Quantifying soil organic carbon after biochar application: how to avoid (the risk of) counting CDR twice?

Abstract

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 offsetting 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 be avoided 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 different soil types, which is extremely laborious and yet to be established 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 be solved 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 effectively avoid double counting.

Full text

Frontiers in Climate 01 frontiersin.org
Quantifying soil organic carbon
after biochar application: how to
avoid (the risk of) counting CDR
twice?
DilaniRathnayake
1,2, 3, Hans-PeterSchmidt
2,3, JensLeifeld
4,
DianeBürge
1, ThomasD.Bucheli
1 and NikolasHagemann
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
osetting 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 beavoided
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 dierent soil types, which is extremely laborious
and yet to beestablished 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 besolved 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 eectively 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, UnitedStates
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 eorts to limit it to 1.5°C (United Nations,
2015). is goal can no longer beachieved by reducing emissions
alone, and carbon dioxide removal (CDR) is necessary to transfer
carbon (C) from the atmosphere into non-atmospheric C-sinks
(Smith etal., 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 beachieved through reduced
tillage, reduced drainage, cover crops, and several other methods,
including biochar (cf. denition in Table 1) application to soil
(Whitman etal., 2010; Blanco-Canqui etal., 2020; COWI, EI, and
IEEP, 2020; Don etal., 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 etal., 2020; Schmidt etal., 2021).
e production and non-oxidative application of biochar itself is
considered a negative emission technology, oen referred to as PyCCS
(Schmidt etal., 2021; Lefebvre etal., 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 eects through land use change
(Werner etal., 2021).
A certication of negative emissions is necessary for their
nancing, which requires the respective carbon removal technique
to bequantiable, deliver additional climate benets, strive to store
carbon for a long time, and contribute to eectively removing
carbon from the atmosphere (COWI, EI, and IEEP, 2020).
Accordingly, the EU suggested four QUantication, Additionality,
Long-term storage, sustainabilITY (QU. A. L. ITY) criteria for
industrial carbon removal certication methodologies (European
Commission, 2022). Today, these carbon removals are certied by
private companies according to their own guidelines or companies
relying on independent third-party certication, e.g., the European
Biochar Certicate (EBC)‘s C-Sink Certicate (EBC, 2021). Either
way, negative emission certicates are generated and sold on the
voluntary market. A core element of a well-functioning market is
that each negative emission can becertied 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 etal., 2015; COWI, EI, and IEEP, 2020), e.g.,
when biochar is certied as a biochar C-sink and as part of SOC
increase, as detailed below.
For biochar, the certication process involves the quantication
of its organic carbon (Corg) content and molar H-to-Corg ratio. Once
soil application is conrmed, a xed portion of biochar-C cannot
beburned or oxidized by other means anymore (e.g., 74–93% when
molar H to Corg ratio < 0.4) and the certication process is completed
(Woolf etal., 2010; Budai etal., 2013; IPCC, 2019; EBC 2021
–
2023;
Rodrigues etal., 2023). In contrast, result-based SOC certication
schemes usually include repeated soil sampling and quantication of
SOC to assess this (temporary) C-sink (Paul etal., 2023). However, as
discussed in detail below, the quantication of SOC typically also
includes biochar-C and other pyrogenic carbon (PyC) contained in
pyrogenic carbonaceous materials (PCM, cf. Table1) such as soot or
wildre-derived char. erefore, e.g., the EBC C-Sink guidelines for
certifying biochar C-sinks require that when farmers purchase
certied biochar, they sign a contract that they are not participating
in a SOC certication program to avoid double counting (EBC, 2021).
is is a weak and hardly veriable requirement that may bedicult
to adhere to with the increasing interest in SOC certication 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 etal., 2020; Oldeld etal., 2022). erefore, the
extent to which non-pyrogenic SOC and non-biochar PyC can
bequantied in routine soil analysis in the presence of biochar should
be evaluated. Routine analysis here means that the method can
beimplemented with a high degree of automation and cost-eectively
TABLE1 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 beburnt 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 etal.,
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 etal., 2015; Hagemann etal.,
2018).
