Аннотация:In a two-year trial the applicability of energy crops for removal of trace hazardous elements from contaminated soil was examined. The experiment was conducted on degraded brown soil in a temperate warm and dry locality (long-term average annual temperature is 7.6 degrees C, long-term annual sum of wet precipitation is 514 mm). In sum four energy plant species were selected as promising crops for hyper-accumulation of hazardous trace elements: 1) Reynoutria sachalinensis Nakai, 2) Malva meluca Graebn., 3) Rumex tianshanicus x Rumex patientia (Uteusch), 4) Sorghum bicolor L. x Sorghum sudanence (hyso). Contamination of soil was modelled by application of sewage sludge from a sewage purification plant to soil in huge total dry matter doses of 110 and 220 t/ha. The trial consisted of five treatment variants: I) the control variant without sludge application; 2) var. I with application of 110 t/ha dry matter of sludge; 3) var. II with application of 220 t/ha dry matter of sludge; 4) var. IIA 220 t/ha sludge in combination with acidification of soil to pH = 3.5 by repeated application of 0.2% solution of phosphoric acid; 5) var. IIB 220 t/ha sludge in combination with chelate agent treatment by repeated application of 0.02% EDTA solution in sum 1000 l/ha for mobilization of hazardous elements in soil. Every variant was repeated four times. The trial sludge had 28.4% dry matter which contains in sum N 2.63%, P 0.829%, K 0.355%, Ca 1.12%, Mg 0.093% and 56.3% of loss of ignition (LOI). High doses of sludge increased the content of plant nutrients and organic matter in soil (Tab. I) and thereby created good conditions for crop production. The total amounts of hazardous elements in sludge and soil before and after application of 110 and 220 t/ha sludge are shown in Tab. II. The sludge application many times increased the content of eleven observed hazardous elements in soil. In accordance with soil content of nutrients the annual average yield of dry matter increased from control up to var. II and decreased in var. IIA and IIB (Tab. III). The maximum yield was achieved by Reynoutria sachalinensis. The difference of nutrient content in each crop between the individual variants of soil treatment is not significant. In contrast with this fact the differences of hazardous element content in observed plants between different soil treatment variants are statistically significant (Tab. IV). Figs 1 to 4 show the ratio increase of hazardous element concentration in individual crops as relative expression in comparison with control variants (100%). The highest concentrations of hazardous elements in aboveground phytomass were determined with Sorghum hyso and Rumex Uteusch.
The results of transfer factor calculation (ratio of plant element concentration to soil element concentration) of each hazardous element are shown in Tab. V. The highest transfer factor was achieved only by Sorghum hyso (Cd, Pb, Ni, Zn, Hg, Mn) and Rumex Uteusch (Cr, Co, As, Cu, Fe). In many cases the transfer factor value in control variant is higher than in sludge treatment variant. Additional acidification or chelate application (var. IIA and IIB) mostly increased both the element content and the value of transfer factor. The results of decontamination factor calculation (ratio of hazardous element amount in crop yield per ha to element amount in 20 cm surface layer of soil per ha, %) are shown in Tab. VI. The ratio increase of taking factor for each element and crop after various treatments is shown in Figs 5 to 8 as relative expression in comparison with control variant (control = 100%). The highest decontamination occurred in Reynoutria sachalinensis (Pb, Cr, Co, As, Hg, Mn). Sorghum hyso specifically removes Cd and Zn, Malva meluca Ni and Rumex Uteusch Cu and Fe. Acidification of soil and using of EDTA significantly increased transfer of hazardous elements from soil to aboveground parts of plants, but at the same time reduced yields of surface phytomass, and thus the total decontamination effect. It was ascertained in the course of two years experiments that decontamination effect of tested energy plants is 14 to 18 times higher than the average effect of traditional agricultural crops. Irrespective of this fact the use of energy crops only for phytoremediation of contaminated soils has not a good perspective because it takes many years to efficiently remove hazardous elements from contaminated soil to crop production. Based on our results, we recommend to use contaminated soil for planting of energy plants, so gradual decontamination would be an added value to phytofuel production.