Effects of Two Types of Biochars on Tungsten (W) Toxicity and Subcellular Distribution of W in Rice Plants (Oryza Sativa L.)

Background: Soils in the vicinity of tungsten mining operations frequently experience contamination with multiple metals. Current literature indicates that remediation strategies have predominantly concentrated on the bioavailability of heavy metals within these soils. However, the bioavailability of tungsten itself has not been sufficiently addressed. Biochar has been shown to contribute to the stabilization of heavy metals; however, research on the bioavailability of tungsten (W) in soil is limited. Methods: This study presents a pot experiment utilizing biochar produced from the corn straw (CB) and beef bone (BB) to investigate its impact on the growth of rice in W-contaminated soil and the bioavailability of W. Results: The results indicated that biochar application enhanced the accumulation of W in rice tissue. Furthermore, the enhancement effect of biochar derived from BB on W in rice shoots and roots was greater than that of CB. This effect can be attributed to several factors. First, the application of biochar raised the soil pH, which in turn increased the mobility of W in alkaline conditions. Consequently, the concentrations of W in the acid-soluble and reducible fractions were elevated, improving the bioavailability of tungsten. Moreover, our study demonstrated that biochar significantly reduced the soil redox potential (Eh) (P < 0.01), with a pronounced negative correlation between the Eh values and the proportion of W in the acid-soluble and reducible fractions. This finding suggests that the reduction in soil Eh facilitated the mobilization of W. The W content in rice roots was positively correlated with the acid-soluble and reducible W in the soil, indicating that biochar application increased the root concentration of tungsten. Furthermore, biochar treatment resulted in a decrease in the proportion of tungsten bound to the root cell walls and an increase in its distribution within the vacuoles and cytoplasm. This redistribution promoted the translocation of tungsten from the roots to the shoots, thereby elevating the tungsten content in the aboveground plant biomass. Conclusion: Biochar application enhanced the translocation of W from roots to shoots, resulting in an increased concentration of tungsten in the aboveground plant biomass. Notably, the biochar produced from beef bone exhibited a more pronounced effect on the accumulation of W in rice shoots and roots compared to that derived from corn straw. Thus, the application of biochar is not recommended for soils contaminated with multiple heavy metals surrounding tungsten mineral deposits, as it may potentially increase the risk of tungsten pollution in the soil. Conversely, biochars are valuable for enhancing the phytoextraction capacity of plants, offering potential strategies for the remediation of tungsten-contaminated soils.