Unexpected 13C-enrichment of organic components from wheat crop soils: evidence for the in situ origin of soil organic matter

Various organic constituents extracted from wheat and from soil organic matter have been analysed for their carbon content, their absolute concentration and their stable carbon isotope ratios. Most organic subfractions from plants, or soil, are 13C-depleted by up to 9.4‰ relative to bulk organic matter, mainly as a result of their higher lipid content. Furthermore, soil organic constituents are unexpectedly 13C-enriched by +1.5‰ to +4.3‰ relative to homologous plant constituents. Indeed, the selective preservation of plant lignin and lipids, following incorporation into the soil biomass, should have led to the accumulation of 13C-depleted compounds. Hence, these results favour the in situ formation of soil organic matter either by recondensation of small molecules or by selective preservation of biopolymers from soil microorganisms.