CHARACTERISATION OF SOIL CARBON USING THERMAL ANALYSIS

Soil is the Earth's second largest reservoir for carbon, containing twice as much C as the atmosphere. C fluxes between terrestrial systems and the atmosphere greatly exceed those between the ocean and the atmosphere (120 Pg C/yr cf. 90 Pg C/year¹), and soil-derived C contributes significantly to ocean sediments. In this context, it is essential for models of C fluxes within the Earth system that soil carbon is well understood, and that its pools of differing turnover and stability can be characterised.

In addition to organic matter derived ultimately from plants (SOM; 1500 Pg C), soils contain 720 Pg C as pedogenic carbonate minerals². These C hosts are dynamic constituents of soils, responding to environmental change such as changes of vegetation. We have developed a novel approach to the characterisation of soil carbon, using thermal analysis coupled to isotope ratio mass spectrometry and quadrupole mass spectrometry, which allows gravimetric determination of organic and inorganic C with simultaneous determination of evolved gas species and carbon isotope ratios.

On heating, cellulose decomposes generally between 300-350°C and lignin between 400-550°C, whilst carbonates decompose at higher temperatures. Weight losses observed for natural samples within these temperature intervals relate directly to the proportions of the major constituents. Thus thermal analysis 'sees' all the carbon within a soil sample.

Using this system, we can distinguish more labile cellulosic and more recalcitrant lignin-related pools within SOM, relating their abundance to changes or differences in land use. We are able to track changes in the proportions of these pools in time series experiments. From simultaneous determination of evolved gas compositions, we observe differences in the behaviour of nitrogen, recognising association between N and carbon pools of differing stability that then relate to N turnover. From C isotope data, we can track changes due to changes in input (eg C4 following C3 vegetation), or due to reaction within the SOM pool (eg fungal degradation of straw³); we can also distinguish soil carbonates from SOM, where these occur as intimate intergrowths that cannot be separated physically.

¹ Smith P 2004 Soil Use & Management 20 212-218 ² Batjes NH 1996 European J Soil Sci 47 151-16 ³ Lopez-Capel et al 2005 JAAP (in press)