How biochar works, and when it doesn't: a review of mechanisms controlling soil and plant responses to biochar

Studies report a wide range of plant responses to biochars due to the diversity of biochars and contexts in which biochars have been applied.

“Studies report a wide range of plant responses to biochars due to the diversity of biochars and contexts in which biochars have been applied.”

Biochar properties vary widely, determined largely by feedstock, HTT, and residence time at HTT, as well as treatments applied before and after pyrolysis.

“Biochar properties vary widely, determined largely by feedstock, HTT, and residence time at HTT, as well as treatments applied before and after pyrolysis (Schimmelpfennig & Glaser, 2012).”

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Meta-analyses found that, on average, biochars increase P availability by a factor of 4.6.

“Meta-analyses found that, on average, biochars increase P availability by a factor of 4.6;”

Biochar reactions in soil occur in three stages: dissolution (1-3 weeks), reactive surface development (1-6 months), and aging (beyond 6 months).

“We describe three stages of reactions of biochar in soil: dissolution (1-3 weeks); reactive surface development (1-6 months); and aging (beyond 6 months).”

Biochar carbon persists in soil for hundreds to thousands of years.

“Biochar carbon persists in soil for hundreds to thousands of years.”

Meta-analyses found that, on average, biochars reduce non-CO2 greenhouse gas emissions from soil by 12%-50%.

“reduce non-CO 2 greenhouse gas emissions from soil by 12%-50%.”

A review of 5400 studies found that wood-based feedstocks generally produced biochars with the highest surface area.

“A review of 5400 studies (Ippolito et al., 2020) found that wood-based feedstocks generally produced biochars with the highest surface area,”

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Biochars can catalyze biotic and abiotic reactions, particularly in the rhizosphere, that increase nutrient supply and uptake by plants.

“Biochars can catalyze biotic and abiotic reactions, particularly in the rhizosphere, that increase nutrient supply and uptake by plants, reduce phytotoxins, stimulate plant development, and increase resilience to disease and environmental stressors.”

Meta-analyses show average crop yield increases of 10%-42% with biochar addition.

“Meta-analyses show average crop yield increases of 10%-42% with biochar addition,”

Biochars can catalyze biotic and abiotic reactions, particularly in the rhizosphere, that stimulate plant development.

“Biochars can catalyze biotic and abiotic reactions, particularly in the rhizosphere, that increase nutrient supply and uptake by plants, reduce phytotoxins, stimulate plant development, and increase resilience to disease and environmental stressors.”

Crop yields increase strongly if site-specific soil constraints and nutrient and water limitations are mitigated by appropriate biochar formulations.

“Crop yields increase strongly if site-specific soil constraints and nutrient and water limitations are mitigated by appropriate biochar formulations.”

The properties of biochar and its effects within agricultural ecosystems largely depend on feedstock and pyrolysis conditions.

“The properties of biochar and its effects within agricultural ecosystems largely depend on feedstock and pyrolysis conditions.”

By increasing pH, porosity, and water availability, biochars can create favorable conditions for root development and microbial functions.

“By increasing pH, porosity, and water availability, biochars can create favorable conditions for root development and microbial functions.”

Meta-analyses found that, on average, biochars decrease plant tissue concentration of heavy metals by 17%-39%.

“decrease plant tissue concentration of heavy metals by 17%-39%;”

Meta-analyses found that, on average, biochars build soil organic carbon through negative priming by 3.8% (range -21% to +20%).

“build soil organic carbon through negative priming by 3.8% (range -21% to +20%);”

Biochars can catalyze biotic and abiotic reactions, particularly in the rhizosphere, that reduce phytotoxins.

“Biochars can catalyze biotic and abiotic reactions, particularly in the rhizosphere, that increase nutrient supply and uptake by plants, reduce phytotoxins, stimulate plant development, and increase resilience to disease and environmental stressors.”

Biochars can catalyze biotic and abiotic reactions, particularly in the rhizosphere, that increase resilience to disease and environmental stressors.

“Biochars can catalyze biotic and abiotic reactions, particularly in the rhizosphere, that increase nutrient supply and uptake by plants, reduce phytotoxins, stimulate plant development, and increase resilience to disease and environmental stressors.”

The greatest crop yield increases with biochar addition occur in low-nutrient P-sorbing acidic soils (common in the tropics).

“with greatest increases in low-nutrient P-sorbing acidic soils (common in the tropics),”

Reviews and meta-analyses show that biochar generally increases water retention.

“increases dissolved and total organic C, CEC, available nutrients, water retention, and aggregate stability;”

• cites El-Naggar et al., 2019Not checked yet

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• cites Lehmann & Joseph, 2015Not checked yet

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Reviews and meta-analyses show that biochar generally increases CEC.

“increases dissolved and total organic C, CEC, available nutrients, water retention, and aggregate stability;”

• cites El-Naggar et al., 2019Not checked yet

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• cites Lehmann & Joseph, 2015Not checked yet

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Applications of biochar-fertilizer mixes at low rates (<1 Mg ha-1 biochar) have also increased yields, particularly when applied as a band near the seed.

“however, applications of biocharfertilizer mixes at low rates (<1 Mg ha -1 biochar) have also increased yields, particularly when applied as a band near the seed (Table S1).”

Biochar can increase microbial activity.

“Biochar can increase microbial activity, accelerate nutrient cycling, and reduce leaching and volatilization of nitrogen (Lehmann & Joseph, 2015).”

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Reviews and meta-analyses show that biochar generally increases dissolved and total organic C.

“increases dissolved and total organic C, CEC, available nutrients, water retention, and aggregate stability;”

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• cites Lehmann & Joseph, 2015Not checked yet

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Biochar can reduce leaching and volatilization of nitrogen.

“Biochar can increase microbial activity, accelerate nutrient cycling, and reduce leaching and volatilization of nitrogen (Lehmann & Joseph, 2015).”

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Reviews and meta-analyses show that biochar generally lowers soil acidity.

“Reviews and meta-analyses show that biochar generally lowers soil acidity and increases buffering capacity;”

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• cites Lehmann & Joseph, 2015Not checked yet

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Biochar can accelerate nutrient cycling.

