13C-dating, the first method to calculate the relative age of molecular substance homologues in soil

This article reports the design of C-13-dating, the first method to calculate the relative age of molecular substance homologues occurring in fractions from the same soil sample. Soil is a major carbon pool impacting modern climate by CO2 release and uptake. Molecular substances that sequester carbon in soils are poorly known due to the absence of methods to study molecular-level C dynamics over agricultural time scales, e. g., 0-200 years. Here, I design a method to calculate the relative age of the plant-derived C-31 n-alkane occurring in 6 fractions from a soil sample naturally C-13-labelled by maize cropping during 23 years. Soil fractions are the bulk soil extract, two humin-encapsulated fractions and three particle-size fractions. Results show that C-31 n-alkane homologues have relative ages ranging from -6.7 years for the humin-encapsulated homologue to +25.1 years for the 200-2,000-mu m fraction homologue. Such a wide variation of 31.8 years evidences temporal pools of molecular substances in soil. This finding also reveals that physical encapsulation can strikingly change the dynamics of a single molecular substance. C-13-dating thus allows to assess the carbon storage potential of molecular substances from crop soils. Such knowledge will help to identify molecular compounds, associated soil pools and agricultural practices that favour carbon sequestration. C-13-dating is further applicable to any environmental sample containing organic matter subjected to a C-13 isotope shift with time. C-13-dating will also help to study the sequestration and delayed release of chemicals in various disciplines, such as pollutants in environmental sciences, pharmaceuticals in medicine, and nutrients in food science.