Glossary Aromatic compound
Inorganic carbon
Feedstock
Labile carbon
Organic carbon
Pyrolysis
Appendix I - Biochar Characteristics (i) Biochar characteristics Molar ratios
Hydrogen to organic carbon molar ratios (H:C(org)) and oxygen to organic carbon molar ratios (O:C(org)) are useful for identifying high-permanence biochars. The lower the ratio, the more stable the final product is and the longer it will take to degrade.23 Where possible, search for H:C(org) ratios over O:C(org) ratios due to their greater accuracy.24 Puro Earth ’s and the European Biochar Certificate (EBC)’s methodology requires H:C(org) ratios of less than 0.7 and O:C(org) molar ratios less than 0.4, which is the point at which the biochar is stable enough to use in carbon storage.25
For medium permanence biochar (with a half-life of 100-1000 years under laboratory conditions), the O:Corg ratios should be between 0.2-0.6. For high permanence biochar (with a half-life greater than 1000 years under laboratory conditions), the ratio should be less than 0.2.26
Fixed carbon content
Fixed carbon content is the proportion of stable carbon in biochar, which is also important to consider. This is a measure of how much carbon the biochar can store, showing the carbon sequestration potential per tonne of biochar.27 Puro Earth requires a fixed carbon content of over 50%, but the higher this number is, the more carbon can be stored per tonne of biochar.28
(ii) Production conditions Pyrolysis temperature
Pyrolysis temperature is the temperature at which feedstock is converted to biochar. Higher temperature pyrolysis is strongly correlated to lower molar ratios, higher fixed carbon content, and greater biochar stability.29 While Puro Earth standards allow for pyrolysis temperatures ranging from 350°C to 1000 °C, higher pyrolysis temperatures indicate better quality, more stable biochar. Pyrolysis temperatures of over 500°C to 600°C are most effective in producing stable biochar; however, this is also influenced by the type of feedstock.30
Feedstock
Feedstock (the material being used to create biochar) also affects the permanence of biochar. Different feedstocks have different permanence depending on their structural stability, ash content, and H:C(org) and O:C(org) ratios.31 Feedstocks can be classified as woody (softwood, hardwood), crop waste (e.g. corn, wheat straw, and rice straw/husk), grassy, and manure (often poultry, pig, and cattle).32 Woody feedstocks tend to be the most stable, and grassy and manure feedstocks the least.33 Crop waste and grassy feedstocks are intermediates between the two but can have wide variability in their stability depending on their ash content; low ash crop wastes such as sugar cane and nut shells are more suitable for carbon storage than rice husks and straws.34
Figure 2: SD, RH, FW, PL and PS indicate biochar of sawdust, rice husk, food waste, poultry litter and paper sludge and adjacent numeric values 1, 2, 3 and 4 indicate different pyrolysis temperatures of 350 °C, 450 °C, 550 °C, 650 °C respectively.35
(iii) Post-production conditions The real-world applications of biochar can also strongly determine its stability in soils. The different geographic regions in which biochar is applied contain a diversity of soil types, moisture levels, mineral contents, and microorganism conditions.36 The decomposition rates of stable carbon remain relatively unchanged by different post-production conditions, but the unstable portion is highly affected by these. Soil type influences how quickly unstable carbon in biochar degrades, with clay soils stabilizing soil carbon faster than mineral or sandy soils.37 The mineral content of the soil also factors in, with soils high in iron and low in calcium promoting biochar stability.38 Temperature is thought to have only minimal effects on biochar decomposition rates; even under climate warming scenarios, soil temperatures are not expected to become high enough to impact carbon stability in biochar.39 However, abrupt soil moisture increases (from flooding or heavy rainfall) can rapidly increase the decomposition rate of unstable biochar; low moisture levels should be maintained where possible.40
Soil microorganisms also play a significant role in biochar decomposition; conditions that enhance microbial activity can decrease the lifespan of biochar in soils.41 High amounts of unstable carbon encourage microorganism activity, this can be remedied through high pyrolysis temperature and woodier feedstocks.42 Biochars that have a high portion of unstable carbon from low pyrolysis temperatures and manure feedstocks can therefore temporarily enhance carbon loss. Ultimately, post-production conditions matter most for biochar with high unstable carbon content, and the net benefit on soil carbon storage is thought to be positive in the long term regardless.43 Estimates of >1000-year half-life times for high-quality biochar remain accurate in temperate climates, but more research is needed on half-lives in a tropical context. However, any differences between laboratory and field estimates are likely to be greatest amongst lower quality biochars with high proportions of unstable biochar.44 More research and field trials are needed to achieve greater precision around biochar’s durability in soils.
