Phytoextraction of Cd and Zn from agricultural soils by Salix ssp. and intercropping of Salix caprea and Arabidopsis halleri

Contamination of agricultural topsoils with Cd above guideline values is of concern in many countries throughout the world. Extraction of metals from contaminated soils using high-biomass, metal-accumulating Salix sp. has been proposed as a low-cost, gentle remediation strategy, but reasonable phytoextraction rates remain to be demonstrated. In an outdoor pot experiment we assessed the phytoextraction potential for Cd and Zn of four willow species (Salix caprea, S. fragilis, S. × smithiana, S. × dasyclados) and intercropping of S. caprea with the hyperaccumulator Arabidopsis halleri on three moderately contaminated, agricultural soils. Large concentrations of Cd (250 mg kg⁻¹) and Zn (3,300 mg kg⁻¹) were determined in leaves of Salix × smithiana grown on a soil containing 13.4 mg kg⁻¹ Cd and 955 mg kg⁻¹ Zn, resulting in bioaccumulation factors of 27 (Cd) and 3 (Zn). Total removal of up to 20% Cd and 5% Zn after three vegetation periods were shown for Salix × smithiana closely followed by S. caprea, S. fragilis and S. × dasyclados. While total Cd concentrations in soils were reduced by up to 20%, 1 M NH₄NO₃-extractable metal concentrations did not significantly decrease within 3 years. Intercropping of S. caprea and A. halleri partly increased total removal of Zn, but did not enhance total Cd extraction compared to single plantings of S. caprea after two vegetation periods.     

Chelate-assisted phytoextraction using canola (Brassica napus L.) in outdoors pot and lysimeter experiments

Phytoextraction is an emerging technology for non-destructive remediation of heavy metal-polluted soils. This study was conducted to test chelate-assisted phytoextraction of Cu, Pb and Zn using EDTA and canola (Brassica napus L. cv. Petranova) on a moderately polluted industrial soil (loamy sand) in the sub-continental climate of Eastern Austria. The effects of the rate (up to 2.1 g kg^–1 soil) and mode (single versus split) of EDTA application on the biomass, water contents and metal concentrations in shoots and roots were investigated along with changes of metal lability in soil and leaching from the root zone in parallel outdoors pot and lysimeter experiments. Labile (1 M NH₄NO₃-extractable) metal concentrations in soil increased considerably upon application of EDTA, indicating enhanced phytoavailability. However, this was also associated with enormously increased metal concentrations in the leachates collected below the root zone. Enhanced metal labilities and leachate concentrations persisted for more than 1 year after harvest. Metal lability was more enhanced by EDTA in rhizosphere relative to bulk soil, indicating interactions of EDTA with root activities. Shoot biomass and water contents of canola were virtually unaffected by EDTA, revealing that canola can tolerate excessive metal concentrations in soil pore water. Metal concentrations in shoots were increased considerably, but were insufficient to obtain reasonable extraction rates. Split applications were generally more effective than the same amounts of EDTA added at once. Metal concentrations in roots decreased after each application of EDTA, possibly indicating metal removal from roots by free protonated EDTA, but increased again within several days. As the application of chelate-assisted phytoextraction is limited by the risk of groundwater pollution, further work should focus on natural, continuous phytoextraction technologies.     

Plant and fertiliser effects on rhizodegradation of crude oil in two soils with different nutrient status

Soils and sediments polluted with crude oil are of major environmental concern on various contaminated sites. Outdoors pot experiments were conducted to test the phytodegradation potential of common reed (Phragmites australis) and poplar (Populus nigra × maximowiczii) in fertilised and non-fertilised control treatments. Two topsoils (E, G) of different texture were mixed with crude oil. Soil analysis included hydrocarbon (HC) measurements, detection of labile phosphorus and mineralised nitrogen as well as dehydrogenase activity. Increased HC degradation by native soil biota was clearly related to higher P availability in soil G and to fertilisation in soil E. Except of the non-fertilised common reed treatment, plants did not enhance crude oil degradation. We found even inhibited degradation of high molecular weight HC in the presence of plants together with declining labile phosphorous concentrations due to planting on soil E. Native soil biota were able to use the whole range of crude oil compounds (C₁₀ to C₆₀) as a carbon source in the presence of sufficient nutrient concentrations in soil. This study is the first to show that reduced HC degradation in the higher molecular weight crude oil fraction (C₂₀ to C₄₀) is likely to be a consequence of decreased phosphorus availability for microorganisms in the plant rhizosphere.