Identification of thermal signature and quantification of charcoal in soil using differential scanning calorimetry and benzene polycarboxylic acid (BPCA) markers
Author:
Hardy Brieuc, Borchard Nils, Leifeld JensORCID
Abstract
Abstract. Black carbon (BC) plays an important role in terrestrial
carbon storage and can sustainably improve soil fertility. However, the
accurate quantification of BC remains critical to fully unravelling the
functions and dynamics of BC in soil. In this study, we explored the
potential of differential scanning calorimetry (DSC) to identify,
characterize and quantify charcoal in the soil of pre-industrial charcoal
kiln sites from various forest and cropland areas in Belgium and Germany.
Pre-industrial charcoals and uncharred soil organic matter (SOM)
demonstrated a distinct thermal signature that could be used to distinguish
between them, with charcoal being more thermally stable than SOM. The DSC
pattern of charcoals was characterized by one to three specific exothermic peaks,
varying in size and position depending on soil conditions. Our data suggest
that the thermal moieties within charcoal depend on the strength of chemical
bonds of C atoms (increasing with the degree of aromatic condensation and
decreasing with weathering) and on the activation energy required to
initiate combustion. Despite the specific thermal features of charcoal, its
decomposition spans a wide range of temperatures that overlaps with the
thermal signature of uncharred SOM. This stresses the challenge of BC
quantification in soil and hinders the use of cut-off temperatures to
accurately quantify charcoal in soil. Therefore, charcoal-C content was
estimated from the relative height of exothermic peaks, attributed either to the
combustion of charcoal or SOM. For a selection of 45 soil samples,
charcoal-C content estimated by DSC was compared to benzene polycarboxylic
acid (BPCA) abundance, a widely used method to quantify BC in soil. The two
methods correlated strongly (R2=0.97), with BPCA C
representing about one-fifth of DSC-derived charcoal C. This reinforces the
view that operationally defined BC content has an absolute quantitative
value only if the recovery rate is controlled, which is very complicated for
many case studies. Overall, our results demonstrate that dynamic thermal
analysis is largely under-exploited despite providing quantitatively
interpretable information across the continuum of SOM.
Funder
Direction Générale Opérationnelle Agriculture, Ressources Naturelles et Environnement du Service Public de Wallonie Fonds Spéciaux de Recherche
Publisher
Copernicus GmbH
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