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

Abstract

International audienceThis article reports the design of 13C-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 C31 n-alkane occurring in 6 fractions from a soil sample naturally 13C-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 C31 n-alkane homologues have relative ages ranging from - 6.7 years for the humin-encapsulated homologue to + 25.1 years for the 200-2000 μm 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. 13C-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. 13C-dating is further applicable to any environmental sample containing organic matter subjected to a 13C isotope shift with time. 13C-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

Extracting the most from terrestrial plant-derived n-alkyl lipids and their carbon isotopes from the sedimentary record: A review

“…These conflicting results highlight the challenges of field-based studies due to possible reworking and multiple sources of n-alkanes within soils (e.g. Lichtfouse, 2011). Upon heating and increasing maturity, n-alkane δ 13 C values appear to not be altered up to the pre-oil generation window.…”

Pesticide resurrection

“…In addition, analytical methods should be developed to identify and quantify bound residues (Loeffler et al 2020). For instance, molecular level 13 C-tracing has allowed to evidence the occurrence of temporal pools of the same organic compound in various fractions of the same soil sample (Lichtfouse et al 1998;Lichtfouse 1999Lichtfouse , 2012. Moreover, cleavage of sedimentary matter with Na 18 OH has shown that DDT metabolites are bound to organic macromolecules (Kalathoor et al 2015).…”

Single Sample Molecular Chronology

“…In 2012, by using the first-order law set up previously to calculate the turnover of individual soil compounds (eq 2), 28 I determined the relative age of individual n-alkanes occurring in six temporal pools of the same soil sample (eq 3): free homologues from the bulk extract, bound homologues from humin extraction, bound homologues from humin pyrolysis, and free homologues in the three particle-size fractions. 4 Results showed relative ages ranging from −6.7 years for the oldest homologues in humin to +25.1 years for the youngest homologues in the 200−2000 μm particle size fraction, compared to the homologues in the bulk soil extract (Figure 3). I named the corresponding method " 13 C-dating", 4…”

“…4 Results showed relative ages ranging from −6.7 years for the oldest homologues in humin to +25.1 years for the youngest homologues in the 200−2000 μm particle size fraction, compared to the homologues in the bulk soil extract (Figure 3). I named the corresponding method " 13 C-dating", 4…”

“…Since most biological and environmental media are most probably made of complex mixtures of various compounds of various ages, such temporal pools should occur widely, yet this field is actually poorly explored. This possible 4,28 The plot shows the percentage of maize-derived carbon in the C 31 n-alkane homologue. The C 31 n-alkane occurring in the bulk soil extract, of δ 13 C of −28.1‰, contains 50.33% maize-derived carbon; it is taken as the time reference and therefore has a relative age of 0.…”