Organic carbon stocks in weathered bedrock—Establishing the soil parent material as a new horizon in soil carbon research

Abstract Soil organic carbon is essential for sustaining soil health and functioning and for achieving the United Nations Sustainable Development Goals. However, soil carbon research to date has focused predominantly on the surface horizons of soil, with fewer studies accounting for the carbon stored down the whole soil profile. Rarer still are assessments of the organic carbon (OC) stored in soil parent materials, in spite of their pervasive presence beneath almost all soil profiles. As a result, our understanding about the storage, functioning, and long‐term stability of OC within soil parent materials is sparse. This exacerbates the challenge of maximizing soil carbon sequestration and achieving Net Zero Carbon targets that require, in the first instance, a holistic account of all carbon pools and sequestration processes, including those that take place across the transition between soils and their parent materials. This study aims to establish the foundation of an evidence base by comparing the stocks of OC measured down agricultural soil profiles with the stocks in their associated parent materials. Sampling of soil and saprolite (i.e., weathered rock) was conducted across three lithologies in the United Kingdom, and thermogravimetric analysis was used to determine both the total OC content and its relative stability (i.e., labile or recalcitrant fractions). OC stocks within saprolite were statistically similar to those measured in overlying soil profiles. This was especially the case for shallow soils where the saprolite is within the effective rooting depth of crops. The ratios of labile to recalcitrant carbon indicated that the OC within saprolite was predominantly persistent but labile when saprolite was close to the land surface. These findings underscore the importance of including soil parent materials in soil carbon studies and opens up new opportunities to design strategies that enhance soil carbon sequestration within the soil parent material zone.