δ ¹³C of plant-derived n-alkanes in soil particle-size fractions

Soil organic carbon can be labelled at natural abundance by the cultivation of C₄ plants, e.g. maize, on soils which have been previously cropped with isotopically distinct C₃ plants, e.g. wheat. Particle-size fractions from soils cultivated for 23 years either with maize or wheat have been analysed for C and N contents, and for δ ¹³C values of bulk C and of plant-derived n-alkanes (C₂₉, C₃₁). The amount of maize-derived organic components has been calculated by isotope balance. After 23 years of maize cropping, δ ¹³C values of n-alkanes from the same soil sample increase with particle size. The results have two implications. First, the input of leaf wax n-alkanes into the soil is occurring preferentially via large particles. Second, in a same soil sample, the C₃₁ n-alkane from the 200-2000 μm particle-size fraction is younger than the C₃₁ n-alkane from the 50-200 and 0-50 μm fractions. In this respect, ¹³C analysis represents a method of relative dating of individual compounds.