Abstract
The aim of this work was to quantify low molecular weight organic acids in the rhizosphere of plants grown in a sewage sludge-treated media, and to assess the correlation between the release of the acids and the concentrations of trace-elements in the shoots of the plants. The species utilized in the experiment were cultivated in sand and sewage sludge-treated sand. The acetic, citric, lactic, and oxalic acids, were identified and quantified by high performance liquid chromatography in samples collected from a hydroponics system. Averages obtained from each treatment, concentration of trace elements in shoots and concentration of organic acids in the rhizosphere, were compared by Tukey test, at 5% of probability. Linear correlation analysis was applied to verify an association between the concentrations of organic acids and of trace elements. The average composition of organic acids for all plants was: 43.2, 31.1, 20.4 and 5.3% for acetic, citric, lactic, and oxalic acids, respectively. All organic acids evaluated, except for the citric acid, showed a close statistical agreement with the concentrations of Cd, Cu, Ni, and Zn found in the shoots. There is a positive relationship between organic acids present in the rhizosphere and trace element phytoavailability.
Highlights
The presence of trace elements in sewage sludge is one of the limitations imposed on its addition to soils
This regulation presents the same limits imposed by the United States (Estados Unidos, 1993), which were calculated from risk evaluations (National Research Council, 1983) based on possible routes that trace elements could follow after they had been added to soils
Dried biomass produced by the majority of the analyzed plants in the control treatments was similar to the sewage sludge treatments due to the constant nutrient supply in all treatments
Summary
The presence of trace elements in sewage sludge is one of the limitations imposed on its addition to soils. The agricultural use of biologically-treated sewage sludge regulation in São Paulo State, Brazil (Cetesb, 1999), limits the maximum concentration of trace elements, as well as the annual and accumulated load rates allowed to be added to soils. This regulation presents the same limits imposed by the United States (Estados Unidos, 1993), which were calculated from risk evaluations (National Research Council, 1983) based on possible routes that trace elements could follow after they had been added to soils. Total concentrations of trace elements may not be a valid argument to limit sewage sludge loads to agricultural fields. Less than 1% of the total amount of trace elements present in biosolids is absorbed by plants (Chang et al, 1997)
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