“Biochar can increase microbial activity, accelerate nutrient cycling, and reduce leaching and volatilization of nitrogen (Lehmann & Joseph, 2015).”

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Reviews and meta-analyses show that biochar generally increases aggregate stability.

“increases dissolved and total organic C, CEC, available nutrients, water retention, and aggregate stability;”

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• cites Lehmann & Joseph, 2015Not checked yet

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Studies reporting positive responses have commonly used biochar application rates of 5-20 Mg ha-1.

“Studies reporting positive responses have commonly used biochar application rates of 5-20 Mg ha -1 (Table 1);”

Reviews and meta-analyses show that biochar generally increases buffering capacity.

“Reviews and meta-analyses show that biochar generally lowers soil acidity and increases buffering capacity;”

• cites El-Naggar et al., 2019Not checked yet

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• cites Lehmann & Joseph, 2015Not checked yet

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Reviews and meta-analyses show that biochar generally reduces bulk density.

“and reduces bulk density (El-Naggar et al., 2019;Lehmann & Joseph, 2015).”

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• cites Lehmann & Joseph, 2015Not checked yet

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A review of 5400 studies found that manure feedstocks produced biochars with the highest N and P content.

“and manure feedstocks produced biochars with the highest N and P content.”

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A review of 5400 studies found that straw-based feedstocks gave the highest cation exchange capacity (CEC).

“straw-based feedstocks gave the highest cation exchange capacity (CEC),”

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Reviews and meta-analyses show that biochar generally increases available nutrients.

“increases dissolved and total organic C, CEC, available nutrients, water retention, and aggregate stability;”

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• cites Lehmann & Joseph, 2015Not checked yet

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As CEC of fresh biochar is relatively low compared with CEC of many soil components, applying biochar typically does not increase the soil CEC immediately.

“As CEC of fresh biochar is relatively low compared with CEC of many soil components, applying biochar typically does not increase the soil CEC immediately (Kharel et al., 2019).”

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Initial rapid dissolution can occur via dissolution of salts, ion exchange, submicrometer particle detachment, and preferential dissolution at crystal imperfections.

“Initial rapid dissolution can occur via dissolution of salts, ion exchange, submicrometer particle detachment, and preferential dissolution at crystal imperfections (Wang et al., 2020).”

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Except in flooded soils, oxygen will diffuse into the pores and react with redox-active organic molecules and minerals, particularly Fe and Mn.

“Except in flooded soils, oxygen will diffuse into the pores and react with redox-active organic molecules (e.g., quinones; Yu & Kuzyakov, 2021) and minerals, particularly Fe and Mn.”

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Fresh biochar typically has a low CEC, as the high temperatures during pyrolysis reduce the concentration of functional groups.

“Fresh biochar typically has a low CEC, as the high temperatures during pyrolysis reduce the concentration of functional groups (e.g., -OH, -COOH, -CH, and -C=O).”

Using methods considered suitable for biochar, CEC ranges from approximately 50 to 200 mmol kg-1.

“Using methods considered suitable for biochar, CEC ranges from approximately 50 to 200 mmol kg -1 (Graber et al., 2017;Mitchell et al., 2013),”

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• cites Mitchell et al., 2013Not checked yet

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When biochar is applied in the form of BCF, the physical and chemical reactions that occur during production of the granules slow the rate and extent of dissolution of N compounds compared with dissolution of mineral fertilizers.

“When biochar is applied in the form of BCF that combines biochar, minerals, and N and P compounds (e.g., urea, ammonium sulfate, diammonium phosphate), the physical and chemical reactions that occur during the production of the granules slow the rate and extent of dissolution of N compounds compared with dissolution of mineral fertilizers (Chen et al., 2017;Shi et al., 2020).”

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• cites Shi et al., 2020Not checked yet

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An exception is flooded rice soils, where biochar application can increase Eh due to the release of O2 from roots.

“An exception is flooded rice soils, where biochar application can increase Eh due to the release of O 2 from roots.”

Biochar can increase soil C levels by decreasing mineralization of existing soil organic matter (SOM).

“In addition, biochar can increase soil C levels by decreasing mineralization of existing soil organic matter (SOM; Wang et al., 2016) and newly added plant C (Weng et al., 2017).”

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Biochar can reduce emissions of the greenhouse gases nitrous oxide and methane.

“Furthermore, biochar can reduce emissions of the greenhouse gases (GHGs), nitrous oxide and methane (Van Zwieten, Kammann, et al., 2015).”

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The extent of changes in soil solution composition depends on the specific biochar and soil.

“The extent of changes in soil solution composition depends on the specific biochar and soil (Mukherjee & Zimmerman, 2013;Schreiter et al., 2020).”

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• cites Schreiter et al., 2020Not checked yet

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High-temperature biochars (HTT >450°C) have relatively lower levels of water-soluble compounds that are dominated by low-molecular weight acids and low-molecular weight neutrals.

“whereas high-temperature biochars (HTT >450°C) have relatively lower levels of watersoluble compounds that are dominated by low-molecular weight acids and low-molecular weight neutrals (Graber et al., 2015;Reynolds et al., 2018;Taherymoosavi et al., 2018).”

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• cites Reynolds et al., 2018Not checked yet

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• cites Taherymoosavi et al., 2018Not checked yet

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Biochar can increase soil C levels by decreasing mineralization of newly added plant C.

“In addition, biochar can increase soil C levels by decreasing mineralization of existing soil organic matter (SOM; Wang et al., 2016) and newly added plant C (Weng et al., 2017).”

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CEC of biochar is more difficult to measure than CEC of soils, due to its pH-dependent variable charge properties and the presence of soluble salts.

“CEC of biochar is more difficult to measure than CEC of soils, due to its pH-dependent variable charge properties and the presence of soluble salts (Graber et al., 2017;Munera-Echeverri et al., 2018).”

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• cites Munera-Echeverri et al., 2018Not checked yet

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Low-temperature biochars can be hydrophobic initially due to accumulation of aliphatic compounds in pores and on the surface.

