Appendix4: Method for Estimating the Change in Mineral Soil Organic Carbon Stocks from Biochar Amendments:
Basis for Future Methodological Development

Appendix 4
Method for Estimating the Change in Mineral Soil
Organic Carbon Stocks from Biochar Amendments: Basis for Future
Methodological Development
This appendix provides a basis for future methodological development of a Tier 1 method for estimating the change
in mineral soil organic C stocks from biochar amendments to soils, rather than complete guidance.
For the purpose of this methodology, biochar is defined as a solid material generated by heating biomass to atemperature in excess of 350°C under conditions of controlled and limited oxidant concentrations to prevent
combustion. These processes can be classified as either pyrolysis (in which oxidants are excluded), or gasification (inwhich oxidant concentrations are low enough to generate syngas). The change in soil organic C stocks from biochar
amendments is estimated separately from other organic amendments over a 100-year time frame. This method does
not deal with pyrolytic organic materials that result from wild fires or open fires, and is only applicable for biochar
added to mineral soils in grasslands and croplands. Biochar is more persistent with only a small portion mineralised
each year at a decreasing rate over many centuries, and therefore the stock change method cannot be used to trackchanges in biochar C stocks over time as is done for other management practices in mineral soils.
The methodology used to estimate biochar C additions to minerals soils is based on a top-down approach in which thetotal amount of biochar generated and added to mineral soil in cropland and grassland1 is required to estimate thecontribution of biochar to annual changes in mineral soil C stocks. Information is not needed on the application rate.
Interactions between biochar C fate and soil type or land management are not considered with this method. However,
the method does require compilers to track the source of feedstock and temperature of the pyrolysis. The total change
in carbon stocks of mineral soils receiving biochar amendments is estimated with Equation 4Ap.1.
EQUATION 4AP.1

ANNUAL CHANGE IN BIOCHAR CARBON STOCK IN MINERAL SOILS RECEIVING BIOCHAR ADDITIONSn(∆BCMineral =∑ BCTOTp • FC p • Fpermp
p =1

)

Where

∆BCMineral = the total change in carbon stocks of mineral soils associated with biochar amendment, tonnes
sequestered C yr-1

BCTOTp= the mass of biochar incorporated into mineral soil during the inventory year for each biochar
production type p , tonnes biochar dry matter yr-1

FC p

= the organic carbon content of biochar for each production type p , tonnes C tonne-1 biochar
dry matter, Table 4Ap.1

Fperm p= fraction of biochar carbon for each production type p remaining (unmineralised) after 100
years, tonnes sequestered C tonne-1 biochar C, Table 4Ap.2n= the number of different production types of biochar

1 This method is not applicable for application of biochar to soils in forest land, settlements, other lands or wetlands. The studies

used in the derivation of Fperm values included only cropland and grassland mineral soils. Thus, the Fperm values provided inTable 4Ap.2 are only applicable to mineral soils under those land uses.

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Global estimates of the organic C content of biochar (𝐹𝐹𝐶𝐶𝑝𝑝 ) as a function of feedstock and heating temperature areprovided in Table 4Ap.1, as well as estimates of the proportion of biochar C that would persist for 100 years (𝐹𝐹𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 )years in Table 4Ap.2 2.
The biochar-C addition is estimated for cropland and grassland, or in total without disaggregation to the amounts
applied in cropland and grassland. If biochar-C is entered without disaggregation, then the C stock change should be
associated with the land use receiving the majority of the biochar.
TABLE 4AP.1
VALUES FOR ORGANIC C CONTENT FACTOR OF BIOCHAR BY PRODUCTION TYPE (𝑭𝑭𝑪𝑪𝒑𝒑 ).
Feedstock

Pyrolysis Production Process

Animal manure

Wood
Herbaceous (grasses, forbs,
leaves; excluding rice husks andrice straw)
Rice husks and rice straw

Nut shells, pits and stones
Biosolids (paper sludge, sewage
sludge)Values for FC p 2

Pyrolysis 10.38 ± 49%

Gasification 1

0.09 ± 53%

Pyrolysis

0.77 ± 42%

Gasification

0.52 ± 52%

Pyrolysis

0.65 ± 45%

Gasification

0.28 ± 50%

Pyrolysis

0.49 ± 41%

Gasification

0.13 ± 50%

Pyrolysis

0.74 ± 39%

Gasification

0.40 ± 52%

Pyrolysis

0.35 ± 40%

Gasification

0.07 ± 50%

Notes:
1

An explanation of the conversion technologies is provided in Annex 2A.2.

All values are presented in the format of the mean value ± the 95% confidence limit expressed as a percentage of the mean (that is ±1.96 * standard error /mean *100).
2

Source:FCp was calculated from the organic carbon content of biochar from regressions by Neves et al. (2011), corrected for ash content using
biochar yield from Woolf et al. (2014). Data on ash, lignin, and carbon content of biomass feedstocks, which are parameters in these
regression equations, were taken from ECN (2018). 3

2 Estimating biochar C remaining for durations of <100 years, such as 20 years, would require additional detailed information on

the chemical nature of the biochar, how it is applied, as well as climatic and edaphic properties of the location it was applied.

3 https://phyllis.nl/Home/Colophon (24/10/2018).

