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Frontiers in Climate 01 frontiersin.org
Quantifying soil organic carbon
after biochar application: how to
avoid (the risk of) counting CDR
twice?
DilaniRathnayake
1,2, 3, Hans-PeterSchmidt
2,3, JensLeifeld
4,
DianeBürge
1, ThomasD.Bucheli
1 and NikolasHagemann
1,2, 3*
1 Agroscope, Environmental Analytics, Zürich, Switzerland, 2 Ithaka Institute, Arbaz, Switzerland, 3 Ithaka
Institute, Goldbach, Germany, 4 Agroscope, Climate and Agriculture Group, Zürich, Switzerland
Pyrogenic carbon capture and storage (PyCCS), which comprises the
production of biomass, its pyrolysis, and the non-oxidative use of the biochar
to create carbon sinks, has been identified as a promising negative emission
technology with co-benefits by improving soil properties. Using biochar as a
soil additive becomes increasingly common as farmers seek methods for soil
improvement and climate change adaptation. Concurrently, there is growing
interest in quantifying soil organic carbon (SOC) at the level of individual plots
to remunerate farmers for their good agricultural practices and the resulting
(temporary) carbon dioxide removal (CDR). However, methods currently applied
in routine analysis quantify SOC, irrespective of its speciation or origin, and
do not allow to distinguish biochar-C from SOC. As certification of PyCCS-
derived CDR is already established using another quantification method (i.e.,
analysis of biochar-C content, tracking and registration of its application, and
osetting of carbon expenditures caused by the PyCCS process), the analysis
of biochar-C as part of SOC may result in double counting of CDR. Hence, the
objectives of this review are (1) to compare the physicochemical properties and
the quantities of biochar and SOC fractions on a global and field/site-specific
scale, (2) to evaluate the established methods of SOC and pyrogenic carbon
(PyC) quantification with regard to their suitability in routine analysis, and (3) to
assess whether double counting of SOC and biochar C-sinks can beavoided
via analytical techniques. The methods that were found to have the potential to
distinguish between non-pyrogenic and PyC in soil are either not fit for routine
analysis or require calibration for dierent soil types, which is extremely laborious
and yet to beestablished at a commercial scale. Moreover, the omnipresence of
non-biochar PyC in soils (i.e., from forest fires or soot) that is indistinguishable
from biochar-C is an additional challenge that can hardly besolved analytically.
This review highlights the risks and limits of only result-based schemes for SOC
certification relying on soil sampling and analysis. Carbon sink registers that
unite the (spatial) data of biochar application and other forms of land-based
CDR are suggested to track biochar applications and to eectively avoid double
counting.
KEYWORDS
pyrogenic carbon capture and storage, carbon sink certification, carbon dioxide
removal, pyrogenic carbonaceous material, black carbon, monitoring, reporting,
verification
OPEN ACCESS
EDITED BY
Carlos Paulo,
SRK Consulting, Canada
REVIEWED BY
José María De La Rosa,
Spanish National Research Council (CSIC),
Spain
Abhishek Kumar,
University of California, Davis, UnitedStates
Puja Khare,
Council of Scientific and Industrial Research
(CSIR), India
Jorge Paz-Ferreiro,
RMIT University, Australia
*CORRESPONDENCE
Nikolas Hagemann
nikolas.hagemann@agroscope.admin.ch;
hagemann@ithaka-institut.org
RECEIVED 23 November 2023
ACCEPTED 18 March 2024
PUBLISHED 04 April 2024
CITATION
Rathnayake D, Schmidt H-P, Leifeld J,
Bürge D, Bucheli TD and Hagemann N (2024)
Quantifying soil organic carbon after biochar
application: how to avoid (the risk of)
counting CDR twice?
Front. Clim. 6:1343516.
doi: 10.3389/fclim.2024.1343516
COPYRIGHT
© 2024 Rathnayake, Schmidt, Leifeld, Bürge,
Bucheli and Hagemann. This is an open-
access article distributed under the terms of
the Creative Commons Attribution License
(CC BY). The use, distribution or reproduction
in other forums is permitted, provided the
original author(s) and the copyright owner(s)
are credited and that the original publication
in this journal is cited, in accordance with
accepted academic practice. No use,
distribution or reproduction is permitted
which does not comply with these terms.
TYPE Review
PUBLISHED 04 April 2024
DOI 10.3389/fclim.2024.1343516
Rathnayake et al. 10.3389/fclim.2024.1343516
Frontiers in Climate 02 frontiersin.org
1 Introduction
Rising temperatures, water scarcity, prolonged droughts, and
unexpected weather events are intensifying more than ever in recent
history. ese impacts are not surprising as the current atmospheric
greenhouse gas (GHG) concentration is higher than at any point in
the last 800,000 years. Altogether, global net anthropogenic GHG
emissions were 59 ± 6.6 Gt CO2eq in 2019, which was 12% higher than
in 2010 (Canadell et al., 2021). To accelerate climate mitigation
activities, a global framework was set to limit global warming to well
below 2°C and pursue eorts to limit it to 1.5°C (United Nations,
2015). is goal can no longer beachieved by reducing emissions
alone, and carbon dioxide removal (CDR) is necessary to transfer
carbon (C) from the atmosphere into non-atmospheric C-sinks
(Smith etal., 2020).
e European Commission recognizes the importance of
industrial negative emission technologies, such as direct air capture
with carbon storage (DACCS) and nature-based solutions for
CDR. e buildup of soil organic matter (SOM) is a crucial element
of nature-based solutions, which can beachieved through reduced
tillage, reduced drainage, cover crops, and several other methods,
including biochar (cf. denition in Table 1) application to soil
(Whitman etal., 2010; Blanco-Canqui etal., 2020; COWI, EI, and
IEEP, 2020; Don etal., 2024). In agriculture, biochar is used, among
others, as a plant nutrient carrier, compost additive, animal bedding
material, and soil conditioner to alleviate nutrient losses, stimulate
buildup of soil organic carbon (SOC), counteract soil erosion, and
improve soil water retention and long-term soil fertility under a
changing climate (Blanco-Canqui etal., 2020; Schmidt etal., 2021).
e production and non-oxidative application of biochar itself is
considered a negative emission technology, oen referred to as PyCCS
(Schmidt etal., 2021; Lefebvre etal., 2023). In 2022, a still rather
modest 100,000 t of CO
2
was removed from the atmosphere via
PyCCS in Europe, but this number is expected to increase to up to 225
Mt. of CO
2
annually by 2036 (EBI, 2023). Globally, PyCCS may
contribute to 6–35% of the negative emissions needed by 2,100
without generating undesirable side eects through land use change
(Werner etal., 2021).
A certication of negative emissions is necessary for their
nancing, which requires the respective carbon removal technique
to bequantiable, deliver additional climate benets, strive to store
carbon for a long time, and contribute to eectively removing
carbon from the atmosphere (COWI, EI, and IEEP, 2020).
Accordingly, the EU suggested four QUantication, Additionality,
Long-term storage, sustainabilITY (QU. A. L. ITY) criteria for
industrial carbon removal certication methodologies (European
Commission, 2022). Today, these carbon removals are certied by
private companies according to their own guidelines or companies
relying on independent third-party certication, e.g., the European
Biochar Certicate (EBC)‘s C-Sink Certicate (EBC, 2021). Either
way, negative emission certicates are generated and sold on the
voluntary market. A core element of a well-functioning market is
that each negative emission can becertied only once. Avoiding
double counting is essential to maintaining the integrity of the
carbon removal systems and to collecting correct data for
greenhouse gas inventories. Double counting of negative emissions
may occur when more than one entity claims the same negative
emission (Schneider etal., 2015; COWI, EI, and IEEP, 2020), e.g.,
when biochar is certied as a biochar C-sink and as part of SOC
increase, as detailed below.
For biochar, the certication process involves the quantication
of its organic carbon (Corg) content and molar H-to-Corg ratio. Once
soil application is conrmed, a xed portion of biochar-C cannot
beburned or oxidized by other means anymore (e.g., 74–93% when
molar H to Corg ratio < 0.4) and the certication process is completed
(Woolf etal., 2010; Budai etal., 2013; IPCC, 2019; EBC 2021
–
2023;
Rodrigues etal., 2023). In contrast, result-based SOC certication
schemes usually include repeated soil sampling and quantication of
SOC to assess this (temporary) C-sink (Paul etal., 2023). However, as
discussed in detail below, the quantication of SOC typically also
includes biochar-C and other pyrogenic carbon (PyC) contained in
pyrogenic carbonaceous materials (PCM, cf. Table1) such as soot or
wildre-derived char. erefore, e.g., the EBC C-Sink guidelines for
certifying biochar C-sinks require that when farmers purchase
certied biochar, they sign a contract that they are not participating
in a SOC certication program to avoid double counting (EBC, 2021).
is is a weak and hardly veriable requirement that may bedicult
to adhere to with the increasing interest in SOC certication and
expanding the use of biochar. In addition, the prevailing
inconsistencies in SOC monitoring protocols and the lack of
standardized protocols in some regions could intensify the double-
counting risk (Smith etal., 2020; Oldeld etal., 2022). erefore, the
extent to which non-pyrogenic SOC and non-biochar PyC can
bequantied in routine soil analysis in the presence of biochar should
be evaluated. Routine analysis here means that the method can
beimplemented with a high degree of automation and cost-eectively
TABLE1 The definition of biochar and related materials.