“Low-temperature biochars can be hydrophobic initially due to accumulation of aliphatic compounds in pores and on the surface;”

After application to soil, water entering biochar pores dissolves soluble organic and mineral compounds on biochar outer and inner surfaces.

“After application to soil, water entering biochar pores dissolves soluble organic and mineral compounds on biochar outer and inner surfaces (Figure 1).”

Anion exchange capacity (AEC) is typically also less than 200 mmol kg-1.

“and anion exchange capacity (AEC) is typically also less than 200 mmol kg -1 (Lawrinenko et al., 2017).”

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Biochars commonly increase soil water holding capacity, particularly in coarse-textured soils.

“Biochars commonly increase soil water holding capacity, particularly in coarse-textured soils, decrease bulk density, and increase porosity, with greater effects observed at rates exceeding 40 Mg ha -1 (see Section 3; Quin et al., 2014).”

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Reported impacts of biochar on germination and seedling growth range from inhibition to stimulation.

“Reported impacts of biochar on germination and seedling growth range from inhibition to stimulation.”

The specific sensitivity of seeds of different plant species to salinity, toxins, hormone-like compounds and water availability also results in very variable results.

“The specific sensitivity of seeds of different plant species to salinity, toxins, hormone-like compounds and water availability also results in very variable results.”

This acceleration leads to oxidation not only of biochar itself but also of SOM and plant residues.

“This acceleration leads to oxidation not only of biochar itself but also of SOM and plant residues (Du et al., 2020)”

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The effects on infiltration were attributed to a biochar-related increase in soil solution Ca and decrease in Na, leading to decreased sodium adsorption ratio.

“The effects were attributed to a biochar-related increase in soil solution Ca and decrease in Na, leading to decreased sodium adsorption ratio (Abrol et al., 2016).”

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High mineral ash biochars and engineered biochars used in BCF generally contain high quantities of small mineral particles <100 μm in or on the C matrix that are easily fragmented from the biochar and are mobile in soil.

“High mineral ash biochars and engineered biochars used in BCF generally contain high quantities of small mineral particles <100 µm (especially silica, alumina, Fe/O and CaCO 3 , CaHPO 4 , and Mg compounds) in or on the C matrix that are easily fragmented from the biochar and are mobile in soil.”

Biochars commonly decrease bulk density.

“Biochars commonly increase soil water holding capacity, particularly in coarse-textured soils, decrease bulk density, and increase porosity, with greater effects observed at rates exceeding 40 Mg ha -1 (see Section 3; Quin et al., 2014).”

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Cultivation and ingestion by soil fauna result in fragmentation and fracturing, creating very small particles (approximately <100 μm).

“Cultivation and ingestion by soil fauna result in fragmentation and fracturing, creating very small particles (approximately <100 µm).”

In acid soils, excess H+ reacts with basic minerals such as calcite and dolomite present within the C lattice of the biochar.

“In acid soils, excess H + reacts with basic minerals such as calcite and dolomite present within the C lattice of the biochar (Amonette & Joseph, 2009).”

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Reduced sealing leads to lower runoff and erosion rates.

“Reduced sealing leads to lower runoff and erosion rates.”

Greater effects on water holding capacity, bulk density, and porosity are observed at rates exceeding 40 Mg ha-1.

“with greater effects observed at rates exceeding 40 Mg ha -1 (see Section 3; Quin et al., 2014).”

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Aqueous extracts of some biochars have been found to stimulate germination and seedling growth.

“Aqueous extracts of some biochars have been found to stimulate germination and seedling growth (Taek-Keun et al., 2012).”

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These biochar-related factors are the reason that biochar feedstock, production HTT, and application amount have a range of impacts on germination speed and rate.

“These biochar-related factors are the reason that biochar feedstock, production HTT, and application amount have a range of impacts on germination speed and rate.”

Other studies that applied a range of woody and manure biochars at rates of 10-40 Mg ha-1 found positive or nil effect on germination.

“Other studies that applied a range of woody and manure biochars at rates of 10-40 Mg ha -1 found positive or nil effect on germination (Das et al., 2020;Gascó et al., 2016;Khan et al., 2014;Mete et al., 2015;Van Zwieten, Kimber, Morris, Downie, et al., 2010).”

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• cites Gascó et al., 2016Not checked yet

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• cites Khan et al., 2014Not checked yet

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Biochar can impact water infiltration into soils, for example, moderating the reduction in infiltration rate that occurs during high-intensity rainstorms in soils prone to surface sealing, as seen at 2% w/w.

“Biochar can also impact water infiltration into soils, for example, moderating the reduction in infiltration rate that occurs during high-intensity rainstorms in soils prone to surface sealing, as seen at 2% w/w by Abrol et al. (2016).”

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Biochars commonly increase porosity.

“Biochars commonly increase soil water holding capacity, particularly in coarse-textured soils, decrease bulk density, and increase porosity, with greater effects observed at rates exceeding 40 Mg ha -1 (see Section 3; Quin et al., 2014).”

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By raising the pH, alkaline biochars alleviate Al and heavy metal toxicity that can reduce root growth in acidic soils.

“For example, by raising the pH, alkaline biochars alleviate Al and heavy metal toxicity that can reduce root growth in acidic soils (Lauricella et al., 2021;Shetty et al., 2020;Van Zwieten, Rose, et al., 2015).”

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• cites Shetty et al., 2020Not checked yet

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• cites Van Zwieten, Rose, et al., 2015Not checked yet

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The physical and chemical properties of biochar surfaces change significantly in Stage 2 through a range of biotic and abiotic processes that take place in the pores exposed after the rapid dissolution phase ends.

“The physical and chemical properties of biochar surfaces change significantly in Stage 2 through a range of biotic and abiotic processes that take place in the pores exposed after the rapid dissolution phase ends (Joseph et al., 2010).”

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Seed germination and early seedling development can be influenced as a result of the effects of biochar on soil physical properties.

“Seed germination and early seedling development can be influenced as a result of the effects of biochar on soil physical properties (Section 2.1.1).”

