THE EFFECT OF SEWAGE-SLUDGE APPLICATION O N THE ACCUMULATION OF MERCURY IN PLANTS

The effect of sewage sludge application on the accumulation of Hg in p lants was investigated in model pot experiments. Two sewage sludges Pr aha and Pisek containing 5.64 +/- 0.51 and 3.89 +/- 0.25 mg.kg(-1) of dry matter, respectively, were applied (Tab. I). Three soils and four crops were tested in the experiment (Tab. II and III). Direct and subs equent effects of sewage sludge application were investigated. Sludge was applied as a fresh material following common agriculture practice. Hg was determined by atomic absorption spectrometer AMA-254. The high est yield effect of sludge application was found at the least fertile soil - fluvisols (Fig. 1 and 2). The sludge applied at the rate 3.33 g dry matter per kg of soil has not almost effected the Hg concentratio n in plants in the first year of planting. The highest Hg content was found in barley straw and in poppy seeds, the lowest in barley grain. Hg concentration increased by 55% on average of all growing crops in t he subsequent planting period when compared to the first one. Similar effect was found at treated and untreated soil, that is why we can ass ume drying of the soil and rewetting next spring made an additional re lease of Hg probably from roots of the first crop or from organic boun ds of Hg in soils. The second part of the experiment was focused on th e determination of available Hg pool by three extractants (2M HNO3, 0. 1M CaCl2, and 0.01M CaCl2) in soil samples taken when crops were harve sted (Tab. V and VI). Results showed almost negligible effect of sewag e sludge application on soil Hg accumulation. Three treatments were ap plied in 1995 and five in 1996 with significant decrease of Hg in soil extracts at sludge treated soils compared to zero treatments. Signifi cant increase of Hg soil concentration was only found at one treatment each year. Fig. 3 and 4 showed the relative amount of extractable Hg by different extractants compared to total Hg content in. soil. The 2M HNO3 released 29 to 66% of total Hg depended on soil and planting per iod. The highest amount was extracted at fluvisols (49% in 1995, and 6 6% in 1996). That soil has the lowest content of organic matter. Suchd ol soil has the highest Hg content and high sorption capacity correspo nding with higher content of humic substances. The 2M HNO3 released 29 % of total Hg in 1995, and 47% in 1996. Both 0.1M CaCl2 and 0.01M CaCl 2 released low amount of Hg at all soils with the maximum from 7 to 12 % at fluvisols again. The effect of growing crop on the extractability of Hg is shown in Fig. 5, 6 and 7. Regardless of soil the highest ext ractable amount of Hg was released at maize treatments and the lowest at poppy treatments. The mentioned results support the idea of importa nt effect of organic compound on the Hg sorption and the effect of gro wing crop on the Hg mobilisation. The relationship among soil Hg relea sed by individual extractants and plant Hg content was described by li near regression (Tab. VII). The results showed weak relationship among almost all tested treatments and a necessity to test extractants rele asing organically bound Hg.