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

Soil organic carbon can be labelled at natural abundance by the cultivation of C-4 plants, e.g, maize, on soils which have been previously cropped with isotopically distinct C-3 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 delta C-13 values of bulk C and of plant-derived n-alkanes (C-29, C-31) The amount of maize-derived organic components has been calculated by isotope balance. After 23 years of maize cropping, delta C-13 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 C-31 n-alkane from the 200-2000 mum particle-size fraction is younger than the C-31 n-alkane from the 50-200 and 0-50 mum fractions. In this respect,C-13 analysis represents a method of relative dating of individual compounds. (C) 2001 Elsevier Science Ltd. All rights reserved.