These effects typically increase with higher biochar rates.

“These effects typically increase with higher biochar rates (Obia et al., 2018).”

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At low biochar rates, low levels of free radicals could be beneficial, as reactive oxygen species can interact with plant hormones that trigger germination.

“However, at low biochar rates, low levels of free radicals could be beneficial, as reactive oxygen species can interact with plant hormones that trigger germination (Gomes & Garcia, 2013).”

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Biochars with high levels of soluble mineral compounds can also cause inhibition at high application rates.

“Biochars with high levels of soluble mineral compounds can also cause inhibition at high application rates.”

At high biochar application rates in the absence of soil (volumetrically equivalent to >40 Mg ha-1, in a petri dish), free radicals from biochar inhibit germination and seedling growth.

“At high biochar application rates in the absence of soil (volumetrically equivalent to >40 Mg ha -1 , in a petri dish), free radicals from biochar inhibit germination and seedling growth (Liao et al., 2014).”

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The effects of biochar in later periods differ from the first stage which is dominated by dissolution of compounds from biochar.

“The effects of biochar in later periods differ from the first stage which is dominated by dissolution of compounds from biochar.”

In stage 2, plant roots intercept and interact with biochar.

“In stage 2, plant roots intercept and interact with biochar.”

Chemical impacts of biochar on soils and soil water solution can also affect seed germination and early seedling development.

“Chemical impacts of biochar on soils and soil water solution can also affect seed germination and early seedling development.”

At high application rates, biochars with high levels of soluble salts could inhibit germination and seedling growth through osmotic stress.

“At high application rates, biochars with high levels of soluble salts could inhibit germination and seedling growth through osmotic stress.”

Stimulation of the gibberellin pathway contributes to the observed promotion of germination and seedling growth by wood biochar in some tomato genotypes.

“French and Iyer-Pascuzzi (2018) found evidence that stimulation of the gibberellin pathway contributes to the observed promotion of germination and seedling growth by wood biochar in some tomato genotypes.”

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Biochars contain polycyclic aromatic hydrocarbons (PAHs) that can inhibit germination at high rates.

“Biochars contain polycyclic aromatic hydrocarbons (PAHs; Gascó et al., 2016;Weidemann et al., 2018), organic pollutants formed during incomplete combustion, that can inhibit germination at high rates.”

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• cites Weidemann et al., 2018Not checked yet

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Certain soluble organic compounds released from biochars can stimulate germination and plant growth.

“Certain soluble organic compounds released from biochars can stimulate germination and plant growth (Sun, Drosos, et al., 2017).”

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Biochars containing karrikins can accelerate germination and early growth of plants.

“Kochanek et al. (2016) showed that biochars containing karrikins, a class of water-soluble organic molecules associated with plant response to fire, can accelerate germination and early growth of plants.”

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At very high rates of application (>50 Mg ha-1), biochars derived from contaminated sludges or feedstock grown in contaminated soils can release heavy metals that inhibit germination.

“At very high rates of application (>50 Mg ha -1 ), biochars derived from contaminated sludges or feedstock grown in contaminated soils can release heavy metals that inhibit germination (Das et al., 2020).”

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C:N ratio does not influence microbial metabolization of biochars.

“Note also that C:N ratio does not influence microbial metabolization of biochars (Torres-Rojas et al., 2020).”

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Recent research indicates that many of the reactions described above related to biochar occur on or in the microagglomerates.

“Recent research indicates that many of the reactions described above related to biochar occur on or in the microagglomerates.”

Some of the nutrients released from fertilizer, especially N and P, can react with the biochar pore surfaces and organo-mineral layers.

“Some of the nutrients released from fertilizer, especially N and P, can react with the biochar pore surfaces and organo-mineral layers (Haider et al., 2020;Hestrin et al., 2019;Joseph et al., 2018;Kammann et al., 2015).”

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• cites Hestrin et al., 2019Not checked yet

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Microagglomerates that form on internal and external biochar surfaces, consisting of nanoparticulate minerals bound with organic molecules, have a significant concentration of -C-O, -C=O, -COOH, or -NH functional groups.

“Microagglomerates that form on internal and external biochar surfaces, consisting of nanoparticulate minerals bound with organic molecules, have a significant concentration of -C-O, -C=O, -COOH, or -NH functional groups (Joseph et al., 2010).”

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Surfaces in nanopores bind molecules more tightly than larger pores.

“Surfaces in nanopores bind molecules more tightly than larger pores (Pignatello et al., 2017).”

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Biochar directly mediates electron transfer by functioning as an electron shuttle.

“Biochar directly mediates electron transfer by functioning as an electron shuttle”

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Adsorption of root exudates by biochar can result in additional adsorption sites for organic molecules.

“and can result in additional adsorption sites for organic molecules (Prendergast-Miller et al., 2014).”

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Biochar indirectly transfers electrons from the valence band to the conduction band in Fe minerals by generating electron-hole pairs producing reactive oxygen species by Fenton and Fenton-like reactions.

“and indirectly transfers electrons from the valence band to the conduction band in the Fe minerals by generating electron-hole pairs producing reactive oxygen species (O ⋅ - 2 , H 2 O 2 , HO ⋅ ) by Fenton and Fenton-like reactions (Yu & Kuzyakov, 2021).”

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When biochar is in contact with root hairs, in the presence of microbes, it has the capacity to store and release nutrient ions and electrons.

“Thus, when biochar is in contact with root hairs, in the presence of microbes, it has the capacity to store and release nutrient ions and electrons (Chew et al., 2020;Sun, Levin, et al., 2017).”

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• cites Sun, Levin, et al., 2017Not checked yet

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In flooded paddy soils, biochar and BCF particles can be encapsulated in an organo-mineral layer on the root surface.

“In flooded paddy soils, biochar and BCF particles can be encapsulated in an organo-mineral layer (Chew et al., 2020) on the root surface.”

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The change in root membrane potential can facilitate uptake of nutrients when required by the plant.

“The change in root membrane potential can facilitate uptake of nutrients when required by the plant.”

