Measurement of the Soil Organic Carbon Pools Isolated Using Bio-Physical-Chemical Fractionation Methods

The studies were performed with samples from different horizons of soddy podzolic soils (Albic Retisols) and typical chernozem (Haplic Chernozems) collected under natural lands and arable fields. The carbon contents in structural (particulate organic matter of 2-0.05 mm in size (C-POM) and mineral-associated organic matter of <0.05 mm in size (C-MAOM)) and process (potentially mineralizable organic matter (C-0) and microbial biomass (C-mic)) pools were determined. In the humus horizon of virgin and arable Albic Retisols, the C-POM, C-MAOM, C-0, and C-mic pools contained 38 and 24, 56 and 72, 5.9 and 5.6, and 1.2 and 1.3% of Corg, respectively. The sizes of these pools in virgin and arable Haplic Chernozems amounted to 42 and 30, 53 and 68, 3.6 and 2.8, and 0.5 and 0.5% of Corg, respectively. Despite a low weight of the POM fraction, the emission potential of C-POM pool is comparable to that of the C-MAOM pool having the large MAOM fraction. A method for quantitative separation of soil organic matter (SOM) into active, intermediate (slow), and passive pools is proposed. The size of the SOM active pool is assessed according to the C-mic and C-0 contents; the size of the passive pool, according to the content of chemically non-oxidizable organic matter in C-POM and C-MAOM fractions. The intermediate pool size is estimated according to the difference between the total organic carbon and the sum of the active and passive pools. The active, intermediate, and passive pools of the studied soils contained 1-7, 51-81, and 13-48% of C-org, respectively, without any significant differences between different land uses.