This dataset contains the carbon and nitrogen content of soil organic matter fractions collected along grassland-to-forest conversion chronosequences. Four chronosequences of grassland-to-forest conversion were used across Scotland, from Alyth to Craik. Soil samples were collected in summer 2018. Soil samples were collected using soil corers. In 2020, soil samples were fractionated in the laboratory, and the elemental composition of bulk soil and soil fraction samples was determined. The goal of the study was to determine the changes in the quantity of soil organic carbon and nitrogen, and the form in which these carbon and nitrogen are stored.