BCF attached to the root or located in the rhizosphere of rice grown in flooded soils significantly alters the pH and Eh around the root.

“BCF attached to the root or located in the rhizosphere of rice grown in flooded soils was observed to significantly alter the pH and Eh around the root”

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P availability can be low in Ca-rich and K-poor feedstocks such as sewage sludge because pyrolysis can convert plant-available organic P into inorganic P that is less available in the short term.

“However, P availability can be low in Ca-rich and K-poor feedstocks such as sewage sludge (Buss et al., 2018(Buss et al., , 2020;;Torres-Rojas et al., 2020;Wang et al., 2019) because pyrolysis can convert plant-available organic P into inorganic P that is less available in the short term (Buss et al., 2020;Rose et al., 2019).”

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• cites Torres-Rojas et al., 2020Not checked yet

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• cites Wang et al., 2019Not checked yet

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Adsorption of root exudates by biochar may cause dissolution of mineral compounds in biochar pores.

“Adsorption of root exudates by biochar may cause dissolution of mineral compounds in biochar pores (Wang et al., 2020)”

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Much of the N within the biochar C matrix (e.g., heterocyclic-N) is unavailable to plants.

“Much of the N within the biochar C matrix (e.g., heterocyclic-N) is unavailable to plants (Clough et al., 2013;Torres-Rojas et al., 2020),”

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• cites Torres-Rojas et al., 2020Not checked yet

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The effect of biochar on P availability is determined by microscale effects on soil pH and soil solution composition, especially Ca content.

“The effect of biochar on P availability is determined by microscale effects on soil pH and soil solution composition, especially Ca content (Buss, Assavavittayanon, et al., 2018;Buss et al., 2018).”

• cites Buss, Assavavittayanon, et al., 2018Not checked yet

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• cites Buss et al., 2018Not checked yet

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Dissolution of mineral compounds in biochar pores can increase nutrient availability.

“which can increase nutrient availability”

Biochar addition to soils resulted in average decreases in plant tissue concentrations of Cd, Pb, Cu, and Zn by 38%, 39%, 25%, and 17%, respectively.

“A meta-analysis found biochar addition to soils resulted in average decreases in plant tissue concentrations of Cd, Pb, Cu, and Zn by 38%, 39%, 25%, and 17%, respectively (Chen et al., 2018).”

• cites Chen et al., 2018Not checked yet

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Surface O-functional groups on biochar can immobilize heavy metals through ion exchange, precipitation, cation and anion metal attraction, reduction, electron shuttling, and physisorption.

“The surface O-functional groups on biochar can immobilize heavy metals through ion exchange, precipitation, cation and anion metal attraction, reduction, electron shuttling, and physisorption (Figure 2; Ahmad et al., 2014;Ding et al., 2014;Liu, Xu, et al., 2018;Tan et al., 2015;Zheng et al., 2020).”

• cites Ahmad et al., 2014Not checked yet

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• cites Ding et al., 2014Not checked yet

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• cites Liu, Xu, et al., 2018Not checked yet

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High-temperature willow biochar adsorbs heavy metals from sewage sludge through physisorption and other mechanisms.

“High-temperature willow biochar was found to adsorb heavy metals from sewage sludge through both physisorption and the mechanisms described above (Bogusz et al., 2019).”

• cites Bogusz et al., 2019Not checked yet

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The optimal rate for disease suppression does not always coincide with the optimum rate for growth response.

“However, the optimal rate for disease suppression does not always coincide with the optimum rate for growth response.”

Priming effect has been observed for abiotic environmental pressures such as salt, heat, cold, toxins, and drought.

“This effect has been observed also for abiotic environmental pressures such as salt, heat, cold, toxins, and drought (Ton & Maunch-Mani, 2003).”

• cites Ton & Maunch-Mani, 2003Not checked yet

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Many studies have shown that biochar can reduce uptake of heavy metal(loid)s by plants.

“Many studies have shown that biochar can reduce uptake of heavy metal(loid)s by plants.”

Plants in a primed state display faster and stronger activation of cellular defense responses upon encountering biotic stresses.

“Plants in a primed state display faster and stronger activation of cellular defense responses, such as earlier oxidative burst and stronger upregulation of defense genes, upon encountering biotic stresses (Conrath et al., 2006).”

• cites Conrath et al., 2006Not checked yet

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Stable precipitates formed in biochars with high P can immobilize Pb through formation of β-Pb9(PO4)6.

“Stable precipitates formed in biochars with high P can immobilize Pb through the formation of β-Pb 9 (PO 4 ) 6”

• cites Cao & Harris, 2010Not checked yet

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• cites Li et al., 2016Not checked yet

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Engineering biochars by adding magnetite nanoparticles can increase AEC and thus adsorb As.

“Engineering biochars through adding magnetite nanoparticles can increase AEC and thus adsorb As (Wan et al., 2020).”

• cites Wan et al., 2020Not checked yet

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Release of Si from biochar, especially straw and rice husk biochars, increases disease resistance and plant growth by suppression of initial infection and pathogen access to plant tissues.

“release of Si from biochar (especially straw and rice husk biochars), reported to increase disease resistance and plant growth (Wang, Wang, et al., 2019) by suppression of initial infection and pathogen access to plant tissues”

• cites Wang, Wang, et al., 2019Not checked yet

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Even feedstocks that contain high contents of heavy metals can reduce the bioavailability of some heavy metals in some soils.

“Even feedstocks that contain high contents of heavy metals can reduce the bioavailability of some heavy metals in some soils.”

Higher alkalinity and calcite in biochar facilitate formation of insoluble hydrocerussite Pb3(CO3)2(OH)2.

“whereas higher alkalinity and calcite in biochar facilitate the formation of insoluble hydrocerussite Pb 3 (CO 3 ) 2 (OH) 2 (Cao & Harris, 2010;Li et al., 2016).”

• cites Cao & Harris, 2010Not checked yet

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• cites Li et al., 2016Not checked yet

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Adsorption by biochar of extracellular pathogenic enzymes and toxins lowers their concentrations in the root zone.

