The dataset details the quantification of three different pools of organic matter (labile, recalcitrant, and refractory) within 1211 down core soil samples collect from across UK saltmarsh sites between 2018 - 2021. Additionally, the dataset presents a measure of organic matter reactivity using the carbon reactivity index (CRI). 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/10.5285/9a62bb8a-dd88-42d2-9af2-25ec338110e6
