity for soil Cd, which intensifies the harm to the ecology during litter transfer.

Variance of soil bacterial community and metabolic profile in the rhizosphere vs. non-rhizosphere of native plant Rumex acetosa L. from a Sb/As co-contaminated area in China

Heavy metals (HMs) contamination poses a serious threat to soil health. However, the rhizosphere effect of native pioneer plants on the soil ecosystem remains unclear. Herein, how the rhizosphere (Rumex acetosa L.) influenced the process of HMs threatening soil micro-ecology was investigated by coupling various fractions of HMs, soil microorganisms and soil metabolism. The rhizosphere effect alleviated the HMs' stress by absorbing and reducing HMs' direct bioavailability, and the accumulation of ammonium nitrogen increased in the rhizosphere soil. Meanwhile, severe HMs contamination covered the rhizosphere effect on the richness, diversity, structure and predicted function pathways of soil bacterial community, but the relative abundance of Gemmatimonadota decreased and Verrucomicrobiota increased. The content of total HMs and physicochemical properties played a more important role than rhizosphere effect in shaping soil bacterial community. Furthermore, As was observed to have a more significant impact compared to Sb. Moreover, plant roots improved the stability of bacterial co-occurrence network, and significantly changed the critical genera. The process influenced bacterial life activity and nutrient cycling in soil, and the conclusion was further supported by the significant difference in metabolic profiles. This study illustrated that in Sb/As co-contaminated area, rhizosphere effect significantly changed soil HMs content and fraction, soil properties, and microbial community and metabolic profiles.

Responses of microbial communities and metabolic profiles to the rhizosphere of Tamarix ramosissima in soils contaminated by multiple heavy metals

Heavy metals (HMs) contamination around smelters poses serious stress to soil microbiome. However, the co-effect of multiple HMs and native vegetation rhizosphere on the soil ecosystem remains unclear. Herein, effects of high HMs level and the rhizosphere (Tamarix ramosissima) on soil bacterial community structure and metabolic profiles in sierozem were analyzed by coupling high-throughput sequencing and soil metabolomics. Plant roots alleviated the threat of HMs by absorbing and stabilizing them in soil. High HMs level decreased the richness and diversity of soil bacterial community and increased numbers of special bacteria. Plant roots changed the contribution of HMs species shaping the bacterial community. Cd and Zn were the main contributors to bacterial distribution in non-rhizosphere soil, however, Pb and Cu became the most important HMs in rhizosphere soil. HMs induced more dominant metal-tolerant bacteria in non-rhizosphere than rhizosphere soil. Meanwhile, critical metabolites varied by rhizosphere in co-occurrence networks. Moreover, the same HMs-tolerant bacteria were regulated by different metabolites, e.g. unclassified family AKYG1722 was promoted by Dodecanoic acid in non-rhizosphere soil, while promoted by Octadecane, 2-methyl- in rhizosphere soil. The study illustrated that high HMs level and rhizosphere affected soil properties and metabolites, by which soil microbial community structure was reshaped.