Terminology Definition
Biochar “Biochar is a porous, carbonaceous material that is
produced by pyrolysis of biomass and is applied in such a
way that the contained carbon remains stored as a long-
term C-sink or replaces fossil carbon in industrial
manufacturing. It is not made to beburnt for energy
generation” (EB C, 2012–2023).
Char “e solid pyrogenic carbonaceous material remaining as
a result of incomplete combustion processes such as those
that occur in natural and man-made re” (Bird etal.,
2015).
Charcoal Charcoal is a pyrogenic carbonaceous material that is
deliberately produced by the pyrolysis of wood, seldomly
of other biomass, that is used as a fuel, reductant, or
chemical/material (Brown etal., 2015; Hagemann etal.,
2018).
Pyrogenic carbon
(PyC) or black C
“e thermochemically altered organic carbon fraction of
pyrogenic carbonaceous material” (Bird etal., 2015).
Pyrogenic
carbonaceous
material (PCM)
PCM is an umbrella term for “all materials that were
produced by thermochemical conversion and contain
some organic C” (Lehmann and Joseph, 2015).
Soot Soot are carbonaceous particles that are unintendedly
formed by the recondensation of hydrocarbons in the gas
phase during the incomplete combustion or pyrolysis of
biogenic or fossil fuels (Lehmann and Joseph, 2015;
Michelsen etal., 2020).
Rathnayake et al. 10.3389/fclim.2024.1343516
Frontiers in Climate 03 frontiersin.org
on a large number of samples to enable a low-cost certication
process. In Europe, costs for SOC determination can beas low as
European €10 or less per sample according to the information
provided by commercial laboratories and farmers. To date, many
chemical, thermal, physical, spectroscopic, and molecular techniques
have been developed for SOC and PyC dierentiation and
quantication (Hammes etal., 2007; Zimmermann and Mitra, 2017).
Hence, this review aims to
i. Compare the physicochemical properties and the quantities of
biochar and SOC fractions on a global and eld/site-specic scale,
ii. Evaluate the established methods of SOC quantication, and
iii. Discuss existing analytical methods for PyC quantication and
their suitability in routine analysis of biochar-C content in
agricultural soils.
is review also aims to evaluate whether double counting of SOC
and biochar C-sinks can beavoided via analytical techniques and to
derive conclusions for reliable approaches for CDR certication when
biochar is applied to soil.
2 Methods
Literature research was performed on Web of Science and Google
Scholar covering the past 20 years. However, several articles published
between 1990 and 1999 were also included due to their importance
for the discussion. e following keywords were used during the
article search: biochar; pyrogenic carbon continuum; soil organic
carbon; soil organic carbon analysis; soil; biochar separation; biochar
quantication in soil; pyrogenic carbon analysis; pyrogenic
carbonaceous material; biochar carbon sink certication; carbon
dioxide removal, CDR; double counting; carbon credits; pyrogenic
carbon stocks; and uxes. Care was taken to prioritize peer-reviewed
publications and those articles that were written in English. However,
several methods-related documents published in German were also
included due to their importance in discussing analytical
developments. Studies related to the PyC analysis in atmospheric
samples were excluded. Priority was given to the recent PyC analysis
involved with biochar-C analysis. Aer the initial screening, the
articles were manually classied into dierent review sections.
3 Carbon speciation, stocks, and
fluxes in soil and biochar
Biochar contains a wide range of organic and mineral components,
whose mass fractions and speciation are determined by both the
feedstock and the pyrolysis process parameters, such as temperature
(Keiluweit etal., 2010; Singh etal., 2012; Bird etal., 2015; Rathnayake
etal., 2020). Carbon speciation includes aliphatic and (polycyclic)
aromatic hydrocarbons that form graphene-like sheets but may also
comprise some residual non-pyrogenic compounds, e.g., from lignin;
ash content can bein the range of 3–90% (Keiluweit etal., 2010;
Hardie etal., 2014; Xiao and Chen, 2017; Ippolito etal., 2020).
SOC comprises a wide range of carbonaceous moieties, including
carbon derived from readily decomposable plant debris and microbial
biomass (i.e., cellulose, sugars, proteins, and lipids), lignin, waxes,
resins, tannins, and secondary metabolites from plants, humic
substances, and PCM (Baldock and Skjemstad, 2000; Six etal., 2002;
Lehmann and Kleber, 2015; Reisser et al., 2016). Pyrogenic
carbonaceous material in soil exhibits overlapping physicochemical
properties to that of biochar and is of both natural (e.g., wildre char)
and anthropogenic (soot) origins (Knicker, 2011; Santín etal., 2017).
e continuum of PCM in soil, their formation pathways, initial
reservoirs, and some basic physicochemical characteristics are
indicated in Figure1.
Both on a global level and the level of an individual plot of land,
both non-pyrogenic and pyrogenic SOC are present and that might
interfere during biochar-C analysis. Global PyC stocks due to PCM in
soil range from 54 to 212 Gt (Supplementary Table S1). Depending on
the re intensity in the past, the PyC fraction in SOC varies globally,
but also varies for land use type and soil texture, with agricultural land
and clay soils rather showing higher PyC (Reisser etal., 2016). Apart
from biochar, PCM enters soils via direct and indirect routes such as
wild or manmade res (land clearing, burning of crop residues, and
unintended res as a result of peatland drainage) and atmospheric
depositions from incomplete combustion (soot), which is more
prominent in upper soil horizons (Sanderman etal., 2021). Tillage,
erosion, and surface runo activities induce the horizontal and
vertical movements of PCM in soils (Qi etal., 2017; Jiménez-González
etal., 2021). e annual global input ux of PyC to the terrestrial
environment via vegetation res and fossil fuel burning ranges from
40 to 383 Mt. Yr
−1
and 2 to 12 Mt. Yr
−1
, respectively. e annual global
biochar input ux is approximately 0.1 Mt. Yr
−1
(Supplementary Table S1), and PyC loss due to remineralization
ranges from 103 to 207 Mt. Yr−1 (Bird etal., 2015).
Depending on the soil types and PyC analytical methods used, the
PyC content in soils could vary from 1 to 37% of SOC (Table2). e
application of 1–10 t ha
−1
biochar to mineral soils with low-to-medium
SOC content (it is uncommon to apply biochar to organic soils or
minerals soil with high SOC) results in 2–18% of SOC being biochar-C
nominally (Figure2). According to that, the initial non-biochar PCM
content in soil (4.3–5.6 t ha−1) could behigher than a low biochar input
(0.7 t ha−1 of biochar-C for 1 t biochar) to the soil.
4 Current methods to quantify SOC in
routine analysis
4.1 Dry oxidation methods
e most widely used methods for determining SOC are based on
dry oxidation. For this purpose, the sample is heated to a dened
temperature, oxidized by pure oxygen, and the resulting CO
2
is
quantied. Inorganic carbon, i.e., carbonates, must beremoved by acid
treatment prior to measurement and deducted to accurately calculate the
C
org
(VDLUFA, 1991; DIN EN 15936). Dry oxidation-based methods are
simple, easy to automate, and have high throughput for smaller sample
amounts (i.e., milligram levels). However, in principle, this method is
similar to the method to measure biochar-C; e.g., the analytical
guidelines of the EBC dene the application of the DIN 51732, a dry
oxidation procedure for solid fuels (Bachmann etal., 2016; Bird etal.,
2017; EBC, 2012
–
2023). Given that the standard method for quantifying
SOC and biochar-C analysis are practically the same, the SOC method
will account for biochar as SOC.
Rathnayake et al. 10.3389/fclim.2024.1343516
Frontiers in Climate 04 frontiersin.org
4.2 Wet oxidation methods
Soil organic carbon can also bequantied by wet oxidation. Here,
a mixture of potassium dichromate and sulfuric acid (sulfochromic
oxidation) is added to the sample, followed by measuring the residual
oxidizing agent (Agroscope, 2020) or the newly formed Cr (III)
(ISO14235). While ISO14235 was withdrawn, sulfochromic oxidation
is still used; e.g., it is the mandatory reference method in Switzerland
(Agroscope, 2020) and the Soil Survey Standard Method in New
South Wales, Australia (Department of Sustainable Natural Resources,
New South Wales, 1990). e advantage of wet chemical approaches
is that organic carbon (including amorphous organic carbon and PyC)
is oxidized in a very targeted manner, whereas carbonates remain
unaected and are thus not detected. However, biochar or any other
non-biochar PyC present in soil can beoxidized under the conditions
applied in the Swiss reference method. us, biochar-C will beat least
partially detected as SOC (Agroscope, 2020). In addition, Hardy and
Dufey (2017) clearly showed that wet oxidation according to the
Walkley–Black method will at least partially oxidize charcoal, with a
potential impact of charcoal aging on its resistance to wet oxidation.
4.3 Analytical precision and minimal
detectable dierence (MDD)
For the Walkley and Black method of wet oxidation, the Global
Soil Laboratory Network quantied a coecient of variation (CV) of
2.7% when the sample contains 1% organic carbon (FAO, 2019).