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Appendix4: Method for Estimating the Change in Mineral Soil Organic Carbon Stocks from Biochar Amendments:
Basis for Future Methodological Development

Production

TABLE 4AP.2
VALUES FOR 𝐅𝐅𝐩𝐩𝐩𝐩𝐩𝐩𝐩𝐩𝐩𝐩 (FRACTION OF BIOCHAR C REMAINING AFTER 100 YEARS)
Value for Fperm

1, 2

p

High temperature pyrolysis and gasification (> 600 °C)

0.89 ± 13%

Medium temperature pyrolysis (450-600 °C)

0.80 ± 11%

Low (350-450 °C)

0.65 ± 15%

Notes:
1
All values are presented in the format of the mean value ± the 95% bootstrap confidence limit expressed as a percentage of the mean(note that the bootstrap confidence intervals are symmetric about the mean to within 2 significant digits and are therefore given as ± apercentage of the mean value).

The studies used in the derivation of 𝐹𝐹𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 values included only cropland and grassland mineral soils. Thus the f 𝐹𝐹𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 values
provided are only applicable to mineral soils under those land uses, and not for forest land, settlements, other land or wetlands.

2

Sources:

Major et al. 2010; Zimmerman 2010; Singh et al. 2012; Zimmerman & Gao 2013; Fang et al. 2014; Herath et al. 2015; Kuzyakov etal. 2014; Dharmakeerthi et al. 2015; Wu et al. 2016

Background Information on Derivation of 𝐅𝐅𝐂𝐂𝐩𝐩 and 𝐅𝐅𝐩𝐩𝐩𝐩𝐩𝐩𝐩𝐩𝐩𝐩 Values

𝐹𝐹𝐶𝐶𝑝𝑝 was calculated using the organic carbon content of biochar on a dry ash-free (daf) basis according to equation 14
from Neves et al. (2011), which was based on a regression (n=128) of data from 26 papers. This daf organic carbon
content was corrected for ash content of the biochar to provide 𝐹𝐹𝐶𝐶𝑝𝑝 as the carbon content per unit mass of biochar
using the regression equation (n=146 from 18 articles) of biochar yield from Woolf et al. (2014). Data on ash (n=1276),
lignin (n=516), and carbon (n=1276) content of biomass feedstocks, which are parameters in these regression
equations, were taken from ECN (2018).
The values for Fpermp were calculated from field and laboratory studies for biochars that were made under different
conversion conditions based on a comprehensive survey of the literature. The amount of biochar C remaining after
100 years was estimated by fitting a two-pool double-exponential model to only those datasets within the list ofreferences that exceeded one year and allowed a two-pool model to be fitted following the rationale outlined by
Lehmann et al. (2015). The data included all available studies that met these stringent quality criteria (Major et al.
2010; Zimmerman 2010; Singh et al. 2012; Zimmerman & Gao 2013; Fang et al. 2014; Herath et al. 2015; Kuzyakov
et al. 2014; Dharmakeerthi et al. 2015; Wu et al. 2016). Fpermp values were adjusted to an ambient temperature of 20°C,
which is higher than current estimates of approximately 10°C average land surface temperature (Rohde et al. 2013)
and therefore conservative since decomposition increases with increasing temperature. Fpermp values were thencalculated for the three categories shown in Table 4Ap.2 as means for each of the three temperature categories (Figure
4Ap.1). Categories were preferred to using a linear regression due to the non-linear relationship between pyrolysis
temperature and biochar C persistence (Whitman et al. 2013; Bird et al. 2015). Long-term field data of naturally
accumulated char and anthropogenically added biochar with unknown production temperatures were assessed
separately as a cross-check on the results (Figure 4Ap.1) and include all available studies with periods exceeding 10
years of observation (Cheng et al. 2008; Hammes et al. 2008; Lehmann et al. 2008; Liang et al. 2008; Nguyen et al.
2008; Vasilyeva et al. 2011; Lutfalla et al. 2017). These data do not utilize isotopes or determine physical losses byleaching and erosion, and therefore do not allow actual mineralization rates to be quantified. Rather, the biochar andchar C remaining can be understood as a minimum value below which persistence will not fall over decadal tomillennial time scales.
Pyrolysis temperature is used for this methodology, as it is more easily available than biochar property measurements
for country-wide estimation. Methods could employ biochar property measurements, e.g., H/Corg (Lehmann et al.
2015) or O/Corg (Spokas 2010) ratios, together with country-specific soil properties, temperatures and moistureregimes, but these properties are not considered in this method.

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Figure 4Ap.1 Fperm calculated from field and laboratory studies for biochars that were made under different
conversion conditions: (a) Fperm estimated for biochars with known production temperatures by fitting a two-pooldouble-exponential model to 59 datasets from eight mineralization experiments that exceeded one year and allowed a
two-pool model to be fitted and adjusted to a decomposition temperature of 20°C recalculated as shown in Lehmann
et al. (2015) (Sources of data include: Major et al. 2010; Zimmerman 2010; Singh et al. 2012; Zimmerman & Gao
2013; Fang et al. 2014; Herath et al. 2015; Kuzyakov et al. 2014; Dharmakeerthi et al. 2015; Wu et al. 2016); (b)Fperm estimated for naturally occurring chars and added biochars with unknown production temperatures using 20
observations from eight long-term field assessments (decadal to millennial time scales) where physical export is notdetermined (Cheng et al. 2008; Hammes et al. 2008; Lehmann et al. 2008; Liang et al. 2008; Nguyen et al. 2008;
Vasilyeva et al. 2011; Lutfalla et al. 2017; mean residence times taken directly from the sources without recalculation).
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Dharmakeerthi RS, Hanley K, Whitman T, Woolf D & Lehmann J. (2015) Organic carbon dynamics in soils with
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Appendix4: Method for Estimating the Change in Mineral Soil Organic Carbon Stocks from Biochar Amendments:
Basis for Future Methodological Development

Herath, H. M. S. K., Camps‐Arbestain, M., Hedley, M. J., Kirschbaum, M. U. F., Wang, T. & van Hale, R. (2015)
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