“adsorption by biochar of extracellular pathogenic enzymes and toxins (released by soil pathogens to dissolve and poison roots) lowering their concentrations in the root zone (Jaiswal, Frenkel, et al., 2018)”

• cites Jaiswal, Frenkel, et al., 2018Not checked yet

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Biochar can elicit systemic resistance in plants against diseases in numerous pathosystems.

“Besides the impacts of biochar on plant growth and development, it has been observed in numerous pathosystems that biochar can elicit systemic resistance in plants against diseases (Frenkel et al., 2017).”

• cites Frenkel et al., 2017Not checked yet

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Chemolithotroph bacteria can grow on surfaces of microagglomerates of clay and Fe nanoparticles and make S and Fe more available to plants.

“Chemolithotroph bacteria can grow on the surfaces of microagglomerates of clay and Fe nanoparticles and make S and Fe more available to plants (Ye et al., 2017).”

• cites Ye et al., 2017Not checked yet

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Biochar induces systemic acquired resistance through upregulation of genes and pathways associated with plant defense and growth.

“induced systemic acquired resistance through upregulation of genes and pathways associated with plant defense and growth (Jaiswal et al., 2020)”

• cites Jaiswal et al., 2020Not checked yet

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Studies showing significant reduction in bioavailability of heavy metals have often applied high rates of biochar, in excess of 10 Mg ha -1.

“Studies showing significant reduction in bioavailability of heavy metals have often applied high rates of biochar, in excess of 10 Mg ha -1 (Chen et al., 2018;Wang et al., 2020).”

• cites Chen et al., 2018Not checked yet

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• cites Wang et al., 2020Not checked yet

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Biochar adsorbs and deactivates plant signaling molecules that induce germination of parasitic weed seeds.

“adsorption and deactivation of plant signaling molecules that induce germination of parasitic weed seeds (Eizenberg et al., 2017)”

• cites Eizenberg et al., 2017Not checked yet

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The impact of biochar on plant disease is a function of biochar dose and type.

“The impact of biochar on plant disease is a function of biochar dose and type (physical/chemical characteristics, as discussed above; Frenkel et al., 2017;Poveda et al., 2021;Rogovska et al., 2017).”

• cites Frenkel et al., 2017Not checked yet

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• cites Poveda et al., 2021Not checked yet

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• cites Rogovska et al., 2017Not checked yet

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Biochar in the growing medium can prime plants for rapid upregulation of defense-related genes.

“Biochar in the growing medium can “prime” plants (Ton & Maunch-Mani, 2003) for rapid upregulation of defense-related genes (Elad et al., 2010;Jaiswal et al., 2014Jaiswal et al., , 2015Jaiswal et al., , 2017Jaiswal et al., , 2020;;Jaiswal, Elad, et al., 2018;Kolton et al., 2017;Kumar et al., 2021;Mehari et al., 2015;Meller Harel et al., 2012).”

• cites Ton & Maunch-Mani, 2003Not checked yet

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• cites Elad et al., 2010Not checked yet

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• cites Jaiswal et al., 2014Not checked yet

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Particles on the surface of biochars consisting of carbon-coated minerals are particularly effective in reducing bioavailability of heavy metals.

“Particles on the surface of biochars consisting of carboncoated minerals are particularly effective in reducing bioavailability of heavy metals (Kumar & Prasad, 2018).”

• cites Kumar & Prasad, 2018Not checked yet

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Biochar can increase mobility of anionic metalloids such as As through a decrease in positively charged sites, which decreases binding sites for As as soil pH increases.

“Biochar can increase the mobility of anionic metalloids such as As (e.g., AsO 3 - 4 , AsO 3 - 3 ; Igalavithana et al., 2017) through a decrease in positively charged sites, which decreases the binding sites for As as soil pH increases (Vithanage et al., 2017).”

• cites Igalavithana et al., 2017Not checked yet

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• cites Vithanage et al., 2017Not checked yet

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Aggregation can protect biochar and newly added organic matter, stabilizing new C for long periods in soil.

“Aggregation can protect biochar and newly added organic matter, stabilizing new C for long periods in soil.”

Rates that are beneficial for plant growth in non-diseased systems can result in disease promotion in pathogen-infected systems.

“Rates that are beneficial for plant growth in non-diseased systems can result in disease promotion in pathogen-infected systems (Jaiswal et al., 2015).”

• cites Jaiswal et al., 2015Not checked yet

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A meta-analysis reported an increase in crop yield over multiple years after a single biochar application where fertilizer was applied.

“In their meta-analysis, Ye et al. (2020) reported an increase in crop yield over multiple years after a single biochar application, where fertilizer was applied.”

• cites Ye et al., 2020Not checked yet

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Porous organomineral heterogeneous microagglomerates with mineral phases including Fe, Al, Si oxides, phosphates, carbonates, and chlorides, and dimensions from 1 to 50 nm, form and bond to the biochar surface.

“Two studies identified the formation of porous organomineral heterogeneous microagglomerates with mineral phases consisting of Fe, Al, Si oxides, phosphates (Ca/Fe/ Al), carbonates (Ca/Mg), and chlorides (K, Na), and dimensions from 1 to 50 nm, bound together by organic compounds and bonded to the biochar surface (Archanjo et al., 2017;Rafiq et al., 2020).”

• cites Archanjo et al., 2017Not checked yet

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• cites Rafiq et al., 2020Not checked yet

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Colloidal aged biochar particles may be more mobile in soil than fresh biochar.

“Colloidal aged biochar particles consisting of microagglomerates and fragments of the C matrix may be more mobile in soil than fresh biochar (Wang, Zhang, et al., 2019).”

• cites Wang, Zhang, et al., 2019Not checked yet

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Improved pepper plant productivity in biochar-treated plots under extreme environmental pressures was attributed to biochar-elicited acclimation responses.

