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

Soil organic matter (SOM) comprises refractory compounds, to which a turnover 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 compounds of organic carbon in arable soils by means of 13C NMR spectroscopy and analysis of carbohydrates. C-depleted soils that are expected to be enriched in refractory compounds are compared with fertilized soils from long-term agroecosystem experiments. In the C-depleted soils, lower proportions of O/N-alkyl C and higher proportions of aromatic and carboxyl C compared with the fertilized counterparts are observed. Ratios of alkyl to O-alkyl C are 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 aromatic C. Yields of wet chemically determined carbohydrates positively correlate with the relative intensities of O-alkyl C in the NMR spectra, and confirm the net decrease of O-alkyl C compounds along with C-depletion. The refractory organic carbon pool in arable soils appears to have a lower contribution of O/N-alkyl C, and a higher proportion of recalcitrant aromatic structures compared with more labile fractions of organic carbon.