Carbon isotope fractionation in various forms of biogenic organic matter: I. Partitioning of carbon isotopes between the main polymers of higher plant biomass

Abstract

Partitioning of carbon isotopes between main polymers of biomass of higher plants was investigated by the example of the structural polymers of wheat plant: lignin, hemicellulose, and cellulose representing the ligno-carbohydrate complex, starch and proteins representing storage compounds, and the lipid fraction. Biopolymers account from 80% (grasses) to 95% (trees) of the biomass of higher plants and are of geochemical interest as biological precursors for the terrigenous organic matter (OM) of sediments and sedimentary rocks. The biomass of algae is also dominated (∼80%) by polymers: proteins, carbohydrates, and lipids. The isotopic heterogeneity of the organs and parts of plants is controlled by carbon isotope composition (from −33.3 to −25.9‰) in biopolymers and their distribution: among various parts of plants. The carbohydrates: starch and hemicellulose are isotopically heaviest (−25.9 and −26.2‰), proteins are slightly poorer in 13C (up to −27.3‰), and lipids (−33.3‰) and lignin (−32.6‰) are isotopically light components of the biomass. The regularity of carbon isotopes partitioning among the large complexes of the biomass of higher plants is reflected in the existence of a common linear trend of δ13C values of biopolymers versus the ranges of thermodynamic β-factor values calculated by the method of isotopic bond numbers for the whole set of monomers in the composition of each polymer studied. The carbohydrates of grain and straw (starch, xylan, and cellulose) form a common C6–C5 pool of the isotopically heaviest polymers of the higher plant biomass (wheat). No significant isotopic effects were observed at the transformation between C6–C5 monomers and their transport between plant organs during grain ripening.