Do SOC Fractions Isolated by Density Fractionation Represent Chemically Different SOC Pools?.

Do SOC fractions isolated by density fractionation represent chemically different SOC pools?

The historical goal of soil organic carbon (SOC) fractionation procedures has been to isolate SOC fractions on the basis of their practical utilization and parse SOC into ‘functional pools’ corresponding to conceptually-defined differential kinetics of SOC (i.e. ‘active’, ‘slow’, and ‘passive’ pools). The turnover rates of SOC pools are known to be modified by temperature, moisture, degree of physical protection, soil texture, and mineralogy. However, models of SOC turnover rely on the assumption that chemical structure represents the principal variable controlling the kinetics of SOC functional pools. The objective of this study was to evaluate differences in the chemical structure of SOC fractions isolated via sequential density fractionation using near edge X-ray absorption fine structure (NEXAFS) spectroscopy for soils at two depth intervals from Bolivia, Ecuador, Cambodia, and the Philippines. Soils from each country were fractionated via sequential density fractionation methods to obtain fractions of <1.8 g/ml, 1.8-2.0 g/ml, and >2.0 g/ml. Total C and N content of the fractions and whole soils was determined by dry combustion. NEXAFS determination of C speciation for fractions and whole soils was conducted at energies between 255 and 310 eV, with spectral images collected every 0.5 eV between 255 and 275 eV; every 0.1 eV between 275 and 296 eV; and every 0.5 eV between 296 and 310 eV.