Bulk elemental (carbon and nitrogen) and stable isotope (delta 13C and delta 15N) data produced from 491 samples collected between 2016-2021 from terrestrial (soil, peat, living biomass, dead biomass), intertidal (saltmarsh vegetation, saltmarsh roots, seagrass biomass, mudflat, faecal matter) and marine (macroalgae, microalgae zooplankton, finfish aquaculture waste) environments across the UK. These samples alongside analytical standard derived from natural materials (lignin, humic acid, cellulose, glucose, protein) were analysed to determine their bulk elemental (organic carbon and nitrogen) and stable isotope (delta 13C org and delta 15N) composition. These values are envisioned to be used to constrain organic carbon sources (terrestrial vs marine) in the natural environment when used alongside isotope mixing models. The work was carried out under the NERC programme - Carbon Storage in Intertidal Environment (C-SIDE), NERC grant reference NE/R010846/1. Full details about this dataset can be found at https://doi.org/10.5285/a445a7a8-528d-4e0b-9094-28cbcd449367