Thermal separation coupled with elemental and isotopic analysis: A method for soil carbon characterisation

Abstract

A new analytical approach (Thermally Based Separation, TBS) for the characterisation of inorganic (IC) and organic (OC) carbon pools in alluvial soils has been tested. The method does not involve any chemical pre-treatment of samples and relies on different thermal stabilities of IC and OC pools, which can be accurately determined with an EA-IRMS analytical system. Elemental (C, g/kg) and isotopic (δ13C, ‰) carbon data were used to check the reliability of the TBS method by calculating the mass balances using the measured IC and OC fractions and total carbon (TC). TBS was applied to 7 soil samples from the Padanian alluvial plain (NE Italy) that were collected from different depths and characterised by textural/geochemical heterogeneity. The method allowed for a mean carbon elemental recovery of 99% (SD = 2%), and mean isotopic deviation (Δ13C) between theoretical (δ13COCXOC + δ13CICXIC)/(XOC + XIC) and measured δ13CTC of 0.2‰ (SD = 0.3‰). A comparison of TBS with other conventional methods for carbon separation provided insights into the possible effects of sample acidification on the soil organic pools. The results suggest a higher robustness of TBS compared to conventional methods in the determination of organic and inorganic carbon pools in soils. This is because TBS bypasses any possible fractionation derived from the hydrolysis of soil organic matter and therefore accurately determines the carbon isotopic composition of the OC and IC fractions. TBS appears to be more robust than conventional methods and is independent from the IC/OC ratio of the samples, from the unpredictable complexity of soil organic matter and from the arbitrary application of the acidification routine. On this basis, TBS represents a very promising approach for a correct and complete characterisation of soil carbon pools.