Assuming soils with low (31.7 t ha−1) and medium (40.6 t ha−1) SOC
contents in the upper 20 cm [cf. Figure 2, soil data for Switzerland
(Leifeld etal., 2005)], the addition of 0.7 t ha
−1
biochar-C (e.g., 1 t of
biochar with 70% C content) increases SOC by 2.2 and 1.7%,
respectively. is change is lower than the CV and thus could not
beaccurately measured. For the dry combustion method used in the
elemental analyzer, Fliessbach etal. (2021) reported a 1–2% CV for a
sample containing 0.73–2.6% of C; i.e., again, the addition of 1 t ha−1
biochar would not berecognized analytically.
However, the accuracy of SOC determination is not solely
determined by analytical precision of organic carbon quantication
but also, e.g., by spatial heterogeneity as well as the variability of soil
bulk density that is needed to derive SOC stocks (t C ha−1) from Corg
(%C; Poeplau etal., 2017, 2020; Wiesmeier etal., 2020). Even under
reasonably optimized conditions (100 samples per plot of 1–2 ha) to
be applied for a sampling of scientic long-term experiments,
Schrumpf etal. (2011) quantied an MDD of 1–2.5 t ha
−1
of SOC for
cropland and grassland sites, respectively. is further conrms that
a single application of 1 t biochar (70% C
org
content) would not
bedetectable, while larger applications (e.g., 3 t ha
−1
biochar) and/or
repeated application will quickly pass this level of MDD within the
typical timeframes of 3–5 years (Wiesmeier etal., 2020) between
repeated SOC quantication to detect stock changes. However, the
MDD is likely to behigher for routine analysis when less than 100
samples are taken per plot for economic reasons.
5 Methods to quantify PyC contents in
soils, their prospects, and limitations
in the quantification of soil biochar-C
content in routine analyses
Pyrogenic carbon in soil consists of a continuum of materials
between the partly charred material and highly graphitized soot-like
structures without clear-cut boundaries (Figure1) (Schmidt etal.,
2001; Knicker, 2011). e existing methods for quantifying, isolating,
and characterizing PCM in soil attempt to dierentiate the inorganic
material, thermally unaltered organic carbon, and PyC using chemical,
FIGURE1
Continuum of pyrogenic carbonaceous materials (PCM) in soil (Hedges etal., 2000; Masiello, 2004; Wiedemeier, 2014; Bird etal., 2015; Santín etal.,
2017; Wagner etal., 2018). This figure was created with BioRender.com.
Rathnayake et al. 10.3389/fclim.2024.1343516
Frontiers in Climate 05 frontiersin.org
thermal, physical, spectroscopic, or molecular marker techniques
(Bird, 2015). Due to technical limitations and variations in treatment
severities, dierent methods isolate and characterize dierent
fractions of the PyC continuum that are unique for the applied
methodologies (Zimmermann and Mitra, 2017). Previous literature
has thoroughly discussed these analytical methods in quantifying
pyrogenic and non-pyrogenic fractions of SOC (Schmidt etal., 2001;
Hammes etal., 2007; Wiedemeier etal., 2013; Bird, 2015; Hardy etal.,
2022). Hence, this section and Table3 only briey summarize the
prospects and limitations of currently available PyC analytical
methods and evaluate their suitability for quantifying soil biochar-C
in the presence of non-biochar PyC in soil with the goal of allowing
quantication of non-biochar SOC in routine analysis.
Physical techniques used for the biochar separation from the soil
are based on its visual appearance or dierences in biochar material
size or density compared to other non-biochar soil organic and
mineral matter. When bigger biochar particles (> 2 mm) are applied
to the soil, it is easier to physically separate by hand picking or sieving
and by combining with microscopic techniques for further
identication and verication (Spokas, 2013; Paetsch etal., 2017).
However, due to the similar color (i.e., black) and depending on the
abundance of non-biochar PCM in the soil, there can bebiases to the
overall quantied biochar content by hand picking or their visual
appearance. Biochar in soil is subject to physical disintegration; i.e.,
particle size is reduced over time (Spokas etal., 2014; Sigmund etal.,
2023), which will result in an underestimation of the soil biochar
content. Alternately, biochar can beseparated by otation due to its
bulk density, which is lower than soil mineral matter. Liquids with
dierent densities, such as water (Sigmund etal., 2017) or sodium
polytungstate solution (Singh etal., 2014), can beused. However,
biochar in soil may form biochar mineral complexes whose density
might besimilar to that of bulk soil (Archanjo etal., 2017; Yang etal.,
2021). Due to the overlapping bulk density, skeletal density, envelope
density, and porosity values of biochar and other non-biochar PCM
(Santín etal., 2017), the sensitivity and precision of density separation
methods can be lower. Physical methods can beextremely time-
consuming and labor-intensive, and sample losses could occur during
sample handling. Hence, physical techniques are irrelevant to biochar
quantication in routine analysis. However, they may beuseful in
research to gain aged biochar for analysis or experiments.
e use of chemical techniques for quantifying biochar in soil
depends on the oxidative resistance of the biochar carbon material
compared to the other non-biochar SOC material. e methods
include the use of NaClO, K2Cr2O7, or UV-based oxidation methods.
However, they could not eectively oxidize hydrophobic non-biochar
SOC and may at least partially oxidize PyC (Hammes etal., 2007;
Knicker et al., 2008; Meredith etal., 2013; Murano et al., 2021).
Furthermore, the K
2
Cr
2
O
7
oxidation method exhibited good
reproducibility and recovery for the chemical oxidation-resistant
elemental carbon (COREC) content in plant char (i.e., plant material
charred at 350°C under oxic conditions) mixed with HF-treated soil.
Nevertheless, that method has not yet been validated for the biochar
produced from various sources and soils with various amounts of
non-biochar PCM.
Chemothermal oxidation method at 375°C (CTO375) followed by
elemental carbon analysis is a relatively simple, inexpensive technique
to isolate and quantify PyC in soils (Gustafsson etal., 2001; Agarwal
and Bucheli, 2011). However, pyrogenic artifacts could beformed if a
sucient amount of oxygen is lacking during the thermal treatment,
lignin may partly survive the treatment, and some of the PyC can
beoxidized entirely during the oxidation step, whereby the extent may
vary for dierent soils as well as for PCM types (Agarwal and Bucheli,
2011; Gerke, 2019; Murano etal., 2021). ermogravimetric analysis
and dierential scanning calorimetry (TGA-DSC) oer the possibility
to identify thermal signatures specic for SOC and biochar/other
non-biochar PCMs. However, biochars show dierent signatures
depending on the pyrolysis temperature, and also, soils may vary in
their background signal (Leifeld, 2007; Hardy etal., 2022). Hence,
unless the baseline soil (without biochar) exhibited distinct thermal
signatures compared to that of biochar, it is harder to quantify biochar
TABLE2 The percentage of PyC in total SOC in dierent soils as reported in previous studies using dierent analytical methods.
Soil type Method used PyC/SOC (%) References
US agricultural soils Hydrogen pyrolysis 2.7–7.7 Lavallee etal. (2019)
US agricultural soils UV oxidation and 13C NMR 10–35 Skjemstad etal. (2002)
Native grassland sites along a
climosequence in North America
BPCA 4–18 Glaser and Amelung (2003)
Iberian top soils MIR 3–20 Jiménez-González etal. (2021)
Southeastern Australian soils MIR 7–37 Wang etal. (2018)
Australian agricultural and pastoral soils CTO375 2–26 Qi etal. (2017)
Various soil types CTO375, microscopy combined with 13C
NMR analysis, UV photooxidation
combined with coulometry
4–6 Cornelissen etal. (2005)
Arable, permanent, and pasture grassland,
forest, and urban soils collected from the
Swiss Soil Monitoring Network
CTO375 1–6 Bucheli etal. (2004)
Peatlands 13C NMR combined with molecular
modeling
12.3–14.7 Leifeld etal. (2018)
NMR, nuclear magnetic resonance; BPCA, benzene polycarboxylic acid; MIR, mid-infrared spectroscopy; CTO375, chemothermal oxidation at 375°C. It is assumed that practically all of the
listed PyC is from non-biochar origin as global biochar application is still extremely low.
Rathnayake et al. 10.3389/fclim.2024.1343516
Frontiers in Climate 06 frontiersin.org
content in soil reliably. is method still needs to bevalidated for
dierent biochars, dierent application rates, and soils with various
amounts of native PCM and mineral compositions.
Hydropyrolysis (HyPy) is a thermal technique with high
reproducibility that can beused to quantify the most stable fraction of
PCM present in soil (Meredith etal., 2012). It measures the fraction
of PyC that contains >7 aromatic ring structures with an H/C molar
ratio of less than 0.5 (stable polycyclic aromatic carbon—SPAC)
(Meredith etal., 2012). Wildre chars exhibit considerably lower
SPAC content (i.e., <30–40% on a dry ash-free basis) than biochar
samples (i.e., SPAC content up to 75% on a dry ash-free weight basis
[Santín etal., 2017]). Using HyPy may result in an underestimation of
biochar’s contribution to SOC as biochar also contains non-SPAC
carbon, especially when produced at rather low temperatures, which
might becertied as a temporary C-sink (Schmidt etal., 2022). us,
HyPy still might not fully exclude double counting of CDR. In
contrast, HyPy might overestimate the contribution of biochar-C to
SOC for soil with high natural PCM content.
