Refractory organic carbon in C-depleted arable soils, as studied by C-13 NMR spectroscopy and carbohydrate analysis

Soil organic matter (SOM) comprises refractory compounds, to which a turnov er time of more than 1000 years has been attributed in SOM models. The goal of this study is to characterize the chemical structure of refractory comp ounds of organic carbon in arable soils by means of C-13 NMR spectroscopy a nd analysis of carbohydrates. C-depleted soils that are expected to be enri ched in refractory compounds are compared with fertilized soils from long-t erm agroecosystem experiments. In the C-depleted soils, lower proportions o f O/N-alkyl C and higher proportions of aromatic and carboxyl C compared wi th the fertilized counterparts are observed. Ratios of alkyl to O-alkyl C a re higher in the depleted soils than in the fertilized ones. Along with the overall C-depletion, the absolute amount of all carbon species was reduced . This net decrease is highest for the O/N-alkyl C and smallest for the aro matic C. Yields of wet chemically determined carbohydrates positively corre late with the relative intensities of O-alkyl C in the NMR spectra, and con firm the net decrease of O-alkyl C compounds along with C-depletion. The re fractory organic carbon pool in arable soils appears to have a lower contri bution of O/N-alkyl C, and a higher proportion of recalcitrant aromatic str uctures compared with more labile fractions of organic carbon. (C) 2000 Els evier Science Ltd. All rights reserved.