TRANSFER OF LEAD, CADMIUM AND MERCURY IN THE SYSTEM - SOIL - PLANT - ANIMAL

At the Grassland Research Station at Zavisin the movement of selected heavy metals (Pb, Cd and Hg) was studied in the system soil - plant - animal at one site in order to find a degree to which the resulting pr oduct of agricultural production (i.e, meat predicted for the human nu trition) in the given region is loaded by potential transfer of heavy metals. The data presented originate from the years 1991 to 1993 and w ere obtained in the grazing ground Podhora at Zavisin near Marianske L azne. This is the site at an altitude of 750 m, characterized by avera ge annual temperature of 6.4 degrees C (12.4 degrees C in the growing season) and average sum of precipitation exceeding 700 mm (of it 400 m m in the growing season). The soil at the site is acid cambisol, evalu ated as medium heavy with acid reaction (pH/KCl 5.0), unsaturated sorp tive complex and low content of low-quality humus. Heavy metals conten t in atmospheric precipitation is monitored at Zavisin since 1992, nam ely within the measuring network of the Research Institute of Water Ma nagement TGM. The bulk method is used, in which sampling vessels are p ermanently exposed and samples are taken always at the end of calendar month. The total wet deposition taken covers also deposition of dust elements. In the years 1986 to 1993 grazing ground was utilized by the system of grazing in short rotation. At the pastures average stand wi th prevalence of ryegrass and smooth-stalked meadow grass. The proport ion of white clover was variable and ranged approximately about 20%. T he stand was measured and sampled under the presence of animals below the portable cages, when the stand was always harvested when reached a height as stated before (13 cm at grazing of cattle and 4 to 6 cm at grazing of sheep). The level of fertilization was falling with time, s ince 1991 no fertilization was applied. Soil and root samples were tak en each year in autumn. Heavy metals content in tissues of animals kep t in given grazing ground was also estimated. Regarding the absence of fertilization, the investigated substances may fall to the given site through the atmospheric precipitation. Lead and cadmium content (from June 1993 also mercury) in precipitation waters and an amount of thes e elements supplied by them are in Tab. I,from which follows that the supply of heavy metals by atmospheric deposition was practically negli gible at the given site and fluctuated distinctly below the literary d ata. It is necessary to pay a great attention to the loading of the so il which decisively participates in the production of foods and presen ts a potential inlet of pollutants into the food chain (Fig. 1). The c ontent of investigated elements in soil of the grazing ground Podhora remained deep below the maximum allowed limits (Tab. II) and in absolu te numbers practically corresponded to average values of agricultural soils of the Czech Republic. This site is from this aspect relatively little loaded by contaminants. As to the accumulation in plant tissues , their content is decreasing in the following order: roots - leaves - shoots underground reserve organs - fruits. The data of Tabs III and IV confirm that the content of investigated heavy metals was in roots of the grazing stand on average of three years three times higher for mercury and cadmium, and ten times higher for lead compared with the f odder. The content of three investigated heavy metals in fodder (Tab. V) did not exceed permitted limit for feeds (Ministry of Agriculture o f CR, 1993) in any case in any of experimental years. However, it foll ows from the literary data that excessive accumulation of hazardous el ements did not take place in cultured crops even at the deposit site o f power-station ash (which covered the topsoil), so that wheat can be cultivated here also for food and fodder purposes. In this connection one can remind that the cadmium content in fodder exceeded a permitted level only rarely when some time before power-station fly ash and its particles were applied to grassland. Fodder from grazing ground Podho ra can be considered as a quality one. Collection of studied elements at the given site (Tab. V) was somewhat lower than their supply by atm ospheric deposition, so in long-time standpoint their accumulation in soil should be considered. In the given case this is really a negligib le amount. Preliminary finding of the content of evaluated heavy eleme nts in animal tissues bred in the grazing ground Podhora (Tab. VI) giv es evidence that meat met hygienic requirements. Conditions for grazin g breeding of animals are here more than suitable.