Koide etal. (2011) recently used an adapted loss on the ignition
(heating sample at 550°C for 4 h) method to determine the soil biochar
content. is method is simple and inexpensive, and no advanced
analytical instruments are involved, only the mue furnace and
balance with the necessary precision. However, this method has to date
only tested for extremely high biochar application rates (20–25 t ha
−1
)
and requires reference samples of both soil and biochar, which both
hinders its application in routine analysis. Nakhli et al. (2019)
signicantly improved the loss-on-ignition method by looking at two
dierent temperatures, but the need for the reference sample remains.
Benzene polycarboxylic acids (BPCA) is a molecular marker
method used in soil PyC analysis (Schmidt etal., 2001). is method
determines the condensed aromatic structure in PCM and does not
produce pyrogenic artifacts during analysis. e conventional BPCA
method is time-consuming, prone to losing parts of the sample during
ltering steps, and highly variable due to the dierent GC instrumental
conditions and calibrations used (Hammes etal., 2007; Wiedemeier
etal., 2016). Overestimation may arise from quantifying humic acid
compounds such as PyC (Chang etal., 2018; Gerke, 2019).
e 13C nuclear magnetic resonance (NMR) quanties 13C atoms
in organic compounds and can identify the chemical bonds (Smernik,
2017). Both soil and biochar contain aliphatic-C and aryl-C. Hence,
this method cannot dierentiate biochar and non-biochar SOC, e.g.,
in humic acids. Methods based on IR are promising due to their
simplicity, low cost, and potential to simultaneously determine
multiple parameters beyond SOC, e.g., inorganic carbon and total
nitrogen with one measurement (Baldock etal., 2013). However, their
widespread application requires comprehensive reference databases
that comprise spectra and calibrations for all types of soil that shall
beinvestigated. In Australia, a national model for an MIR-based SOC
determination was built based on 20,495 soil samples from 4,526
locations across the country (Baldock etal., 2013). Such comprehensive
reference databases might even allow us to distinguish biochar and
non-biochar PyC, as the site-specic reference spectra might also
cover the dierent background concentrations of PyC. However, this
needs to beproven separately. It also needs to beveried in detail to
what extent the semi-persistent carbon fraction of biochar can also
bedetected by NIRS/MIRS as part of the biochar-C pool. In essence,
IR-based methods are promising and have the potential to even reduce
costs per sample, but require a tremendous investment to build the
reference that would have to beaccomplished by public actors, like the
Commonwealth Scientic and Industrial Research Organization
in Australia.
Isotopic carbon chemistry can also be used to delineate the
biochar-C in the soil if their
13
C isotopic signatures are substantially
dierent from the soils they applied (Bird etal., 2015). However, in
reality, framers may apply biochar produced from various feedstock
materials into the soil, originating potentially from both C3 and C4
plants with contrasting isotopic signatures, which hinders the
application of isotope-based methods in routine analyses (Chalk and
Smith, 2022).
Generally, all of these methods require considerable background
knowledge about the site and biochar application history, partly high-
tech equipment, or, in the case of near or mid-infrared (NIR/MIR)
spectroscopy a site/soil-specic reference library, which is not yet
routinely available. us, in the foreseeable future, it will hardly
bepossible to assess the MDD between biochar and other non-biochar
PCM in biochar-applied soils with sucient sensitivity, selectivity, and
precision using currently available PyC analytical methods in
routine analysis.
6 C-sink registry
An analytical solution to quantify non-biochar SOC is in itself
only necessary as long as there is no conrmed or independently
veriable information as to whether, and if so at what dosage,
biochar-C has been applied to a specic piece of land. Once such
information is available, total SOC can bedetermined with state-of-
the-art routine analysis (cf. Section 3), and the already-certied
biochar-C could be simply subtracted arithmetically. Advanced
biochar-based C-sink certication methods require the registration of
FIGURE2
An exemplary illustration of the soil organic carbon (SOC) stocks and
its composition from non-pyrogenic SOC, non-biochar pyrogenic
carbon (PyC), and biochar carbon after application of 0, 1, and
10 t ha−1 biochar with 0.7 g g−1 carbon (biochar-C) content. The low
(31.7 t ha−1) and medium (40.6 t ha−1) SOC stock values were based on
the SOC concentrations in the top20 cm of Swiss arable soils
(Leifeld etal., 2005). Non-biochar PyC was assumed to be13.7% of
the SOC (Reisser etal., 2016). Percentages indicate the contribution
of biochar to total SOC (i.e., analytically determined SOC including
pyrogenic SOC).
Rathnayake et al. 10.3389/fclim.2024.1343516
Frontiers in Climate 07 frontiersin.org
biochar C-sinks in a public carbon sink registry containing all
necessary information, such as biochar-C content and molar H to C
org
ratio, the amount and date of applied biochar, and localization of the
biochar C-sink (EBC, 2021; Etter etal., 2021; Puro.earth, 2024).
However, farmers can apply biochar without C-sink certication and
without entry into a carbon registry. In that case, no double accounting
would occur when the total SOC of the soil is analyzed. However, if
biochar C-sinks are valorized for CO
2
-emission osets on the
voluntary market without registering the localized biochar application,
the risk of double accounting is high. erefore, biochar-based
C-sinks may not becertied without listing on a public C-sink registry
to make the above information available. When SOC-based C-sinks
are certied, it is necessary to query the state or regional C-sink
registry to verify how much C-sink-certied biochar was applied
TABLE3 Prospects and limitations of existing PyC analytical methods in distinguishing soil applied biochar-carbon from non-biochar PyC and from
amorphous SOC.
PyC analytical method Prospects Limitations Key references
Microscopic assessments, hand
picking, or using solutions with
dierent densities.
Simple and can beapplied when
suciently large PCMs are presented in
soils.
Time-consuming and labor-intensive (thus, this
can beexpensive). Possible overlaps between
dierent PCMs.
Singh etal. (2014) and Sigmund etal.
(2017)
Chemical oxidation using NaClO
or K2Cr2O7 and UV
photooxidation
Simple and well-established with detailed
protocols and reference methods.
Inecient removal of non-PyC substances
present in SOC (i.e., lipids and waxy
compounds) and limited selectivity between
biochar and non-biochar PyC.
Simpson and Hatcher (2004), Hammes
etal. (2007), Knicker etal. (2008),
Meredith etal. (2013), and Murano
etal. (2021)
CTO375 followed by elemental
carbon analysis.
Simple and inexpensive methodology
and easiness of controlling operating
conditions.
Pyrogenic artifacts could beformed if a
sucient amount of oxygen is lacking during
the thermal treatment. is method exhibits
high selectivity toward soot-like PyC.
Gustafsson etal. (2001), Bucheli etal.
(2004), Agarwal and Bucheli (2011),
Gerke (2019), and Murano etal. (2021)
ermogravimetry with
dierential scanning calorimetry
(TG-DSC)
Simple and inexpensive methodology. Dicult to reliably quantify amorphous and
less stable PCM (i.e., charred wood and straw)
present in soils due to overlapping signals.
Hence, there is a limited selectivity between
biochar and non-biochar PyC.
Leifeld (2007), Plante etal. (2009), and
Hardy etal. (2022)
Adapted loss on the ignition
method
Simple and inexpensive and no advanced
analytical instruments are involved (only
the mue furnace and balance with
necessary precision).
is method requires initial soil and biochar
samples and biochar application history. When
biochar promotes the formation of non-biochar
SOC over time, the biochar content might
beoverestimated due to SOC increase.
Koide etal. (2011)
Hydropyrolysis (HyPy) followed
by elemental carbon analysis
HyPy is a matrix-independent, high-
precision, and highly reproducible
technique for PyC quantication. It has
been used to calibrate and validate the
other PyC detection techniques.
Not yet widely used and established method.
Possible overlap with soot.
Ascough etal. (2009), Meredith etal.
(2012), and Cotrufo etal. (2016)
Benzene polycarboxylic acids
(BPCA)
is method does not produce pyrogenic
artifacts during analysis.
Time-consuming method, prone to losing parts
of the sample during ltering steps. In addition,
some PyC structures can bedestroyed during
the extraction resulting in underestimation of
PyC.
Schmidt etal. (2001) and Cerqueira
etal. (2015)
NMR spectroscopy 13C NMR quanties 13C atoms in organic
compounds, can identify the chemical
bond, and is used to characterize
biochars.
is method is time-consuming, expensive, and
requires specic knowledge in instrumental
handling and data evaluation and hence does
not allow application in routine analysis.
Smernik (2017)
Near and mid-infrared
spectroscopy
Rapid, economically viable, no chemicals
involved, and non-destructive.
ese methods need to bereferenced against
materials with known PyC content, calibrated,
and validated with a wide range of PyC and soil
types to beused in routine analysis with higher
precision.
Jauss etal. (2017), Jiménez-González
etal. (2021), and Pressler etal. (2022)
Isotopic methods Economically viable, no chemicals
involved, and non-destructive.
Considerable background knowledge about the
site and biochar application history is necessary,
which does not allow application in routine
analysis.
Gustafsson etal. (2001), Bird etal.
(2015), Ascough etal. (2016), Paetsch
etal. (2017), and Pulcher etal. (2022)
Rathnayake et al. 10.3389/fclim.2024.1343516
Frontiers in Climate 08 frontiersin.org
during the SOC certication period in order to subtract the already-
certied C-sink from the SOC C-sink certication.