Pyrogenic carbon
(PyC) or black C
“e thermochemically altered organic carbon fraction of
pyrogenic carbonaceous material” (Bird etal., 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 etal., 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 certication
process. In Europe, costs for SOC determination can beas 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 dierentiation and
quantication (Hammes etal., 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-specic scale,
ii. Evaluate the established methods of SOC quantication, and
iii. Discuss existing analytical methods for PyC quantication 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 beavoided via analytical techniques and to
derive conclusions for reliable approaches for CDR certication 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
quantication in soil; pyrogenic carbon analysis; pyrogenic
carbonaceous material; biochar carbon sink certication; 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. Aer the initial screening, the
articles were manually classied into dierent 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 etal., 2010; Singh etal., 2012; Bird etal., 2015; Rathnayake
etal., 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 bein the range of 3–90% (Keiluweit etal., 2010;
Hardie etal., 2014; Xiao and Chen, 2017; Ippolito etal., 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 etal., 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., wildre char)
and anthropogenic (soot) origins (Knicker, 2011; Santín etal., 2017).
e continuum of PCM in soil, their formation pathways, initial
reservoirs, and some basic physicochemical characteristics are
indicated in Figure1.
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 etal., 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 etal., 2021). Tillage,
erosion, and surface runo activities induce the horizontal and
vertical movements of PCM in soils (Qi etal., 2017; Jiménez-González
etal., 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 etal., 2015).
Depending on the soil types and PyC analytical methods used, the
PyC content in soils could vary from 1 to 37% of SOC (Table2). 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 (Figure2). According to that, the initial non-biochar PCM
content in soil (4.3–5.6 t ha−1) could behigher 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 dened
temperature, oxidized by pure oxygen, and the resulting CO
2
is
quantied. Inorganic carbon, i.e., carbonates, must beremoved 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 dene the application of the DIN 51732, a dry
oxidation procedure for solid fuels (Bachmann etal., 2016; Bird etal.,
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 bequantied 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
unaected and are thus not detected. However, biochar or any other
non-biochar PyC present in soil can beoxidized under the conditions
applied in the Swiss reference method. us, biochar-C will beat 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 dierence (MDD)
For the Walkley and Black method of wet oxidation, the Global
Soil Laboratory Network quantied a coecient 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 etal., 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
beaccurately measured. For the dry combustion method used in the
elemental analyzer, Fliessbach etal. (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 berecognized analytically.
However, the accuracy of SOC determination is not solely
determined by analytical precision of organic carbon quantication
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 etal., 2017, 2020; Wiesmeier etal., 2020). Even under
reasonably optimized conditions (100 samples per plot of 1–2 ha) to
be applied for a sampling of scientic long-term experiments,
Schrumpf etal. (2011) quantied an MDD of 1–2.5 t ha
−1
of SOC for
cropland and grassland sites, respectively. is further conrms that
a single application of 1 t biochar (70% C
org
content) would not
bedetectable, 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 etal., 2020) between
repeated SOC quantication to detect stock changes. However, the
MDD is likely to behigher 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 (Figure1) (Schmidt etal.,
2001; Knicker, 2011). e existing methods for quantifying, isolating,
and characterizing PCM in soil attempt to dierentiate the inorganic
material, thermally unaltered organic carbon, and PyC using chemical,
FIGURE1
Continuum of pyrogenic carbonaceous materials (PCM) in soil (Hedges etal., 2000; Masiello, 2004; Wiedemeier, 2014; Bird etal., 2015; Santín etal.,
2017; Wagner etal., 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, dierent methods isolate and characterize dierent
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 etal., 2001;
Hammes etal., 2007; Wiedemeier etal., 2013; Bird, 2015; Hardy etal.,
2022). Hence, this section and Table3 only briey 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
quantication of non-biochar SOC in routine analysis.
Physical techniques used for the biochar separation from the soil
are based on its visual appearance or dierences 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
identication and verication (Spokas, 2013; Paetsch etal., 2017).
However, due to the similar color (i.e., black) and depending on the
abundance of non-biochar PCM in the soil, there can bebiases to the
overall quantied 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 etal., 2014; Sigmund etal.,
2023), which will result in an underestimation of the soil biochar
content. Alternately, biochar can beseparated by otation due to its
bulk density, which is lower than soil mineral matter. Liquids with
dierent densities, such as water (Sigmund etal., 2017) or sodium
polytungstate solution (Singh etal., 2014), can beused. However,
biochar in soil may form biochar mineral complexes whose density
might besimilar to that of bulk soil (Archanjo etal., 2017; Yang etal.,
2021). Due to the overlapping bulk density, skeletal density, envelope
density, and porosity values of biochar and other non-biochar PCM
(Santín etal., 2017), the sensitivity and precision of density separation
methods can be lower. Physical methods can beextremely time-
consuming and labor-intensive, and sample losses could occur during
sample handling. Hence, physical techniques are irrelevant to biochar
quantication in routine analysis. However, they may beuseful 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 eectively oxidize hydrophobic non-biochar
SOC and may at least partially oxidize PyC (Hammes etal., 2007;
Knicker et al., 2008; Meredith etal., 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 etal., 2001; Agarwal
and Bucheli, 2011). However, pyrogenic artifacts could beformed if a
sucient amount of oxygen is lacking during the thermal treatment,
lignin may partly survive the treatment, and some of the PyC can
beoxidized entirely during the oxidation step, whereby the extent may
vary for dierent soils as well as for PCM types (Agarwal and Bucheli,
2011; Gerke, 2019; Murano etal., 2021). ermogravimetric analysis
and dierential scanning calorimetry (TGA-DSC) oer the possibility
to identify thermal signatures specic for SOC and biochar/other
non-biochar PCMs. However, biochars show dierent signatures
depending on the pyrolysis temperature, and also, soils may vary in
their background signal (Leifeld, 2007; Hardy etal., 2022). Hence,
unless the baseline soil (without biochar) exhibited distinct thermal
signatures compared to that of biochar, it is harder to quantify biochar
TABLE2 The percentage of PyC in total SOC in dierent soils as reported in previous studies using dierent analytical methods.
