Identifying and quantifying geogenic organic carbon in soils – the case of graphite

Abstract. A widely overlooked source of carbon (C) in the soil environment is organic carbon (OC) of geogenic origin, e.g. graphite, occurring mostly in metamorphic rocks. Appropriate methods are not available to quantity graphite and to differentiate it from other organic and inorganic C sources in soils. This methodological shortcoming also complicates studies on OC in soils formed on graphite-containing bedrock, because of the unknown contribution of a very different soil OC source. In this study, we examined Fourier-transform infrared (FTIR) spectroscopy, Thermogravimetric analysis (TGA) and the smart combustion methods for their ability of identifying and quantifying graphitic C in soils. For this purpose, several artificial soil samples with graphite, CaCO3 and plant litter as usual C components were created. A graphitic standard was mixed with pure quartz and a natural soil for calibration and validation of the methods over a graphitic C range of 0.1 to 4 %. Furthermore, rock and soil material from both a graphite bearing schist and a schist without natural graphite were used for method validation. FTIR: As specific signal intensities of distinct graphite absorption bands were missing, calibration could only be performed on general effects of graphite contents on the energy transmitted through the samples. The use of samples from different mineral origin yielded significant matrix effects and hampered the prediction of geogenic graphite contents in soils. TGA: Thermogravimetric analysis, based on changes in mass loss due to differences in thermal stabilities, are suggested as a useful method for graphite identification, although (calcium) carbonate and graphitic C have a similar thermal stability. However, the quantitative estimation of the graphite contents was challenging as dehydroxylation (mass loss) of a wide range of soil minerals occur in a similar temperature range. Smart combustion: The method is based on measuring the release of C during a combustion program, quantified by a non-dispersive infrared detector (NDIR) being part of a commercial elemental analyser, whereby carbonates and graphitic C could be separated by switching between oxic and anoxic conditions during thermal decomposition. Samples were heated to 400 °C under oxygen rich conditions, after which further heating was done under anoxic conditions till 900 °C. The residual oxidizable carbon (ROC), hypothesized to be graphitic C, was measured by switching back to oxygenic conditions at 900 °C. Test samples showed promising results for quantifying graphitic C in soils. For the purpose of quantifying graphitic C content in soil samples, smart combustion was the most promising method of those who have been examined in this study. However, caution should be taken with carbonate rich soils as increasing amounts of carbonate resulted in an underestimation of graphitic C content.