Availability of heavy metals to corn plants and the leaching of chemical species in acid soil continuously amended with biosolids

Author(s)

Ana Rosa Martins dos Anjos

• Document type: Doctoral Thesis
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 1999-11-12
• Advisor: Maria Emilia Mattiazzo

Abstract

In this paper data are presented from a pot experiment using biosolid- amended soil for evaluation: 1) the leaching of K+, Ca2+, Mg2+, heavy metals and nitrates: 2) the accumulation capacity of soils for heavy metals and 3) biovailability of these heavy metals to corn plants. Pots used in the experiment had a 0,5 m3 capacity and were filled with soils; Rhodic Hapludox (clayey) and Typic Hapludox (coarse loamy). The statistic design adopeted was a randomized block design with four replications and four treatments. Biosolids were applied to soils every two every two months in amounts corresponding to 78 Mg ha-1 (dry weight basis), during a 12 months period totalizing 5 additions for a total of 388 Mg ha-1. Control treatments consisted of soil only. Soil sampling was made before biosolid application. Leachate was collected, sample and analyzed. Rainfall during this period was measured. Heavy metal availability to corn plants was evaluated after then final addition of biosolids. Correlations between level of metal present in plant parts and available level in soil estimated using various extractants: 0.1 mol L-1 HCl; Mehlich 3; DTPA 5 mmol L-1 pH = 7.3 were made. Total metal level in soil was analyzed by acqua regia digestion in a microwave. During plant growth, measurements of plant high and stem diameter were made weekly. Leaf sampling was made when plants were at 50% flowering. At harvesting the plants parts were separated and analyzed. The results for flowers, leaves, stem, sheath, grain corn cob and straw were presented separately. From the results, we concluded that the rate and frequency of biosolid application (78 Mg ha-1 caused an increase in soil electric conductivity, but the values were not close to those for saline soil limits. There was also an increase in nitrate, K+, Mg2+ and Ca2+ leaching. Presence of heavy metals was not observed in the leachate. The possibility of N-NO3 leaching to groundwater makes the biosolids application to soil dependent of the biosolids content of N. Dry matter production was higher in treatments with biosolids, but this increase did not correspond to an increase in corn productivity compared to the control that was fertilized with a mineral formula. Differences between treatments with biosolids as a function of soil type was also observed. Probably this was due to the high salt concentration in the Typic Hapludox soil. Concentration of Cd, Cr, Ni, and Pb in plant parts were below detection, but Cu and Zn addition by biosolids increased the level in plant parts with the exception of grain and and cob. Mehlich 3 was the most effective extractant for the determination of Cu and Zn availability to corn plants. Althought no Cd, Cr, Cu, Ni, Pb and Zn leaching was observed nor the transfer of Cd, Cr, Ni and Pb to the food chain, care must be taken about changes in soil properties that could cause movement of those metals. (AU)