Natural 13C abundance of the organic matter pools in haplic chernozem

Abstract

Natural ¹³C abundance of different organic matter (OM) pools of Haplic Chernozem (Streleskaya steppe plot of the Central Chernozemic State Biospheric Reserve) isolated by granulo-densitometric fractionation was studied. Natural variations of the ¹³C/¹²C ratio (δ¹³C) allowed to confirm the sequence of OM decomposition in soils, which follows a continuum from fresh and partially decomposed organic materials in the discrete OM (Free and Occluded) to significantly/completely processed in the mineral-associated OM (Clay and Res). This is reflected in the increase of the δ¹³C value during passing from discrete to mineral–associated OM. All studied OM pools are subjected to microbial processing, but in different extent. Free OM, despite the fact that it consists of well-recognized plant material, is not always the most fresht and most undecomposed organic material. The «lighter» isotope signature of occluded OM as compared to free OM, despite its obvious higher degree of microbial processing, may be a consequence of its physical protection in microaggregates (unstable under the sonication) within the soil structural units. The most microbial processed mineral-associated organic material (Clay and Residue fractions) is characterized by the «heaviest» isotopic signature. The isotopic composition of carbon in the whole soil is mainly determined by the Residue fraction due to its quantitatively domination in the bulk soil.

The conceptual scheme allows to quantify the carbon (C) fluxes of in the OM pool / soil system. It was revealed that the main C fluxes in the OM pools go from the Free OM to the Residue fraction through unstable under the sonication microaggregates. The most probable C flows into the whole soil — unstable under the sonication microaggregates, as well as the Residue fraction, which are characterized by the lowest Δ¹³C values (difference between Δ¹³C of «sourse» and «product»).