The use of sewage sludge in agriculture increases the resistance of soils to erosion, the ion exchange and it is as an excellent source of nutrients, particularly nitrogen and phosphorus. However, the sludge can also cause harmful changes to the edaphic system. Our objective is to assess the distribution of metals Cd, Cu, Ni, Pb and Zn in five geochemical compartments in an Oxisol that has received sewage sludge in different doses for six sequential crops. Also, we aim to verify if there is remobilization of these metals among the compartments of the OM when it is decomposed, and in another experiment, in the same area with application of nitrogen fertilizer, to study the percolation of these metals in the soil solution up to 5 m deep. The quantities of metals in these geochemical compartments and remobilization when part of OM is degraded are related to the theory of chemical time bomb. Soil samples will be collected in an area with a history of sewage sludge application at the layers 0-10, 10-20 and 20-40 cm. The samples will be submitted to sequential extraction and fractionation of soil OM before and after incubation to stimulate the degradation of OM. In order to study the quality of the humic acid fraction several spectroscopic techniques will be applied, such as: laser-induced fluorescence spectroscopy, electron paramagnetic resonance, nuclear magnetic resonance, fourier transform infrared and fluorescence spectroscopy Ultraviolet-visible. The metal concentrations in the extracted fractions will be quantified by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). Soil solution samples will be collected at different depths, reaching up to 5 m, and they will be analyzed for: pH, Eh, electrical conductivity, temperature and turbidity, dissolved organic carbon, cations and anions.
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