Tracking systems are already established as a part of C-sink
registers or in energy attribute certicates, which are issued to
conrm the use of renewable energy (NREL, 2015). In addition,
registers are used for certicates from emission avoidance and
forestry; e.g., carbon credits created within the clean development
mechanism had to belisted in the international transaction log
provided by the United Nations Framework Convention on Climate
Change (Lovell, 2010). Here, avoiding double use of certicates is
the major goal, and spatial information is not included, which
would be added information to C-sink registers. Currently,
dedicated C-sink registers are oered, e.g., by C Capsule (Sheeld,
UnitedKingdom), Puro.earth (Helsinki, Finland), and the Global
Carbon Register Foundation (Arbaz, Switzerland). All providers
mentioned above oer global applicability but dier in the details
of the registered information. Each register may contain entries
from all over the world. us, a single register query is currently
insucient to obtain the necessary information. is could
beovercome either through geographical exclusivity of the registers
or through a global (e.g., International Organization for
Standardization (ISO)) standard for data structure and query
procedures of such registers so that even information that may
be distributed across dierent databases can be retrieved in a
uniform manner. Geographical exclusivity could beachieved in the
form of national registers run by federal oces or independent
non-prot, non-governmental institutions (foundations) with a
governmental mandate that would exclude the operation of any
other non-connected register organization in the country. A
description of the certication process for biochar and SOC C-sink
using a carbon sink register to avoid double counting is explained
in more detail in the Supplementary material with the aid of a
hypothetical example (Supplementary Text 1).
7 Conclusion
Biochar production and biochar application to agricultural soils
to build up SOC are two synergistic CDR methods. However, their
combination is a challenge to the monitoring, reporting, and
verication (MRV) of SOC buildup as there is a signicant risk of
double counting when biochar is applied, and SOC buildup is
remunerated based on soil sampling and quantication of organic
carbon. Based on this review, standard analytical methods for soil
carbon are t for purpose but cannot distinguish between SOC and
PyC including biochar. More advanced methods can distinguish
PyC from non-pyrogenic SOC but are complex, expensive, and not
suitable for routine analysis. Moreover, none of these PyC analytical
methods can suciently distinguish biochar-C from non-biochar
PyC present in soil. us, there is a considerable risk of double
counting of CDR when the production of biochar-C is certied and,
at the same time, SOC certication based on result-based payments
with sampling and organic carbon quantication performed on
land to which biochar was applied. Hence, this risk can eciently
beaddressed at the governance level using the following two steps:
(1) all biochar-based carbon sinks shall beregistered in a public
C-sink register, and (2) when certifying SOC increase as CDR, the
public C-sink register shall beconsulted to control that no certied
biochar was applied to the respective eld during the certication
period. If biochar was applied, the SOC increase needs to
be corrected arithmetically for the already-certied
biochar-C. Similarly, SOC measurements taken for SOC-based
CDR certication should be registered and geo-referenced to
improve their MRV and to enable proper global carbon accounting
considering the dierent types, permanence, and potential control
periods of C-sinks. We, therefore, suggest the use of C-sink registers
as a cost-eective tool to avoid double counting of CDR and
improve overall carbon accounting for climate change mitigation.
ese registers should beimplemented on regional, national, or
supranational (e.g., EU) levels and should claim exclusivity for this
respective area.
Author contributions
DR: Investigation, Writing – original dra. H-PS: Writing –
review & editing. JL: Writing – review & editing. DB: Writing – review
& editing. TB: Writing – review & editing. NH: Conceptualization,
Funding acquisition, Writing – review & editing.
Funding
e author(s) declare that nancial support was received for the
research, authorship, and/or publication of this article. is study was
supported by the Road4Schemes project under the H2020 European
Joint Programme SOIL (EJP-SOIL, 862695) project. Open access
funding by Agroscope.
Conflict of interest
H-PS and NH are members of the scientic advisory board of
Carbon Standards International AG. H-PS is co-founder of the Global
Carbon Register Foundation.
e remaining authors declare that the research was conducted in
the absence of any commercial or nancial relationships that could
beconstrued as a potential conict of interest.
Publisher’s note
All claims expressed in this article are solely those of the authors
and do not necessarily represent those of their aliated organizations,
or those of the publisher, the editors and the reviewers. Any product
that may be evaluated in this article, or claim that may be made by its
manufacturer, is not guaranteed or endorsed by the publisher.
Supplementary material
e Supplementary material for this article can befound online
at: https://www.frontiersin.org/articles/10.3389/fclim.2024.1343516/
full#supplementary-material
Rathnayake et al. 10.3389/fclim.2024.1343516
Frontiers in Climate 09 frontiersin.org
References
Agarwal, T., and Bucheli, T. D. (2011). Adaptation, validation and application of the
chemo-thermal oxidation method to quantify black carbon in soils. Environ. Pollut. 159,
532–538. doi: 10.1016/j.envpol.2010.10.012
Agroscope. (2020). Schweizerische Referenzmethoden der Forschungsanstalten
Agroscope, Bestimmung des organisch gebundenen Kohlenstos (Corg), Version 1.2.
Available at: https://ira.agroscope.ch/en-US/publication/46276 (Accessed August 03,
2023).
Archanjo, B. S., Mendoza, M. E., Albu, M., Mitchell, D. R. G., Hagemann, N.,
Mayrhofer, C., et a l. (2017). Nanoscale analyses of the surface structure and composition
of biochars extracted from eld trials or aer co-composting using advanced analytical
electron microscopy. Geoderma 294, 70–79. doi: 10.1016/j.geoderma.2017.01.037
Ascough, P. L., Bird, M. I., Brock, F., Higham, T. F. G., Meredith, W., Snape, C. E., et al.
(2009). Hydropyrolysis as a new tool for radiocarbon pre-treatment and the
quantication of black carbon. Quat. Geochronol. 4, 140–147. doi: 10.1016/j.
quageo.2008.11.001
Ascough, P. L., Bird, M. I., Meredith, W., and Snape, C. E. (2016). Dates and fates of
pyrogenic carbon: using spectroscopy to understand a “missing” global carbon sink.
Spectrosc. Eur. 28, 6–9.
Bachmann, H. J., Bucheli, T. D., Dieguez-Alonso, A., Fabbri, D., Knicker, H.,
Schmidt, H. P., et al. (2016). Toward the standardization of biochar analysis: the COST
action TD1107 Interlaboratory comparison. J. Agric. Food Chem. 64, 513–527. doi:
10.1021/acs.jafc.5b05055
Baldock, J. A., Hawke, B., Sanderman, J., and Mac Donald, L. M. (2013). Predicting
contents of carbon and its component fractions in Australian soils from diuse
reectance mid-infrared spectra. Soil Res. 51, 577–595. doi: 10.1071/SR13077
Baldock, J. A., and Skjemstad, J. O. (2000). Role of the soil matrix and minerals in
protecting natural organic materials against biological attack. Org. Geochem. 31,
697–710. doi: 10.1016/S0146-6380(00)00049-8
Bird, M. (2015). “Test procedures for biochar analysis in soils,” in Biochar for
Environmental Management. New York: Routledge, 711–748.
Bird, M., Keitel, C., and Meredith, W. (2017). “Analysis of biochars for C, H, N, O and
S by elemental analyzer” in Biochar: A guide to analytical methods. eds. B. Singh, M.
Camps-Arbestain and J. Lehmann (Clayton: CSIRO Publishing), 39–50.
Bird, M. I., Wynn, J. G., Saiz, G., Wurster, C. M., and McBeath, A. (2015). e
pyrogenic carbon cycle. Annu. Rev. Earth Planet. Sci. 43, 273–298. doi: 10.1146/annurev-
earth-060614-105038
Blanco-Canqui, H., Laird, D. A., Heaton, E. A., Rathke, S., and Acharya, B. S. (2020).
Soil carbon increased by twice the amount of biochar carbon applied aer 6 years: eld
evidence of negative priming. GCB Bioenergy 12, 240–251. doi: 10.1111/gcbb.12665
Brown, R., del Campo, B., Boateng, A. A., Garcia-Perez, M., Masek, O., Lehmann, J.,
et al. (2015). “Fundamentals of biochar production” in Biochar for environmental
management: Science, technology and implementation (New York: Routledge), 39–61.
Bucheli, T. D., Blum, F., Desaules, A., and Gustafsson, Ö. (2004). Polycyclic aromatic
hydrocarbons, black carbon, and molecular markers in soils of Switzerland. Chemosphere
56, 1061–1076. doi: 10.1016/j.chemosphere.2004.06.002
Budai, A., Zimmerman, A. R., Cowie, A. L., Webber, J. B. W., Singh, B. P., Glaser, B.,
et al. (2013). Biochar carbon stability test method: an assessment of methods to
determine biochar carbon stability. Available at: https://biochar-international.org/wp-
content/uploads/2018/06/IBI_Report_Biochar_Stability_Test_Method_Final.pdf
(Accessed August 01, 2023).
Canadell, J. G., Monteiro, P. M. S., Costa, M. H., Da, L. C., Cox, P. M., Eliseev, A. V.,
et al. (2021). Global carbon and other biogeochemical cycles and feedbacks. Available
at: https://hal.science/hal-03336145 (Accessed August 02, 2023).
