13C natural abundance for analysis of steps of organic carbon transformation in soil: application for various ecosystems

Following the developed concept of carbon (C) flows during soil organic matter (SOM) formation, from which the probable C pathways between the aggregates and SOM fractions can be suggested based on the natural changes of the ¹³C/¹²C ratios, we have prepared the review based on 42 publications. The data were collected from the existing databases using the following keywords: “soil organic matter fractions and ¹³C”, “density fractionation and ¹³C”, and “soil aggregates and ¹³C”; publications contained the data from forest, shrubland, grassland, and cropland ecosystems that were located in the Temperate, Mediterranean, subtropical and tropical climatic zones were chosen; only the top 20 cm were considered. Besides the δ¹³C data, the main soil properties, including pH, total C and nitrogen contents, texture, and the dominant type of soil minerals, were collected. All data for the isotopic composition of aggregates (>2000, 250-2000, 52-250, and <53 µm) and density fractions (<1.4, 1.4-1.6, 1.8-2.0, and >2.2 g cm^-3) were normalized to the δ¹³C values of bulk soils. The preliminary analyses have shown that the isotopic composition of density fractions separated from the soils allocated in temperate and Mediterranean climates followed the previously established order, namely was getting heavier with the increase of particle densities. In contrast, density fractions separated from the soils of subtropical and tropical zones did not show prominent trends, or isotopic composition showed the enrichment in ¹²C with increased particle density. The isotopic composition of fractions separated from forest soils was also found with more minor variations compared to cropland and grassland. The data related to the probability of C flow between the density fractions and aggregates during SOM formation were also calculated and will be presented, as well as the concept explaining the effect of land use and climatic variables on the changes of the isotopic composition of density fractions and aggregates.