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

Abstract

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 quantify 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 method for their ability to identify and quantify graphitic C in soils. For this purpose, several artificial soil samples with graphite, CaCO3 and plant litter as the 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 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 origins 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, is 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 occurs 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) as 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 which 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.

Peat Soil Burning in the Mezzano Lowland (Po Plain, Italy): Triggering Mechanisms and Environmental Consequences

“…We used the Elementar SoliTOC Cube elemental analyzer, in compliance with the DIN 19539 standard, to carry out the temperature‐dependent differentiation of total carbon (TC) in the investigated soil samples. This method is also known as “smart combustion” (Zethof et al., 2019 ). Powdered samples were loaded in stainless steel crucibles and placed in an 80‐position autosampler.…”

“…This area is renowned for peat burning since historical times (Cremonini et al, 2008;Martinelli et al, 2015), and is characterized by peat deposits exposed to the Mediterranean climate with frequent and persistent droughts (Marchina et al, 2017;2019). To assess the effects of burning, we applied an analytical technique based on the distinctive thermal stability of the various carbon bearing-phases (Mörchen et al, 2019;Natali et al, 2020;Zethof et al, 2019Zethof et al, , 2020 as well as isotope ratio mass spectrometry (Natali et al, 2018b). With the obtained results we provide constraints on the effects of burning in the surrounding environment, evaluating the smoldering combustion effects on the local soil carbon stock.…”

Prokaryotic Community Composition and Extracellular Polymeric Substances Affect Soil Microaggregation in Carbonate Containing Semiarid Grasslands

“…Inorganic carbon was determined by measuring total carbon (TC) before and after acid treatment with excess of HCl, i.e., inorganic C = TC before -TC after , using an elemental CN analyzer (Vario EL, Elementar, Langenselbold, Germany). The same device was used to measure total N. The samples from the RH site contained graphitic C, which was measured by smart combustion method (Zethof et al, 2019) using the soli-TOC cube (Elementar, Langenselbold, Germany). Graphitic C was used to estimate the non-geogenic organic C. In summary: organic C = (TC-inorganic C) -graphitic C. Texture was determined after oxidizing the organic material with H 2 O 2 and removal of carbonates using HCl, after which samples were wet sieved <63 µm and the silt/clay fraction further quantified by a Sedimat 4-12 (UGT, Müncheberg, Germany).…”

Geogenic organic carbon in terrestrial sediments and its contribution to total soil carbon

“…There are many soil-and substrate-specific factors that might influence the OC contribution from current vegetation to sedimentary OC, such as potential rooting depth or pore distribution. No method has yet been established to allow a direct quantification of GOC in different soils or sediments, apart from promising methods to quantify the graphitic part of GOC in soils (Zethof et al, 2019). The only reliable approach for distinguishing between both sources is the use of 14 C. Since the deposition of sediments mostly took place > 50 000 years BP (before present), they do not contain 14 C, which has a mean half-life time of 5730 years (Libby, 1952).…”