Cerqueira, W. V., Rittl, T. F., Novotny, E. H., and Netto, A. D. (2015). High throughput
pyrogenic carbon (biochar) characterisation and quantication by liquid
chromatography. Anal. Methods 7, 8190–8196. doi: 10.1039/C5AY01242B
Chalk, P., and Smith, C. J. (2022). 13C methodologies for quantifying biochar stability
in soil: a critique. Eur. J. Soil Sci. 73, 1–12. doi: 10.1111/ejss.13245
Chang, Z., Tian, L., Li, F., Zhou, Y., Wu, M., Steinberg, C. E. W., et al. (2018). Benzene
polycarboxylic acid — a useful marker for condensed organic matter, but not for only
pyrogenic black carbon. Sci. Total Environ. 626, 660–667. doi: 10.1016/j.
scitotenv.2018.01.145
Cornelissen, G., Gustafsson, O. R., Bucheli, T. D., Jonker, M. T. O., Koelmans, A. A.,
and Van Noort, P. C. M. (2005). Extensive sorption of organic compounds to black
carbon, coal, and kerogen in sediments and soils: mechanisms and consequences for
distribution, bioaccumulation, and biodegradation. Environ. Sci. Technol. 39, 6881–6895.
doi: 10.1021/es050191b
Cotrufo, M. F., B oot, C., Abiven, S., Foster, E. J., Haddix, M., Reisser, M., et al. (2016).
Quantication of pyrogenic carbon in the environment: an integration of analytical
approaches. Org. Geochem. 100, 42–50. doi: 10.1016/j.orggeochem.2016.07.007
COWI, EI, and IEEP. (2020). Analytical support for the operationalisation of an EU
carbon farming initiative: Lessons learned from existing result-based carbon farming
schemes and barriers and solutions for implementation within the EU. Report to the
European Commission, DG Climate. Available at: https://climate.ec.europa.eu/
document/download/b0fc5b79-92b3-4ec1-89ba-3846158e904a_en?lename=policy_
forest_carbon_report_en.pdf (Accessed July 15, 2023).
Department of Sustainable Natural Resources, New South Wales. (1990). Soil survey
standard test method organic carbon. Available at: https://www.environment.nsw.gov.
au/resources/soils/testmethods/oc.pdf (Accessed August 05, 2023).
DIN 51732. Testing of solid fuels – determination of total carbon, hydrogen and
nitrogen content – instrumental methods. Available at: https://www.din.de/de/
mitwirken/normenausschuesse/nmp/veroeentlichungen/wdc-beuth:din21:205570833
(Accessed September 25, 2023).
DIN EN 15936. Soil, waste, treated biowaste and sludge – determination of total
organic carbon (TOC) by dry combustion. Available at: https://www.din.de/de/
mitwirken/normenausschuesse/naw/wdc-beuth:din21:344580989 (Accessed September
25, 2023).
Don, A., Seidel, F. J. L., Leifeld, J., Kätterer, T., Martin, M., Pellerin, S., et al. (2024).
Carbon sequestration in soils and climate change mitigation – denitions and pitfalls.
Glob. Chang. Biol. 30:e16983. doi: 10.1111/gcb.16983
EBC. (2021). Certication of the carbon sink potential of biochar, Ithaka Institute,
Arbaz, Switzerland. Available at: https://www.european-biochar.org/media/doc/139/c_
en_sink-value_2-1.pdf (Accessed August 02, 2023).
EBC. (2012-2023). European biochar certicate - guidelines for a sustainable
production of biochar.’ carbon standards international (csi), frick, switzerland. Available
at: https://www.european-biochar.org/media/doc/2/version_en_10_3.pdf (Accessed
August 25, 2023).
EBI. (2023). European biochar industry consortium e.V., Freiburg European biochar
market report 2022/23. Available at: https://www.biochar-industry.com/market-
overview/ (Accessed October 26, 2023).
Etter, H., Vera, A., Aggarwal, C., Delaney, M., and Manley, S. (2021). Methodology for
biochar utilization in soil and non-soil applications: Version 1.0. Available at: https://
verra.org/wp-content/uploads/2021/08/210803_VCS-Biochar-Methodology-v1.0-.pdf
(Accessed October 26, 2023).
European Commission. (2022). Commission sta working document: Impact
assessment report accompanying the document proposal for a regulation of the
european parliament and of the council establishing a union certication framework for
carbon removals. SWD. Available at: https://data.consilium.europa.eu/doc/document/
ST-15557-2022-ADD-3/en/pdf (Accessed November 21, 2023).
FAO (2019). Standard operating procedure for soil organic carbon Walkley-black
method titration and colorimetric method. Available at: https://www.fao.org/3/
ca7471en/ca7471en.pdf (Accessed October 25, 2023).
Fliessbach, A., Tresch, S., and Steens, M. (2021). Review on the techniques and
requirements for monitoring stock changes of soil organic carbon. Available at: https://
www.bafu.admin.ch/dam/bafu/en/dokumente/boden/externe-studien-berichte/review-
on-the-techniques-and-requirements-for-monitoring-stock-changes-of-soil-organic-
carbon.pdf.download.pdf/PoBourgeois_PROJEKT3.pdf (Accessed October 26, 2023).
Gerke, J. (2019). Black (pyrogenic) carbon in soils and waters: a fragile data basis
extensively interpreted. Chem. Biol. Technol. Agric. 6, 1–8. doi: 10.1186/
s40538-019-0151-6
Glaser, B., and Amelung, W. (2003). Pyrogenic carbon in native grassland soils along
a climosequence in North America. Global Biogeochem. Cycles 17, 1–8. doi:
10.1029/2002gb002019
Gustafsson, Ö., Bucheli, T. D., Kukulska, Z., Andersson, M., Largeau, C., Rouzaud, J. N.,
et al. (2001). Evaluation of a protocol for the quantication of black carbon in sediments.
Global Biogeochem. Cycles 15, 881–890. doi: 10.1029/2000GB001380
Hagemann, N., Spokas, K., Schmidt, H. P., Kägi, R., Böhler, M. A., and Bucheli, T. D.
(2018). Activated carbon, biochar and charcoal: linkages and synergies across pyrogenic
carbon’s ABCs. Water (Switzerland) 10, 1–19. doi: 10.3390/w10020182
Hammes, K., Schmidt, M. W. I., Smernik, R. J., Currie, L. A., Ball, W. P., Nguyen, T. H., et al.
(2007). Comparison of quantication methods to measure re-derived (black-elemental)
carbon in soils and sediments using reference materials from soil, water, sediment and the
atmosphere. Global Biogeochem. Cycles 21, 1–18. doi: 10.1029/2006GB002914
Hardie, M., Clothier, B., Bound, S., Oliver, G., and Close, D. (2014). Does biochar
inuence soil physical properties and soil water availability? Plant Soil 376, 347–361. doi:
10.1007/s11104-013-1980-x
Hardy, B., Borchard, N., and Leifeld, J. (2022). Identication of thermal signature and
quantication of charcoal in soil using dierential scanning calorimetry and benzene
polycarboxylic acid (BPCA) markers. Soil 8, 451–466. doi: 10.5194/soil-8-451-2022
Hardy, B., and Dufey, J. E. (2017). Geoderma the resistance of centennial soil charcoal
to the “Walkley-black” oxidation. Geoderma 303, 37–43. doi: 10.1016/j.
geoderma.2017.05.001
Hedges, J. I., Eglinton, G., Hatcher, P. G., Kirchman, D. L., Arnosti, C., Derenne, S.,
et al. (2000). e molecularly-uncharacterized component of nonliving organic matter
in natural environments. Org. Geochem. 31, 945–958. doi: 10.1016/
S0146-6380(00)00096-6
Rathnayake et al. 10.3389/fclim.2024.1343516
Frontiers in Climate 10 frontiersin.org
IPCC. (2019). Appendix 4 method for estimating the change in mineral soil organic
carbon stocks from biochar amendments: basis for future methodological development.
In IPCC, 2019 renement to the 2006 IPCC guidelines for National Greenhouse gas
Inventories. Available at: https://www.ipcc-nggip.iges.or.jp/public/2019rf/pdf/4_
Volume4/19R_V4_Ch02_Ap4_Biochar.pdf (Accessed September 25, 2023).
Ippolito, J. A., Cui, L., Kammann, C., Wrage-Mönnig, N., Estavillo, J. M.,
Fuertes-Mendizabal, T., et al. (2020). Feedstock choice, pyrolysis temperature and type
inuence biochar characteristics: a comprehensive meta-data analysis review. Biochar 2,
421–438. doi: 10.1007/s42773-020-00067-x
ISO14235. (1998). soil quality — determination of organic carbon by sulfochromic
oxidation. Available at: https://www.iso.org/standard/23140.html (Accessed October 06,
2023).
