Origin of the enriched labile fraction (ELF) in Russian Chernozems with different site history

It was assumed that protected soil organic matter (SOM) inside small macroaggregates is indicated by the enriched labile fraction (ELF) (2.07–2.22 g cm−3 of aggregated fine silt). The ELF should be of fungal origin and sensitively reflect management effects on SOM. To test these hypotheses, we have isolated ELF from mollic epipedons (0–10 cm) of Chernozems under native steppe, meadow, forest, and long-term arable land. The ELF was characterized by C, N, amino-sugar and 13C-CP/MAS-NMR measurements. The results showed that the glucosamine-to-muramic acid ratio was higher in ELF than in other aggregate-density fractions. This indicated that microbial residues in the ELF, stronger than in other fractions, are of fungal origin. The remaining SOM composition as identified by 13C-CP/MAS-NMR spectroscopy was not different from that of the other density fractions. The ELF was therefore enriched in fungal debris but it did not solely consist of microbial N. Nevertheless, ELF seemed to reflect land-use effects on SOM. The C and N loss from ELF due to arable land use or afforestation resulted in a relative preservation of its bacterial-derived muramic acid relative to glucosamine, however, due to the lack of independent replicates such effects could not be statistically evaluated.