16 Feb 2022
16 Feb 2022
Status: this preprint is currently under review for the journal SOIL.

Thermal signature and quantification of charcoal in soil by differential scanning calorimetry and BPCA markers

Brieuc Hardy1,2, Nils Borchard3,4, and Jens Leifeld5,6 Brieuc Hardy et al.
  • 1Sustainability, Systems & Prospectives Department – Soil, Water and Integrated Crop Production —Walloon Agricultural Research Center, Gembloux, Belgium
  • 2Earth and Life Institute – Environmental Sciences, Université Catholique de Louvain, Croix du Sud 2, 1348 Louvain-la-Neuve, Belgium
  • 3German Agricultural Society e.V., Eschborner Landstraße 122, 60489 Frankfurt, Germany
  • 4Humboldt University Berlin, Faculty of Life Sciences, Invalidenstrasse 42, 10115 Berlin, Germany
  • 5Agroscope, Climate and Agriculture Group, Reckenholzsstrasse 191, 8046 Zurich, Switzerland
  • 6Environmental Geoscience, Universität Basel, Basel, Switzerland

Abstract. Black carbon (BC) plays an important role in terrestrial carbon storage and can improve sustainably soil fertility. Nevertheless, the accurate quantification of BC remains a critical issue to fully unravel the functions and dynamics of BC in soil. In this work, we explored the potential of differential scanning calorimetry (DSC) to identify, characterize and quantify charcoal in the soil of pre-industrial charcoal kiln sites (CKS) from a diversity of forest and cropland soils in Belgium and Germany. Pre-industrial charcoals and uncharred soil organic matter (SOM) demonstrated a distinct thermal signature that allowed their discrimination, with charcoal being more thermally stable than SOM. The DSC pattern of charcoals is characterized by one to three specific exotherms varying in size and position depending on soil conditions. From our data, we assume 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 onset the combustion. Despite the specific thermal features of charcoal, its decomposition spans over 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 exotherms 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 acids (BPCA) pattern, 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 remind us 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 rapidly and at low cost quantitatively interpretable information all over the continuum of soil organic matter.

Brieuc Hardy et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on soil-2021-146', Anonymous Referee #1, 24 Feb 2022
    • AC1: 'Reply on RC1', Brieuc Hardy, 16 Mar 2022
  • RC2: 'Comment on soil-2021-146', Alexander Bonhage, 06 Mar 2022
    • AC2: 'Reply on RC2', Brieuc Hardy, 16 Mar 2022
      • RC4: 'Reply on AC2', Alexander Bonhage, 24 Mar 2022
        • AC4: 'Reply on RC4', Brieuc Hardy, 24 Mar 2022
  • RC3: 'Comment on soil-2021-146', Anonymous Referee #3, 15 Mar 2022
    • AC3: 'Reply on RC3', Brieuc Hardy, 23 Mar 2022

Brieuc Hardy et al.

Brieuc Hardy et al.


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Short summary
Soil amendment with artificial black carbon (BC, biomass transformed by incomplete combustion) has the potential to mitigate climate change. Nevertheless, the accurate quantification of BC in soil remains a critical issue. Here, we successfully used dynamic thermal analysis (DTA) to quantify centennial BC in soil. We demonstrate that DTA is largely under-exploited despite providing rapidly and at low cost quantitative information all over the range of soil organic matter.