“Improved pepper plant productivity in biochar-treated plots in a multi-year trial conducted under extreme environmental pressures (high evaporation demand and vapor pressure deficit, high daytime temperatures (heat stress) at planting and low nighttime temperatures at fruiting, brackish water irrigation) was attributed to biochar-elicited acclimation responses in the plants (Kumar, Elad, et al., 2”

• cites Kumar, Elad, et al., 2018Not checked yet

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A substantial fraction of biochar in Terra Preta soils remained in particulate form, protected by Fe and Al oxides.

“Observations of Terra Preta soils identified that a substantial fraction of the biochar remained in particulate form, protected by Fe and Al oxides (Glaser et al., 2000).”

• cites Glaser et al., 2000Not checked yet

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Disturbance through cultivation, wetting-drying and freeze-thaw cycles, and ingestion by soil fauna can lead to fragmentation of biochar particles and oxidation of biochar surfaces.

“Disturbance through cultivation, exposure to wetting-drying and freeze-thaw cycles, and ingestion by soil fauna can lead to further fragmentation of biochar particles and oxidation of biochar surfaces exposed through detachment of microagglomerates (Wang et al., 2020).”

• cites Wang et al., 2020Not checked yet

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Aged high-temperature wood biochar particles retain plant-available N as nitrates and ammonium adsorbed onto organo-mineral microagglomerates.

“Aged high-temperature wood biochar particles retain plant-available N as nitrates and ammonium, adsorbed onto the organo-mineral microagglomerates (Haider et al., 2020).”

• cites Haider et al., 2020Not checked yet

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Reduction in uptake of Cd and Pb from a highly contaminated soil was sustained over 3 years after a single application of wheat straw biochar.

“For example, the reduction in uptake of Cd and Pb from a highly contaminated soil was sustained over 3 years after a single application of wheat straw biochar (Bian et al., 2014).”

• cites Bian et al., 2014Not checked yet

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Biochar particles can be protected within soil microaggregates.

“Biochar particles can also be protected within the soil microaggregates (Figure 3).”

Simultaneous occurrence of Fe(II) and Fe(III) as magnetite and hematite could make N and P more available through redox cycling of Fe.

“Simultaneous occurrence of Fe(II) and Fe(III) present as magnetite and hematite could make N and P more available through redox cycling of Fe (Haider et al., 2020).”

• cites Haider et al., 2020Not checked yet

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Tests with heat stress and biochar in Arabidopsis indicated early microstresses primed plants to cope better with subsequent acute heat stress.

“Tests with heat stress and biochar in Arabidopsis indicated early microstresses primed the plants to cope better with subsequent acute heat stress.”

Bioavailability of heavy metals can increase or decrease as biochar ages in soil.

“The bioavailability of heavy metals has been observed to increase or decrease as biochar ages in soil (Wang et al., 2020).”

• cites Wang et al., 2020Not checked yet

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Terra Preta soils provide evidence of very long-term reactions of biochar in soil.

“The biochar-enriched anthropogenic Terra Preta soils associated with pre-Columbian settlements in the Brazilian Amazon (Steiner et al., 2009) provide evidence of very long-term reactions of biochar in soil.”

• cites Steiner et al., 2009Not checked yet

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• AssumptionRests on an assumption: Terra Preta soils are valid analogues for long-term aging of modern biochar.

Application of biochar to contaminated soils could reduce heavy metal bioavailability via direct interactions and indirect interactions that immobilize heavy metals through modification of soil properties.

“Application of biochar to contaminated soils could reduce heavy metal bioavailability via (1) direct interactions between biochar and heavy metals, and (2) indirect interactions that immobilize heavy metals through modification of soil properties (see Section 2.2.3)”

Rasse et al. (2017) found a similar rate of decomposition of Miscanthus biochar between laboratory and field conditions over a 90 day incubation period.

“although Rasse et al. (2017) found a similar rate of decomposition of Miscanthus biochar between laboratory and field conditions over a 90 day incubation period.”

• cites Rasse et al., 2017Not checked yet

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Increase in soil CEC following biochar application can reduce bioavailability of cationic heavy metals.

“Increase in soil CEC following biochar application can also reduce the bioavailability of cationic heavy metals (Mohamed et al., 2017).”

• cites Mohamed et al., 2017Not checked yet

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Biochar increased P availability on average by a factor of 4.6.

“A meta-analysis by Glaser and Lehr (2019) found availability of P increased on average by a factor of 4.6 in response to biochar application.”

• cites Glaser and Lehr, 2019Not checked yet

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Selecting appropriate biochar type suited to soil properties, plant type, and specific heavy metal can result in effective remediation while safeguarding food quality.

“Therefore, selecting the appropriate biochar type to address heavy metal contamination, suited to the soil properties, type of plant, and specific heavy metal, can result in effective remediation while safeguarding food quality.”

This response most likely reflects physicochemical and microbial changes that improve soil health as biochar ages, rather than a simple soil pH response.

“This response most likely reflects the physicochemical and microbial changes that improve soil health as biochar ages (Section 2.3), rather than a simple soil pH response.”

Due to high persistence, biochar can contribute significantly to long-term C sequestration.

“Due to this high persistence, biochar can contribute significantly to long-term C sequestration (Lehmann, 2007).”

• cites Lehmann, 2007Not checked yet

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Remediation efficacy of biochar for heavy metal contamination depends on type of biochar, biogeochemical properties of soil, plant species, and specific heavy metal.

“the remediation efficacy depends on the type of biochar, biogeochemical properties of soil, plant species, and the specific heavy metal (Albert et al., 2020;Palansooriya et al., 2020).”

• cites Albert et al., 2020Not checked yet

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• cites Palansooriya et al., 2020Not checked yet

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In two studies using BCF, N partial factor productivity increased by 37%-74%.

“Similarly, in two studies using BCF, N partial factor productivity increased by 37%-74% (Joseph, Graber, et al., 2013;Qian et al., 2014).”

• cites Joseph, Graber, et al., 2013Not checked yet

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• cites Qian et al., 2014Not checked yet

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Mean residence times calculated from incubations are around 10 times lower than under field conditions.

“Kuzyakov et al. (2009) suggested that mean residence times calculated from incubations (Table 2), which maintain optimal conditions for decomposition, are around 10 times lower than under field conditions (Kuzyakov et al., 2009)”

• cites Kuzyakov et al., 2009Not checked yet

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Grass and straw biochars increase water-holding capacity to a greater extent than woody biochars.

