Recycling organic residual by-products (ORB) in agriculture is a common practice. Indeed, it provides agronomic benefits as providing nutrients for crops. However, it may also induce environmental issues. Among these environmental issues are the high potentially toxic heavy metal concentrations that can be observed in ORB. Agricultural recycling of ORB is also a complex undertaking in which many choices must be made. For example, what type of ORB to use? Which quantity? Is a treatment of ORB necessary? What treatment (composting, anaerobic digestion…)? Etc… These choices should be based on a comprehensive assessment of potential benefits (methane potential, nutrient availability, carbon storage) and negative effects due to the presence of contaminants. So far, those choices are made mainly according to agronomical reasons and the negatives effects are only taken into account according to the regulation. While it is now well accepted that the fate of heavy metals in the environment is affected by their speciation, the regulation on heavy metals for agricultural recycling of ORB is solely based on total concentration in the ORB and in the soil. This is mainly due to the lack of understanding regarding the heavy metal speciation in ORB, the impact of treatments on heavy metal speciation, and the scarcity of knowledge on the fate of heavy metal following ORB soil application. These questions are the key scientific challenges that the study aims to meet. The methodological approach used in this study permitted to assess Zn speciation in key points of the agricultural recycling system: in ORB that have been treated or not, and within the soils that had received ORB applications. We used a combination of state-of-the-art physical techniques: X-ray powder diffraction (XRD), scanning electron microscopy with X-ray microanalysis (SEM-EDS), and X-ray absorption spectroscopy (XAS). We performed short-term lab experiment and long-term field experiment. The main results of this s