Jauss, V., Sullivan, P. J., Sanderman, J., Smith, D. B., and Lehmann, J. (2017). Pyrogenic
carbon distribution in mineral topsoils of the northeastern UnitedStates. Geoderma 296,
69–78. doi: 10.1016/j.geoderma.2017.02.022
Jiménez-González, M. A., De la Rosa, J. M., Aksoy, E., Jeery, S., Oliveira, B. R. F., and
Verheijen, F. G. A. (2021). Spatial distribution of pyrogenic carbon in Iberian topsoils
estimated by chemometric analysis of infrared spectra. Sci. Total Environ. 790:148170.
doi: 10.1016/j.scitotenv.2021.148170
Keiluweit, M., Nico, P. S., and Johnson, M. G. (2010). Dynamic molecular structure
of plant biomass-derived black carbon (biochar). Environ. Sci. Technol. 44, 1247–1253.
doi: 10.1021/es9031419
Knicker, H. (2011). Pyrogenic organic matter in soil: Its origin and occurrence, its
chemistry and survival in soil environments. Quaternar y Int. 243, 251–263. doi:
10.1016/j.quaint.2011.02.037
Knicker, H., Wiesmeier, M., and Dick, D. P. (2008). A simplied method for the
quantication of pyrogenic organic matter in grassland soils via chemical oxidation.
Geoderma 147, 69–74. doi: 10.1016/j.geoderma.2008.07.008
Koide, R. T., Petprakob, K., and Peoples, M. (2011). Quantitative analysis of biochar
in eld soil. Soil Biol. Biochem. 43, 1563–1568. doi: 10.1016/j.soilbio.2011.04.006
Lavallee, J. M., Conant, R. T., Haddix, M. L., Follett, R. F., Bird, M. I., and Paul, E. A.
(2019). Selective preservation of pyrogenic carbon across soil organic matter fractions
and its inuence on calculations of carbon mean residence times. Geoderma 354:113866.
doi: 10.1016/j.geoderma.2019.07.024
Lefebvre, D., Fawzy, S., Aquije, C. A., Osman, A. I., Draper, K. T., and Trabold, T. A.
(2023). Biomass residue to carbon dioxide removal: quantifying the global impact of
biochar. Biochar 5:65. doi: 10.1007/s42773-023-00258-2
Lehmann, J., and Joseph, S. (2015). Biochar for environmental management: Science,
technology and implementation. New York: Routledge.
Lehmann, J., and Kleber, M. (2015). e contentious nature of soil organic matter.
Nature 528, 60–68. doi: 10.1038/nature16069
Leifeld, J. (2007). ermal stability of black carbon characterised by oxidative
dierential scanning calorimetry. Organ. Geochem. 38, 112–127. doi: 10.1016/j.
orggeochem.2006.08.004
Leifeld, J., Alewell, C., Bader, C., Krüger, J. P., Mueller, C. W., Sommer, M., et al. (2018).
Pyrogenic carbon contributes substantially to carbon storage in intact and degraded
northern peatlands. L. Degrad. Dev. 29, 2082–2091. doi: 10.1002/ldr.2812
Leifeld, J., Bassin, S., and Fuhrer, J. (2005). Carbon stocks in Swiss agricultural soils
predicted by land-use, soil characteristics, and altitude. Agricult. Ecosyst. Environ. 105,
255–266. doi: 10.1016/j.agee.2004.03.006
Lovell, H. C. (2010). Governing the carbon oset market. Wiley Interdiscip. Rev. Clim.
Chang. 1, 353–362. doi: 10.1002/wcc.43
Masiello, C. A. (2004). New directions in black carbon organic geochemistry. Mar.
Chem. 92, 201–213. doi: 10.1016/j.marchem.2004.06.043
Meredith, W., Ascough, P. L., Bird, M. I., Large, D. J., Snape, C. E., Song, J., et al. (2013).
Direct evidence from hydropyrolysis for the retention of long alkyl moieties in black
carbon fractions isolated by acidied dichromate oxidation. J. Anal. Appl. Pyrolysis 103,
232–239. doi: 10.1016/j.jaap.2012.11.001
Meredith, W., Ascough, P. L., Bird, M. I., Large, D. J., Snape, C. E., Sun, Y., et al. (2012).
Assessment of hydropyrolysis as a method for the quantication of black carbon using
standard reference materials. Geochim. Cosmochim. Acta 97, 131–147. doi: 10.1016/j.
gca.2012.08.037
Michelsen, H. A., Colket, M. B., Bengtsson, P., D’Anna, A., Desg roux, P., Haynes, B. S.,
et al. (2020). A review of terminology used to describe soot formation and evolution
under combustion and pyrolytic conditions. Am. Chem. Soc. Nano 14, 12470–12490.
doi: 10.1021/acsnano.0c06226
Murano, H., Liu, G., Wang, Z., Tanihira, Y., Asahi, T., and Isoi, T. (2021). Quant ication
methods of pyrogenic carbon in soil with soil as a complex matrix: comparing the
CTO-375 and Cr2O7 methods. Soil Sci. Plant Nutr. 67, 380–388. doi:
10.1080/00380768.2021.1925960
Nakhli, S. A. A., Panta, S., Brown, J. D., Tian, J., and Imho, P. T. (2019). Quantifying
biochar content in a eld soil with varying organic matter content using a two-
temperature loss on ignition method. Sci. Total Environ. 658, 1106–1116. doi: 10.1016/j.
scitotenv.2018.12.174
NREL. (2015). Renewable electricity: how do youknow youare using it? Available at:
https://www.nrel.gov/docs/fy15osti/64558.pdf (Accessed January 20, 2024).
Oldeld, B. E. E., Eagle, A. J., Rubin, R. L., Rudek, J., and Gordon, D. R. (2022).
Crediting agricultural soil carbon sequestration; regional consistency is necessary for
carbon credit integrity. Science 375, 1222–1225. doi: 10.1126/science.abl7991
Paetsch, L., Mueller, C. W., Rumpel, C., Angst, Š., Wiesheu, A. C., Girardin, C., et al.
(2017). A multi-technique approach to assess the fate of biochar in soil and to quantify
its eect on soil organic matter composition. Org. Geochem. 112, 177–186. doi: 10.1016/j.
orggeochem.2017.06.012
Paul, C., Bartkowski, B., Dönmez, C., Don, A., Mayer, S., Steffens, M., et al.
(2023). Carbon farming: are soil carbon certificates a suitable tool for climate
change mitigation? J. Environ. Manag. 330:117142. doi: 10.1016/j.jenvman.2022.
117142
Plante, A. F., Fernández, J. M., and Leifeld, J. (2009). Geoderma application of thermal
analysis techniques in soil science. Geoderma 153, 1–10. doi: 10.1016/j.
geoderma.2009.08.016
Poeplau, C., Jacobs, A., Don, A., Vos, C., Schneider, F., Wittnebel, M., et al. (2020). Stocks
of organic carbon in German agricultural soils—key results of the rst comprehensive
inventory. J. Plant Nutr. Soil Sci. 183, 665–681. doi: 10.1002/jpln.202000113
Poeplau, C., Vos, C., and Don, A. (2017). Soil organic carbon stocks are systematically
overestimated by misuse of the parameters bulk density and rock fragment content. Soil
3, 61–66. doi: 10.5194/soil-3-61-2017
Pressler, Y., Boot, C. M., Abiven, S., Lugato, E., and Francesca Cotrufo, M. (2022).
Continental-scale measurements of soil pyrogenic carbon in Europe. Soil Res. 60,
103–113. doi: 10.1071/SR19396
Pulcher, R., Balugani, E., Ventura, M., Greggio, N., and Marazza, D. (2022). Inclusion
of biochar in a C dynamics model based on observations from an 8-year eld
experiment. Soil 8, 199–211. doi: 10.5194/soil-8-199-2022
Puro.earth (2024). Biochar Methodology, version 3. Available at: https://7518557.fs1.
hubspotusercontent-na1.net/hubfs/7518557/Supplier%20Documents/Puro.earth%20
Biochar%20Methodology.pdf (Accessed March 24, 2024).
Qi, F., Naidu, R., Bolan, N. S., Dong, Z., Yan, Y., Lamb, D., et al. (2017). Pyrogenic
carbon in Australian soils. Sci. Total Environ. 586, 849–857. doi: 10.1016/j.
scitotenv.2017.02.064
Rathnayake, D., Maziarka, P., Ghysels, S., Mašek, O., Sohi, S., and Ronsse, F. (2020).
How to trace back an unknown production temperature of biochar from chemical
characterization methods in a feedstock independent way. J. Anal. Appl. Pyrolysis
151:104926. doi: 10.1016/j.jaap.2020.104926
Reisser, M., Purves, R. S., Schmidt, M. W. I., and Abiven, S. (2016). Pyrogenic carbon
in soils: a literature-based inventory and a global estimation of its content in soil organic
carbon and stocks. Front. Earth Sci. 4, 1–14. doi: 10.3389/feart.2016.00080
Rodrigues, L., Budai, A., Elsgaard, L., Hardy, B., Keel, S. G., Mondini, C., et al. (2023).
e importance of biochar quality and pyrolysis yield for soil carbon sequestration in
practice. Eur. J. Soil Sci. 74, 1–11. doi: 10.1111/ejss.13396
Sanderman, J., Baldock, J. A., Dangal, S. R. S., Ludwig, S., Potter, S., Rivard, C., et al.
(2021). Soil organic carbon fractions in the Great Plains of the United States: an
application of mid-infrared spectroscopy. Biogeochemistry 156, 97–114. doi: 10.1007/
s10533-021-00755-1
Santín, C., Doerr, S. H., Merino, A., Bucheli, T. D., Bryant, R., Ascough, P., et al.