Appendix II - Biochar Co-benefits Environmental Factors
The impact of applying biochar to soil on increasing crop yields depends on a number of environmental factors like soil type, soil pH, fertilizer inputs, and soil fertility.45 A meta-analysis on crop yields found that in tropical climates, where acidic, nutrient-poor, soils are more common, biochar elicited a 25% increase in yield , but had almost no effect in temperate climates.46 Another meta-analysis found the effects of biochar on crop yields ranging from a reduction in crop yields by 28% to increases of 39% . Their results reinforce that positive effects on crop yields are most common in soils that are either acidic or neutral, or those that have a coarse to medium texture. This suggests that biochar increases crop yields where it is able to improve the water holding capacity of the soil or produce a liming effect (where calcium and magnesium are added to decrease soil acidity and add nutrients).47 Positive effects may be further reinforced when biochar is applied alongside an inorganic fertilizer, which can increase yields by a further 10%.48
Biochar characteristics
There are two main characteristics of biochar that influence its suitability for agricultural use: feedstock and pyrolysis temperature, which determines organic carbon and mineral content, pH, and surface area.49 Feedstocks that are less suited to carbon sequestration appear more beneficial for increasing agricultural outputs, due to their high mineral and unstable carbon content. In a 2022 meta-analysis, high-nutrient, manure-based biochars resulted in the greatest crop increases; lower-nutrient woody and crop-waste biochars still provided positive effects but performed significantly lower. Lower pyrolysis temperatures (between 401°C and 500°C) also had the greatest positive effects on crop yield, with no significant effect on yield observed when pyrolysis temperatures were greater than 600 °C.50 This is because the mineral compounds that form at higher temperatures can be inaccessible for plant uptake.51
While these are helpful guidelines, additional long-term field studies on the effects of biochar in soils under different contexts are needed to address our uncertainty regarding the co-benefits of biochar purposed for carbon sequestration. The recent Biochar Research Network Act of 2022 will establish a national biochar research network to quantify the characteristics that influence biochar co-benefits and carbon sequestration and understand the trade-offs between these uses.
23 H:Corg and O:Corg molar ratios that use organic carbon content should be used instead of H:C and O:C molar ratios that measure total carbon content where possible, as they are a more reliable indicator of permanence. This is because inorganic carbon aromatic compound Leng et al. 2019 ;
24 H:Corg is considered a better measure than O:Corg as it can be measured directly, whereas O:Corg has to be derived from other measurements (O = 100 − C − H − N − S − ash) and so is often overestimated, see introduction in Klasson 2017 and section 3.1.2., Leng et al. 2019 :
25 See 1.1.6., Puro.earth 2022 ; section 7.2 &7.3, European Biochar Certificate 2022 ; Appendix 7, International Biochar Initiative 2015 ; introduction, Klasson 2017
26 See conclusion, Spokas 2010.
27 See section 4.1.1., Leng et al. 2019 .
28 See section 1.1.1., Puro Earth 2021
29 See section 3.2., McBeath et al. 2015 ; section 3.3., Enders et al. 2012
30 See ‘Carbon, hydrogen, and oxygen’ section, Ippolito et al, 2020
31 See abstract, introduction, materials and methods Chaturvedi et al. 2020 ; ‘Biomass chemical characterization’, Veiga et al. 2017 ; section 3.1., Enders et al. 2012 ;
32 See ‘Feedstock choice’ section, Ippolito et al, 2020
33 See abstract and conclusions, Hassan et al. 2020
34 See section 3.1.2., Conz et al. 2017 ; section 3.1., Leng & Huang, 2018 ; ‘The amounts and rates of biochar decomposition’, Wang et al., 2015
35 Pariyar et al, 2020
36 See ‘Future perspective’, Spokas, 2010 ; ‘Abiotic factors and processes’, Wang et al. 2015.
37 See highlights, Fang et al. 2015 ; conclusions and implications, Fang et al. 2013 .
38 See conclusion, Yang et al. 2021
39 See “Temperature sensitivity of biochar decomposition” section. Lehmann et al. 2012 .
40 See conclusion, Yang et al. 2021 .
41 See ‘Biological decomposition’ Lehmann et al. 2012 ; ‘Microorganisms and biochar stability’, Ameloot et al. 2013
42 The portion of labile (unstable) carbon in biochar is determined largely by pyrolysis temperature and feedstock type. Higher pyrolysis temperatures and feedstocks with higher lignin content (hard woods) will lead to a lower portion of labile carbon. See ‘Influence of native SOM: priming and co-metabolism’, Ameloot et al. 2013 ; section 4.1, Zimmerman et al. 2010 .
43 Biochars that have a high portion of labile carbon can enhance carbon mineralisation in soil (the process of turning the carbon in organic matter into carbon dioxide) in early stages, facilitating carbon loss. See ‘Influence of biochar production and application conditions” Ameloot et al. 2013 ; introduction & conclusion Zimmerman et al. 2010 .
44 Kuzyakov et al., 2014.
45 See section 3.2.2. Adhikari et al., 2018.
46 “Here we use a global-scale meta-analysis to show that biochar has, on average, no effect on crop yield in temperate latitudes, yet elicits a 25% average increase in yield in the tropics.” Jeffery et al., 2017 .
47 “The greatest (positive) effects with regard to soil analyses were seen in acidic (14%) and neutral pH soils (13%), and in soils with a coarse (10%) or medium texture (13%). This suggests that two of the main mechanisms for yield increase may be a liming effect and an improved water holding capacity of the soil, along with improved crop nutrient availability.” Jeffery el al., 2011.
48 “Biochar + inorganic fertilizer increased yield by an additional 10%.” Bai et al., 2022.
49 See ‘Biochar: Production and Characteristics’. Al-Wabel et al., 2017.
50 See sections 4.1 and 4.3. Bai et al., 2022.
51 “Specifically, with increasing pyrolysis temperature one typically observes increasing biochar C, P, K, Ca, ash content, pH, specific surface area (SSA), and decreasing N, H, and O content.” Ippolito et al., 2020.
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