Multidimensional Characterization of Soil Carbon Pools Using Stable Isotope and Quadrupole Mass Spectrometry Coupled to Thermal Analysis.
David A. C. Manning, Elisa Lopez-Capel, and Maggie White. School of Civil Engineering and Geosciences, Univ of Newcastle, Newcastle upon Tyne, United Kingdom
Our understanding of the turnover of soil carbon and how soil carbon pools interact with the atmosphere (hence influence atmospheric CO2) is fundamental to soil management and its consequences for environmental change. Modeling of soil carbon depends on reliable and meaningful characterization and quantification of both soil organic matter and carbonate/oxalate minerals within soil. Thermal analysis is ideal for this task, as different C hosts decompose thermally at different temperatures in a heating cycle. Relatively labile aliphatic-dominant material (e.g. cellulose) decomposes in "air" (80% He or N2, 20% O2) between 300 and 350°C, whereas material containing an increasing proportion of aromatic C (e.g. lignin) decomposes between 400 and 650°C (Lopez-Capel et al. 2005a). ‘Black' carbon can be investigated using this technique. Charcoals, produced strictly by combustion, show no thermal activity (apart from loss of, for example, absorbed water) until the heating experiment has reached the temperature at which they formed. Similarly, coals and soots can be distinguished using their different thermal decomposition characteristics. In addition, thermal analysis readily distinguishes carbonate minerals (calcite decomposes at 700-800°C) and oxalate minerals (multiple step decomposition; 150 - 600°C). Importantly, this technique involves no chemical separation, and is unique in its ability to 'see' all of the carbon within a single sample. We have extended the technique by coupling a thermal analysis system to (a) an isotope ratio mass spectrometer and (b) a quadrupole mass spectrometer. This provides the capability of simultaneous determination for discrete bulk components of composite soil organic matter samples of: (1) mass loss, (2) C isotope ratio and (3) evolved gas molecular composition. We are now able to distinguish relative contributions made to discrete SOM pools of carbon from, for example, C3 and C4 plant inputs (Lopez Capel et al., 2005b). To illustrate this, in an experiment using dung from cattle fed on separate C3 and C4 diets, increased contributions of dung-derived C (δ13Cv-PDB = -25.7‰ for C3 dung and -15.4‰ for C4 dung) to the refractory pool could be quantified using the thermal analysis system. Evolved gas analysis also showed that nitrogen is principally associated with more refractory components in this system, implying a slower turnover than for C. Similarly, the fungal degradation of wheat straw (Lopez-Capel et al., in press) shows initial isotopic heterogeneity consistent with its plant origins (δ13Cv-PDB = -23.8 ‰ for cellulosic material; -26.1 ‰ for ligninic material), and becomes homogenous with heavier δ13C values (-21.0 ‰) as lignin is preferentially degraded by fungal growth. In this case, the behaviour of N 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. Thus N shows contrasting behavior as fungal degradation proceeds. 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 modeling of carbon turnover within soil systems. It has the particular advantage that chemical procedures are not needed in sample preparation, allowing internally consistent data to be collected for different organic matter bulk types and intimately intergrown carbonate/oxalate minerals. References: (i) Lopez-Capel, E., Abbott, G. D. Thomas, K. M. and Manning, D. A. C. Fungal degradation of lignocellulose in wheat straw using simultaneous thermal analysis mass spectrometry and stable carbon isotopes. Journal of Analytical and Applied Pyrolysis, in press. (ii) Lopez-Capel, E., Bol, R. and Manning, D. A. C. Application of simultaneous thermal analysis mass spectrometry and stable carbon isotope analysis in a carbon sequestration study. Rapid Communications in Mass Spectrometry, 19, 3192-8, 2005b. (iii) Lopez-Capel, E. Sohi, S., Gaunt, J. L. and Manning, D. A. C. Use of thermogravimetry-differential scanning calorimetry to characterize modelable soil organic matter fractions. Soil Science Society of America Journal, 69, 136-140, 2005a; correction: SSSAJ, 69, 930.