In:
SOIL, Copernicus GmbH, Vol. 5, No. 2 ( 2019-12-19), p. 383-398
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.
Type of Medium:
Online Resource
ISSN:
2199-398X
DOI:
10.5194/soil-5-383-2019
DOI:
10.5194/soil-5-383-2019-supplement
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2019
detail.hit.zdb_id:
2834892-8
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