pH-DependentBioavailability Speciation and Phytotoxicityof Tungsten (W) in Soil Affect Growth and Molybdoenzyme Activity ofNodulated Soybeans

摘要

Increasing use of tungsten (W)-based products opened new pathways for W into environmental systems. Due to its chemical alikeness with molybdenum (Mo), W is expected to behave similarly to its “twin element”, Mo; however, our knowledge of the behavior of W in the plant–soil environment remains inadequate. The aim of this study was to investigate plant growth as well as W and nutrient uptake depending on soil chemical properties such as soil pH and texture. Soybean (Glycine max cv. Primus) was grown on two acidic soils differing in soil texture that were either kept at their natural soil pH (pH of 4.5–5) or limed (pH of ≥7) and amended with increasing concentrations of metallic W (control and 500 and 5000 mg kg^–1). In addition, the activity of molybdoenzymes involved in N assimilation (nitrate reductase) and symbiotic N2 fixation (nitrogenase) was also investigated. Our results showed that the risk of W entering the food web was significantly greater in high-pH soils due to increased solubility of mainly monomericW. The effect of soil texture on W solubility and phytoavailabilitywas less pronounced compared to soil pH. Particularly at intermediateW additions (W 500 mg kg^–1), symbiotic nitrogenfixation was able to compensate for reduced leaf nitrate reductaseactivity. When W soil solution concentrations became too toxic (W5000 mg kg^–1), nodulation was more strongly inhibitedthan nitrogenase activity in the few nodules formed, suggesting amore-efficient detoxification and compartmentalization mechanism innodules than in soybean leaves. The increasing presence of polymericW species observed in low-pH soils spiked with high W concentrationsresulted in decreased W uptake. Simultaneously, polymeric W specieshad an overall negative effect on nutrient assimilation and plantgrowth, suggesting a greater phytotoxicity of W polymers. Our studydemonstrates the importance of accounting for soil pH in risk assessmentstudies of W in the plant–soil environment, something thathas been completely neglected in the past.