The Biogeochemistry of tungsten (W) in the plant-soil environment

Tungsten (W) is an economically important transition metal that finds a broad scope of applications ranging from household necessities to high-end technology goods. However, in the past decades, increasing industrial and military use of W-based products (particularly ammunition, as well as drilling, milling and cutting tools) opened new pathways of W into natural systems and raise the need for a better understanding of the behaviour of W in the environment. Soils play a particularly important role in controlling the bioavilability of pollutants and their entry into the food web via plant uptake as they serve as filter and buffer systems. However, compared to other trace metals, knowledge about the fate of W in the plant-soil environment is rather sketchy. This project therefore aims at providing crucial information on W solubility, speciation and partitioning in soils governed by important soil chemical properties like pH. Considering the chemical similarity of W and Mo, an essential micronutrient involved in the plant N cycle, we will also explore W uptake and partitioning within the plant and the effect of elevated W concentrations on plant biomass production, N assimilation, symbiontic N2 fixation and on plant metabolic reactions. By linking operationally defined chemical W pools in soil with W accumulation in the plant biomass, results of this project will provide a sound basis for assessing the potential entry of W into food web. Furthermore, the impact of plant-soil interactions (rhizosphere processes) on W solubility in the close vicinity of roots will be elucidated using 2D imaging techniques. By combining conventional techniques with novel methods and high-end analytical tools this project will deliver completely new insights into the behaviour of W in natural systems and provide valuable information on W fluxes in the plant-soil environment. Furthermore results will serve as basis for future risk assessment and management of W contamination in the environment.

Increasing use of tungsten (W) based products opened new pathways of W into environmental systems. W shares a strong chemical alikeness with molybdenum (Mo), an essential plant nutrient, and is therefore expected to interfere with Mo-dependent biochemical processes in plants like nitrogen (N) assimilation and symbiotic N2 fixation. A thorough understanding of the solubility and bioavailability of W in soils as well as W toxicity to plants is therefore crucial for environmental risk assessment. The aim of this project was to deliver detailed information on W solubility, speciation and partitioning in soils as well as on W plant toxicity and related plant stress reactions. Our results revealed a strong dependency of W solubility on soil pH, with W solubility being significantly greater in alkaline, high pH soils (pH 7-8) resulting in a greater risk of W entering the food web in these soils. Oxides and hydroxides were identified as the main mineral surfaces responsible for W sorption and therefore W retention in soils. We confirmed the formation of W polymers (i.e. large molecule composed of many repeated subunits) in acidic soils (pH 4-5) and at high W contamination. Even though W exhibited a moderate toxicity to plants, these W polymers were found to be more toxic to plants than single W molecules (monomers). Although molybdenum is essential for healthy plant growth, it can also be toxic if concentrations are too high. When comparing plant growth performance under tungsten and molybdenum contamination, W was found to be more toxic to plants than molybdenum. Molybdoenzymes are involved in N uptake (nitrate reductase) and N2 fixation (nitrogenase) by symbiotic bacteria in root nodules of soybean. Studying the activity of these molybdoenzymes in soybean grown on W contaminated soils revealed that symbiotic nitrogen fixation was able to compensate for reduced nitrate reductase activity. These results indicate a more efficient detoxification/ compartmentalization mechanism in nodules than in soybean leaves. Screening the protein and metabolite concentration in soybean tissue showed that W induced typical heavy metal stress responses and confirmed that nodulated soybeans were better able to cope with W contamination. W ammunition is used as 'green' alternative to toxic lead (Pb) in hunting shots. Using chemical imaging techniques, we observed a greater mobility of Pb compared to W during W and Pb ammunition weathering in soils. However, we also found high release of Ni derived from the W ammunition alloy which should not be neglected in environmental risk assessment. Overall, our results revealed important insights on W plant toxicity mechanisms and demonstrated the importance of considering soil pH in risk assessment studies of W in the plant-soil environment, which has been completely neglected in the past.