Soil type Method used PyC/SOC (%) References
US agricultural soils Hydrogen pyrolysis 2.7–7.7 Lavallee etal. (2019)
US agricultural soils UV oxidation and 13C NMR 10–35 Skjemstad etal. (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 etal. (2021)
Southeastern Australian soils MIR 7–37 Wang etal. (2018)
Australian agricultural and pastoral soils CTO375 2–26 Qi etal. (2017)
Various soil types CTO375, microscopy combined with 13C
NMR analysis, UV photooxidation
combined with coulometry
4–6 Cornelissen etal. (2005)
Arable, permanent, and pasture grassland,
forest, and urban soils collected from the
Swiss Soil Monitoring Network
CTO375 1–6 Bucheli etal. (2004)
Peatlands 13C NMR combined with molecular
modeling
12.3–14.7 Leifeld etal. (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 bevalidated for
dierent biochars, dierent application rates, and soils with various
amounts of native PCM and mineral compositions.
Hydropyrolysis (HyPy) is a thermal technique with high
reproducibility that can beused to quantify the most stable fraction of
PCM present in soil (Meredith etal., 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 etal., 2012). Wildre 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 etal., 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 becertied as a temporary C-sink (Schmidt etal., 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 etal. (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 mue 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)
signicantly improved the loss-on-ignition method by looking at two
dierent temperatures, but the need for the reference sample remains.
Benzene polycarboxylic acids (BPCA) is a molecular marker
method used in soil PyC analysis (Schmidt etal., 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 dierent GC instrumental
conditions and calibrations used (Hammes etal., 2007; Wiedemeier
etal., 2016). Overestimation may arise from quantifying humic acid
compounds such as PyC (Chang etal., 2018; Gerke, 2019).
e 13C nuclear magnetic resonance (NMR) quanties 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 dierentiate 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 etal., 2013). However, their
widespread application requires comprehensive reference databases
that comprise spectra and calibrations for all types of soil that shall
beinvestigated. 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 etal., 2013). Such comprehensive
reference databases might even allow us to distinguish biochar and
non-biochar PyC, as the site-specic reference spectra might also
cover the dierent background concentrations of PyC. However, this
needs to beproven separately. It also needs to beveried in detail to
what extent the semi-persistent carbon fraction of biochar can also
bedetected 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 beaccomplished by public actors, like the
Commonwealth Scientic 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
dierent from the soils they applied (Bird etal., 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-specic reference library, which is not yet
routinely available. us, in the foreseeable future, it will hardly
bepossible to assess the MDD between biochar and other non-biochar
PCM in biochar-applied soils with sucient 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 conrmed or independently
veriable information as to whether, and if so at what dosage,
biochar-C has been applied to a specic piece of land. Once such
information is available, total SOC can bedetermined with state-of-
the-art routine analysis (cf. Section 3), and the already-certied
biochar-C could be simply subtracted arithmetically. Advanced
biochar-based C-sink certication methods require the registration of
FIGURE2
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 top20 cm of Swiss arable soils
(Leifeld etal., 2005). Non-biochar PyC was assumed to be13.7% of
the SOC (Reisser etal., 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 etal., 2021; Puro.earth, 2024).
However, farmers can apply biochar without C-sink certication 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 osets on the
voluntary market without registering the localized biochar application,
the risk of double accounting is high. erefore, biochar-based
C-sinks may not becertied without listing on a public C-sink registry
to make the above information available. When SOC-based C-sinks
are certied, it is necessary to query the state or regional C-sink
registry to verify how much C-sink-certied biochar was applied
TABLE3 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
dierent densities.
Simple and can beapplied when
suciently large PCMs are presented in
soils.
Time-consuming and labor-intensive (thus, this
can beexpensive). Possible overlaps between
dierent PCMs.
