A detailed study of the near-surface composition and structure of Nb, the current state-of-the-art material for superconducting radio-frequency accelerator (SRF) cavities, is of extreme importance in order to understand the consequences of different treatments applied during their production as well as its effects in conditions resembling real operation. Recently, different thermal treatments involving the presence of nitrogen achieved remarkable performance increases of such devices, however, the physical and chemical phenomena involved are still elusive. In this work, single-crystalline Nb(100) and large-grain Nb surfaces were studied in and ex-situ during notable treatments such as the Nitrogen Infusion and Nitrogen Doping procedures as well as during treatments in UHV and in hydrogen atmospheres. A wide variety of surface sensitive techniques such as Grazing-Incidence X-ray Diffraction (GIXRD), X-ray Reflectivity (XRR), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffuse Scattering, Scanning Electron Microscopy (SEM), Energy-dispersive X-ray Spectroscopy (EDX) and Atomic Force Microscopy (AFM) were employed in order to retrieve detailed information on the formation of new phases, consumption of natural surface oxides and on the behavior of subsurface interstitial species such as oxygen and nitrogen. The Nitrogen Infusion treatment first promotes the partial reduction of the native Nb(100) oxides, Nb2O5, NbO2 and NbO, into solely NbO. Upon cooling to 120 °C, no evidence of nitrogen-rich layers was observed after nitrogen exposure times up to 48 hours. An oxygen enrichment below the oxide/Nb interface and posterior diffusion towards the Nb matrix was detected upon annealing up to 250 °C in ultra-high-vacuum (UHV). Nitrogen introduction to the the system at 250 °C promotes neither N diffusion into the Nb matrix nor the formation of new subsurface layers. Upon further heating to 500 °C in nitrogen atmosphere, a new subsurface NbxNy layer was detected. The Nitrogen Doping treatment promotes the growth of crystalline NbO and β-Nb2N, while several minor phases such as NbON, NbN and NbNxOy are also observed. Mild temperature (120-150 °C) annealing in hydrogen atmosphere was observed to introduce hydrogen into Nb, causing the precipitation of niobium hydrides upon cooling to cryogenic temperatures, allied with a segregation of interstitial oxygen atoms towards the surface. The formation of such structures can be mitigated by annealing cycles as low as 125 °C in UHV for 2 hours. The insights provided by this work can potentially be implemented in real cavity treatments in order to facilitate the current employed procedures.