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Seeing soil carbon: use of thermal analysis in the characterization of soil C reservoirs of differing stability

¹ School of Civil Engineering and Geosciences, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK
² SerCon Ltd., Unit 1A Wistaston Road Business Centre, Wistaston Road, Crewe, Cheshire CW2 7RP, UK

^* E-mail: david.manning{at}ncl.ac.uk

To understand the rates of turnover of soil carbon, and hence interactions between soil carbon pools and atmospheric CO₂ levels, it is essential to be able to quantify and characterize soil organic matter and mineral hosts for C. Thermal analysis is uniquely suited to this task, as different C compounds decompose during a heating cycle at different temperatures. In ‘air’ (80% He or N₂, 20% O₂), relatively labile cellulosic material decomposes between 300 and 350°C and more refractory lignin and related materials decompose between 400 and 650°C. Calcite and other common soil carbonate minerals decompose at 750–900°C. Using thermal analysis connected to a quadrupole mass spectrometer and to an isotope ratio mass spectrometer, it is possible to simultaneously determine mass loss during combustion, evolved gas molecular compositions, and carbon isotope ratios for evolved CO₂. As an example of the potential of the technique, the evolution of a fungally-degraded wheat straw shows initial isotopic heterogeneity consistent with its plant origins (–23.8 v-PDB for cellulosic material; –26.1 v-PDB for ligninic material), which homogenizes at heavier ¹³C values (–21.0 v-PDB) as lignin is preferentially degraded by fungal growth. Simultaneously, it is shown that the evolution of nitrogen compounds is initially dominated by decomposition of aliphatic N within the cellulosic component, but that with increasing fungal degradation it is the ligninic component that contributes N to evolved gases, derived presumably from pyrrolic and related N groups produced during soil degradation through condensation reactions. Overall, the use of thermal analysis coupled to quadrupole and stable isotope mass spectrometry appears to have considerable potential for the characterization of discrete carbon pools that are amenable to the modelling of carbon turnover within soil systems.

KEYWORDS: soil, organic matter, carbon turnover, thermal analysis, carbon sequestration, carbon isotopes