Singh etal. (2014) and Sigmund etal.
(2017)
Chemical oxidation using NaClO
or K2Cr2O7 and UV
photooxidation
Simple and well-established with detailed
protocols and reference methods.
Inecient 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
etal. (2007), Knicker etal. (2008),
Meredith etal. (2013), and Murano
etal. (2021)
CTO375 followed by elemental
carbon analysis.
Simple and inexpensive methodology
and easiness of controlling operating
conditions.
Pyrogenic artifacts could beformed if a
sucient amount of oxygen is lacking during
the thermal treatment. is method exhibits
high selectivity toward soot-like PyC.
Gustafsson etal. (2001), Bucheli etal.
(2004), Agarwal and Bucheli (2011),
Gerke (2019), and Murano etal. (2021)
ermogravimetry with
dierential scanning calorimetry
(TG-DSC)
Simple and inexpensive methodology. Dicult 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 etal. (2009), and
Hardy etal. (2022)
Adapted loss on the ignition
method
Simple and inexpensive and no advanced
analytical instruments are involved (only
the mue 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
beoverestimated due to SOC increase.
Koide etal. (2011)
Hydropyrolysis (HyPy) followed
by elemental carbon analysis
HyPy is a matrix-independent, high-
precision, and highly reproducible
technique for PyC quantication. 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 etal. (2009), Meredith etal.
(2012), and Cotrufo etal. (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 bedestroyed during
the extraction resulting in underestimation of
PyC.
Schmidt etal. (2001) and Cerqueira
etal. (2015)
NMR spectroscopy 13C NMR quanties 13C atoms in organic
compounds, can identify the chemical
bond, and is used to characterize
biochars.
is method is time-consuming, expensive, and
requires specic 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 bereferenced against
materials with known PyC content, calibrated,
and validated with a wide range of PyC and soil
types to beused in routine analysis with higher
precision.
Jauss etal. (2017), Jiménez-González
etal. (2021), and Pressler etal. (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 etal. (2001), Bird etal.
(2015), Ascough etal. (2016), Paetsch
etal. (2017), and Pulcher etal. (2022)

Rathnayake et al. 10.3389/fclim.2024.1343516
Frontiers in Climate 08 frontiersin.org
during the SOC certication period in order to subtract the already-
certied C-sink from the SOC C-sink certication.
Tracking systems are already established as a part of C-sink
registers or in energy attribute certicates, which are issued to
conrm the use of renewable energy (NREL, 2015). In addition,
registers are used for certicates from emission avoidance and
forestry; e.g., carbon credits created within the clean development
mechanism had to belisted in the international transaction log
provided by the United Nations Framework Convention on Climate
Change (Lovell, 2010). Here, avoiding double use of certicates 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 oered, e.g., by C Capsule (Sheeld,
UnitedKingdom), Puro.earth (Helsinki, Finland), and the Global
Carbon Register Foundation (Arbaz, Switzerland). All providers
mentioned above oer global applicability but dier in the details
of the registered information. Each register may contain entries
from all over the world. us, a single register query is currently
insucient to obtain the necessary information. is could
beovercome 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 dierent databases can be retrieved in a
uniform manner. Geographical exclusivity could beachieved in the
form of national registers run by federal oces or independent
non-prot, 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 certication 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
verication (MRV) of SOC buildup as there is a signicant risk of
double counting when biochar is applied, and SOC buildup is
remunerated based on soil sampling and quantication 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 suciently 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 certied and,
at the same time, SOC certication based on result-based payments
with sampling and organic carbon quantication performed on
land to which biochar was applied. Hence, this risk can eciently
beaddressed at the governance level using the following two steps:
(1) all biochar-based carbon sinks shall beregistered in a public
C-sink register, and (2) when certifying SOC increase as CDR, the
public C-sink register shall beconsulted to control that no certied
biochar was applied to the respective eld during the certication
period. If biochar was applied, the SOC increase needs to
be corrected arithmetically for the already-certied
biochar-C. Similarly, SOC measurements taken for SOC-based
CDR certication should be registered and geo-referenced to
improve their MRV and to enable proper global carbon accounting
considering the dierent types, permanence, and potential control
periods of C-sinks. We, therefore, suggest the use of C-sink registers
as a cost-eective tool to avoid double counting of CDR and
improve overall carbon accounting for climate change mitigation.
ese registers should beimplemented 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 scientic 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
beconstrued as a potential conict 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 aliated 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 befound 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
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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 Certicate
GHG Greenhouse gas
HyPy Hydropyrolysis
MDD Minimum detectable dierence
MIR Mid-infrared spectroscopy
MRV Monitoring, reporting, and verication
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 dierential scanning calorimetry