This work aimed to evaluate the chemical changes that occur in the structure of organic matter in poultry litter (PL) during its composting, and the evolution in the structure of humic substances (HS) formed during this process, and thus determine the relationship between the structural changes of organic matter during composting with the bioavailability of heavy metals present in PL and with the capacity of this residue to adsorb Pb and Cu metals. In chapter 1, a preliminary study was carried out on the role of the PL organic matter structure in changes in the solubility of heavy metals present in this residue during the initial stage of composting. In the first thirty days of composting, the favoring of more stable interactions between heavy metals and the more recalcitrant organic fraction of PL reduced the solubility of these elements and, consequently, the polluting potential of PL. In chapter 2, a characterization of the changes in the organic structure of the PL and in the humic fractions formed during the entire composting process through chemical and spectroscopic techniques of structural elucidation is presented. It was found that the stabilization of organic matter during PL composting involves the constant incorporation of fulvic acids (FA) into humic acids (HA), with the preservation of nitrogenous compounds and the replacement of aliphatic and carboxylic structures by aromatic compounds in the HA , and increased functionalization and polarity of FA. The evolution of humic structures in PL during composting favored the preservation of nitrogen structures and the high presence of carbohydrates in this residue at the end of the process. Chapter 3 presents the action of the structure of organic matter and humified fractions of PL, and their transformations during the entire composting, on the bioavailability of heavy metals in the environment. The most recalcitrant chemical forms of heavy metals increased during composting due to the affinity of these elements with the oxidized structures of the HS formed during the composting process. However, the association of Pb with the C_Alk-O and C_Alk-di-O structures of the HS formed during composting contributed to the increase in its water-soluble fraction. Given the role played by the functional groups of the PL structure, and their changes promoted by the composting process, chapter 4 describes the phenomena of adsorption of Cu and Pb, by selecting models, in PL compounds with different composting times, in addition to identifying the type of bond between metals and compounds and the influence of their structure on the adsorption of these metals. Due to the heterogeneity of the PL surface structure, the linear model proved to be more adequate to describe the process of adsorption of Cu and Pb in this residue, at all times of composting. Due to the structural characteristics developed during composting, non-composted PL showed a higher affinity for Pb, while composted PL showed a higher affinity for Cu. However, regardless of the composting time, the adsorption of Cu and Pb on the PL occurred predominantly in a specific way. However, the non-composted PL presented a larger part of the Pb sorbed in a specific way, while the PL with longer composting time presented a larger part of the Cu sorbed in a specific way. In chapter 5, the transfer of heavy metals to the arugula plant (Eruca sativa) when it is fertilized with PL in different stages of composting is presented. The application of PL, regardless of the composting time, reduced the Mn contents and increased the Cu contents in the plant. However, the application of uncomposted PL and composted for only 30 days reduced the Cu bioaccumulation factor in the shoot (BCF-PA) and in the root (BCF-root), demonstrating that the application of poorly humified PL reduces the ability of plant to accumulate Cu, although an increase of this metal was observed in the plant due to the increase in bioavailable Cu in the soil. The greater affinity of Pb for structures of high lability contributed to its increase in arugula plant fertilized with PL. The increase in the water-soluble and exchangeable fractions of Zn in the soil resulting from the application of PL, regardless of the composting time, also favored the increase in Zn contents in the rocket. Thus, the increase in high lability structures in SH (C_Alk-O and C_Alk-di-O) with compost, as they act as precursors in the formation process of these SH, reflecting on the final characteristics of the compost, may be favoring non-specific adsorption of Pb, increasing the participation of this metal in the water-soluble fraction and contributing to the absorption of this metal by the arugula plant.