(2017). Carbon sequestration potential and physicochemical properties dier between
wildre charcoals and slow-pyrolysis biochars. Sci. Rep. 7, 11233–11211. doi: 10.1038/
s41598-017-10455-2
Schmidt, H. P., Abiven, S., Hageman, N., and Meyer Zu Drewer, J. (2022). Permanence
of soil applied biochar. An executive summary for global biochar carbon sink
certication. Arbaz, Switzerland. Available at: www.biochar-journal.org/en/ct/109
Schmidt, H. P., Kammann, C., Hagemann, N., Leifeld, J., Bucheli, T. D., Sánchez
Monedero, M. A., et al. (2021). Biochar in agriculture – a systematic review of 26 global
meta-analyses. GCB Bioenergy 13, 1708–1730. doi: 10.1111/gcbb.12889
Schmidt, M. W. I., Skjemstad, J. O., Czimczik, C. I., and Prentice, K. M. (2001).
Comparative analysis of black carbon in soils. Global Biogeochem. Cycles 15, 163–167.
doi: 10.1029/2000GB001284
Schneider, L., Kollmuss, A., and Lazarus, M. (2015). Addressing the risk of double
counting emission reductions under the UNFCCC. Clim. Chang. 131, 473–486. doi:
10.1007/s10584-015-1398-y
Schrumpf, M., Schulze, E. D., Kaiser, K., and Schumacher, J. (2011). How accurately
can soil organic carbon stocks and stock changes bequantied by soil inventories?
Biogeosci. Discus. 1, 1193–1212. doi: 10.5194/bg-8-1193-2011
Sigmund, G., Bucheli, T. D., Hilber, I., Micić, V., Kah, M., and Hofmann, T. (2017).
Eect of ageing on the properties and polycyclic aromatic hydrocarbon composition of
biochar. Environ Sci Process Impacts 19, 768–774. doi: 10.1039/c7em00116a
Sigmund, G., Schmid, A., Schmidt, H. P., Hagemann, N., Bucheli, T. D., and
Hofmann, T. (2023). Small biochar particles hardly disintegrate under cryo-stress.
Geoderma 430:116326. doi: 10.1016/j.geoderma.2023.116326
Simpson, M. J., and Hatcher, P. G. (2004). Overestimates of black carbon in soils and
sediments. Naturwissenschaen 91, 436–440. doi: 10.1007/s00114-004-0550-8
Singh, B. P., Cowie, A. L., and Smernik, R. J. (2012). Biochar carbon stability in a clayey
soil as a function of feedstock and pyrolysis temperature. Environ. Sci. Technol. 46,
11770–11778. doi: 10.1021/es302545b
Rathnayake et al. 10.3389/fclim.2024.1343516
Frontiers in Climate 11 frontiersin.org
Singh, B., Fang, Y., Cowie, B. C. C., and omsen, L. (2014). NEXAFS and XPS
characterisation of carbon functional groups of fresh and aged biochars. Org. Geochem.
77, 1–10. doi: 10.1016/j.orggeochem.2014.09.006
Six, J., Conant, R. T., Paul, E. A., and Paustian, K. (2002). Stabilization mechanisms of
SOM implications for C saturation of soils.Pdf. Plant Soil 241, 155–176. doi:
10.1023/A:1016125726789
Skjemstad, J. O., Reicosky, D. C., Wilts, A. R., and McGowan, J. A. (2002). Charcoal
carbon in US agricultural soils. Soil Sci. Soc. Am. J. 66, 1249–1255. doi: 10.2136/
sssaj2002.1249
Smernik, R. J. (2017). “Analysis of biochars by 13C nuclear magnetic resonance
spectroscopy” in Biochar: A guide to analytical methods (Clayton: CSIRO Publishing),
151–161.
Smith, P., Soussana, J. F., Angers, D., Schipper, L., Chenu, C., Rasse, D. P., et al. (2020).
How to measure, report and verify soil carbon change to realize the potential of soil
carbon sequestration for atmospheric greenhouse gas removal. Glob. Chang. Biol. 26,
219–241. doi: 10.1111/gcb.14815
Spokas, K. A. (2013). Impact of biochar eld aging on laboratory greenhouse gas
production potentials. GCB Bioenergy 5, 165–176. doi: 10.1111/gcbb.12005
Spokas, K. A., Novak, J. M., Masiello, C. A., Johnson, M. G., Colosky, E. C.,
Ippolito, J. A., et al. (2014). Physical disintegration of biochar: an overlooked process.
Environ. Sci. Technol. Lett. 1, 326–332. doi: 10.1021/ez500199t
United Nations (2015). Paris Agreement. Available at: https://unfccc.int/les/
essential_background/convention/application/pdf/english_paris_agreement.pdf
(Accessed August 20, 2023).
VDLUFA (1991). Verband Deutscher Landwirtschalicher Untersuchungs-und
Forschungs-anstalten eV). Methodenbuch, Band I: Die Untersuchung von Böden.
Available at: http://www.methodenbuch.de/index.php?option=com_content&view=art
icle&id=7&Itemid=108&lang=de (Accessed August 11, 2023).
Wagner, S., Rudolf, J., and Stubbins, A. (2018). Dissolved black carbon in aquatic
ecosystems. Limnol. Oceanogr. Lett. 3, 168–185. doi: 10.1002/lol2.10076
Wang, X., Sanderman, J., and Yoo, K. (2018). Climate-dependent topographic eects
on pyrogenic soil carbon in southeastern Australia. Geoderma 322, 121–130. doi:
10.1016/j.geoderma.2018.02.025
Werner, C., Lucht, W., G erten, D., and Kammann, C. (2021). Potential of land-neutral
negative emissions through biochar sequestration. Earth’s Futur. 10:2583. doi:
10.1029/2021EF002583
Whitman, T., Scholz, S. M., and Lehmann, J. (2010). Biochar projects for mitigating
climate change: an investigation of critical methodology issues for carbon accounting.
Carbon Manag. 1, 89–107. doi: 10.4155/cmt.10.4
Wiedemeier, D. B. (2014). New insights into pyrogenic carbon by an improved
benzene Polycarboxylic acid molecular marker method. Available at: https://www.zora.
uzh.ch/id/eprint/105332/1/2014_esis_Daniel_Wiedemeier_III%20.pdf (Accessed
August 29, 2022).
Wiedemeier, D. B., Hilf, M. D., Smittenberg, R. H., Haberle, S. G., and Schmidt, M. W.
I. (2013). Improved assessment of pyrogenic carbon quantity and quality in
environmental samples by high-performance liquid chromatography. J. Chromatogr. A
1304, 246–250. doi: 10.1016/j.chroma.2013.06.012
Wiedemeier, D. B., Lang, S. Q., Gierga, M., Abiven, S., Bernasconi, S. M.,
Früh-Green, G. L., et al. (2016). Characterization, quantication and compound-specic
isotopic analysis of pyrogenic carbon using benzene polycarboxylic acids (Bpca). J. Vis.
Exp. 2016, 1–9. doi: 10.3791/53922
Wiesmeier, M., Mayer, S., Carsten, P., Katharina, H., Axel, D., Uwe, F., et al. (2020).
CO2 certicates for carbon sequestration in soils: methods, management practices and
limitations. Leibniz Cent. Landsc. Res. doi: 10.20387/BONARES-NE0G-CE98
Woolf, D., Amonette, J. E., Street-Perrott, F. A., Lehmann, J., and Joseph, S. (2010).
Sustainable biochar to mitigate global climate change. Nat. Commun. 1, 1–9. doi:
10.1038/ncomms1053
Xiao, X., and Chen, B. (2017). A direct observation of the ne aromatic clusters and
molecular structures of biochars. Environ. Sci. Technol. 51, 5473–5482. doi: 10.1021/acs.
est.6b06300
Yang, F., Xu, Z., Huang, Y., Tsang, D. C. W., Ok, Y. S., Zhao, L., et al. (2021).
Stabilization of dissolvable biochar by soil minerals: release reduction and organo-
mineral complexes formation. J. Hazard. Mater. 412:125213. doi: 10.1016/j.
jhazmat.2021.125213
Zimmermann, A. R., and Mitra, S. (2017). Trial by re: on the terminology and
methods used in pyrogenic organic carbon research. Front. Earth Sci. 5:95. doi: 10.3389/
feart.2017.00095
Rathnayake et al. 10.3389/fclim.2024.1343516
Frontiers in Climate 12 frontiersin.org
Glossary
BPCA Benzene polycarboxylic acids
CDR Carbon dioxide removal
Corg Organic carbon
CTO375 Chemothermal oxidation at 375°C
DACCS Direct air carbon capture and storage
DIN Deutsches Institut für Normung (German Institute for Standardization)
EBC European Biochar Certicate
GHG Greenhouse gas
HyPy Hydropyrolysis
MDD Minimum detectable dierence
MIR Mid-infrared spectroscopy
MRV Monitoring, reporting, and verication
NIR Near-infrared spectroscopy
NMR Nuclear magnetic resonance
PCM Pyrogenic carbonaceous material
PyC Pyrogenic carbon
PyCCS Pyrogenic carbon capture and storage
SOC Soil organic carbon
SOM Soil organic matter
TG-DSC ermogravimetry with dierential scanning calorimetry