The rapid expansion of Canada goldenrod (Solidago canadensis L.) in China has drawn considerable attention as it may not only decrease vegetation diversity but also alter soil nutrient cycling in the affected ecosystems. Soil extracellular enzymes mediate nutrient cycling by catalyzing the organic matter decomposition; however, the mechanisms by which alien plant invasion may affect soil extracellular enzymes remain unclear. The objective of this study was to investigate the responses of soil extracellular enzyme activities and ecoenzymatic stoichiometry to S. canadensis invasion. Several extracellular enzymatic activities related to carbon, nitrogen, and phosphorus cycling were measured using a fluorometric method. Ecoenzymatic stoichiometry was used as a proxy of soil microbial metabolic limitations. S. canadensis invasion appeared to be associated with decreased activities of enzymes and with substantial conversions of microbial metabolic carbon and nitrogen limitations. The changes in the activities of extracellular enzymes and the limitations of microbial metabolism were correlated with the alterations in the nutrient availability and resource stoichiometry in the soil. These findings reveal that the alterations in soil available nutrients associated with S. canadensis invasion may regulate extracellular enzymatic activities and cause microbial metabolic limitations, suggesting that S. canadensis invasion considerably affects biogeochemical cycling processes.

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In this study, the natural zeolite and rice husk biochar were mixed as a combination amendment for metal immobilization in a Cd, Pb, As, and W co-contaminated soil. A 90 day incubation study was conducted to investigate the effects of amendments on toxic metal in soil. Zeolite, biochar, and their combination application increased the soil pH and cation exchange capacity. A combination of amendments decreased the bioavailability of Cd, Pb, As, and W. Besides, the potential drawback of biochar application on As and W release was overcome by the combination agent. Zeolite, biochar, and combination treatment decreased total bioavailability toxicity from 335.5 to 182.9, 250.5, and 143.4, respectively, which means that combination was an optimum amendment for soil remediation. The results of the Community Bureau of Reference sequential extraction and scanning electron microscopy–energy-dispersive spectrometry images confirmed the Cd and Pb adsorption onto biochar. However, As and W immobilization was dominantly controlled by zeolite. It appears that the combination of amendments is an efficient amendment to remediate Cd, Pb, As, and W co-contamination in soil, although the combination of amendments has a lower stabilization rate for W than for zeolite.

Soil with heavy metals’ contamination has caused worldwide concern, and there is an increasing interest in the application of washing agents for the remediation of soils with heavy metals’ contamination. The review summarizes the recent findings about soil washing with different washing agents. For soil washing technologies, the solubilizing capability, toxicity, and biocompatibility of agents are essential concerns. Washing agents can enhance heavy metals’ desorption and removal from soil. Inorganic acids/bases/salts, synthetic surfactants, and synthetic chelators are often limited due to their adverse effects on soil. Biosurfactants, HAs (humic acids), and LMWOA (low-molecular-weight organic acids) are suggested washing agents, but the limitation of their low production needs to be conquered. Moreover, both washing with a mixture agent and sequential washing have often been adopted to improve the overall capacity of the washing agent for decontamination. Mixture washing can obtain the synergetic effect for soil washing and increase washing efficiency. Sequential washing can apply an agent with a high heavy metals removal rate. However, this may cause environmental risks in the early stage, and then remove the washing agent injected in the early stage by the secondary washing stage. Overall, the already known cases reveal the good prospect of soil washing for soil remediation.

The potential ecological risk index (RI), proposed by the Swedish geochemist Hakanson, has been widely used for quantitatively evaluating the pollution degree caused by various pollutants. As an emerging contaminant, tungsten (W) poses ecological risks to the environment, and the quantitative assessment of the risk is of extraordinary significance. However, the lack of a determined W toxicity coefficient has limited the use of the RI in evaluating the W pollution degree. In this study, the toxicity coefficient of W (= 2.00) was calculated based on Hakanson’s theory, then verified via a case study conducted by 23 sediment samples in Taojiang River near a W mining area in southern Jiangxi. The risk factor (Ew) and geoaccumulation index (Igeo) of W, and RI and Nemerow comprehensive pollution index (NCPI) of eight heavy metals at each sampling site were calculated and compared, respectively. The results showed consistent correlations and trends of pollution levels for the investigated sites, which means the rationality of assigning a toxicity coefficient of 2.00 for W. These results can contribute to the use of the RI method for the scientific evaluation of W pollution levels.