“Grass and straw biochars increase water-holding capacity to a greater extent than woody biochars (Burrell et al., 2016;Kroeger et al., 2020).”

• cites Burrell et al., 2016Not checked yet

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• cites Kroeger et al., 2020Not checked yet

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Biochar application can alter soil Eh, impacting speciation, mobility, and bioavailability of anionic heavy metalloids such as As.

“Biochar application can also alter soil Eh, impacting the speciation, mobility, and bioavailability of anionic heavy metalloids such as As (Yuan et al., 2017).”

• cites Yuan et al., 2017Not checked yet

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The very slow decomposition of biochar compared to unpyrolyzed biomass is attributed to its aromatic structure resulting from chemical transformations during carbonization.

“The very slow decomposition of biochar in comparison to unpyrolyzed biomass is attributed to its aromatic structure, which results from chemical transformations of biomass during carbonization.”

The majority of C in biochar has a mean residence time in the range of hundreds and thousands of years.

“Unlike other forms of biomass that are rapidly decomposed in soil, the majority of C in biochar has a mean residence time in the range of hundreds and thousands of years (Schmidt et al., 2011;Wang et al., 2016).”

• cites Schmidt et al., 2011Not checked yet

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• cites Wang et al., 2016Not checked yet

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Negative effects on yields can result from reduction in soil N and P availability, especially at high rates of high temperature biochars.

“Negative effects can result from reduction in soil N and P availability (Nielsen et al., 2014;Prommer et al., 2014) especially at high rates of high temperature biochars (Kammann et al., 2015) through binding mechanisms described in Section 2.2.”

• cites Nielsen et al., 2014Not checked yet

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• cites Prommer et al., 2014Not checked yet

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• cites Kammann et al., 2015Not checked yet

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The kinetics of formation of the fused aromatic C structure depend on rate of heating, ratio of lignin to cellulose and hemicellulose, time at HTT, and mineral content.

“The kinetics of formation of the fused aromatic C structure depend on the rate of heating, the ratio of lignin to cellulose and hemicellulose, time at the HTT, and mineral content (Budai et al., 2014;Leng & Huang, 2018;Rawal et al., 2016).”

• cites Budai et al., 2014Not checked yet

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• cites Leng & Huang, 2018Not checked yet

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• cites Rawal et al., 2016Not checked yet

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Yield responses were greater in the third year after a single application when fertilizer was applied with biochar.

“Importantly, Ye et al. (2020) identified that yield responses were greater in the third year after a single application, when fertilizer was applied with the biochar.”

• cites Ye et al., 2020Not checked yet

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Heavy metals may be present in biochar produced from feedstocks such as sewage sludge and treated timber.

“Heavy metals may be present in biochar produced from feedstocks such as sewage sludge and treated timber.”

Movement of biochar through the soil profile can increase persistence in some soil types.

“The movement of biochar through the soil profile can increase persistence in some soil types (Singh et al., 2015).”

• cites Singh et al., 2015Not checked yet

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The largely amorphous structure of biochars pyrolyzed at temperatures in excess of 400-450°C has been found to be persistent.

“The largely amorphous structure of biochars pyrolyzed at temperatures in excess of 400-450°C has been found to be persistent.”

Minerals present in biochar, especially Si and P, can increase persistence of biochar-C.

“Minerals present in biochar, especially Si and P, can increase persistence of biochar-C (Xu et al., 2017).”

• cites Xu et al., 2017Not checked yet

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Biochar can lower denitrification by decreasing total denitrification activity.

“and (ii) decreasing total denitrification activity (Cayuela et al., 2013;Weldon et al., 2019).”

• cites Cayuela et al., 2013Not checked yet

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• cites Weldon et al., 2019Not checked yet

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• AssumptionRests on an assumption: Microbiological N transformations are the main source of N2O emissions from soil, with autotrophic nitrification and heterotrophic denitrification being the main N2O formation pathways.

Biochar can lower denitrification by facilitating the last step of denitrification, the transformation of N2O to N2.

“Biochar can lower denitrification (the reduction of NO - 3 to N 2 ) by: (i) facilitating the last step of denitrification (the transformation of N 2 O to N 2 )”

• cites Cayuela et al., 2013Not checked yet

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• cites Weldon et al., 2019Not checked yet

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• AssumptionRests on an assumption: Microbiological N transformations are the main source of N2O emissions from soil, with autotrophic nitrification and heterotrophic denitrification being the main N2O formation pathways.

N2O can be transformed to NH3, pyridine, or pyrrole compounds on biochar surfaces, thus decreasing N2O emissions.

“It has also been shown that N 2 O can be transformed to NH 3 , pyridine, or pyrrole compounds on biochar surfaces, thus decreasing N 2 O emissions (Quin et al., 2015).”

• cites Quin et al., 2015Not checked yet

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• AssumptionRests on an assumption: Microbiological N transformations are the main source of N2O emissions from soil, with autotrophic nitrification and heterotrophic denitrification being the main N2O formation pathways.

The decrease in denitrification may result from a decrease in availability of nitrate and bioavailable C substrate.

“The decrease in denitrification may result from decrease in availability of NO 3 -and bioavailable C substrate (Fiorentino et al., 2019;Hagemann, Kammann, et al., 2017;Heaney et al., 2020).”

• cites Fiorentino et al., 2019Not checked yet

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• cites Hagemann, Kammann, et al., 2017Not checked yet

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• cites Heaney et al., 2020Not checked yet

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• AssumptionRests on an assumption: Microbiological N transformations are the main source of N2O emissions from soil, with autotrophic nitrification and heterotrophic denitrification being the main N2O formation pathways.

In a subtropical pasture, a 13C-depleted hardwood biochar initiated positive priming up to 0.15 Mg C ha -1 over 62 days, switching to negative priming after 188 days in presence of plants.

“In a subtropical pasture on a Rhodic Ferralsol, a 13 C-depleted hardwood biochar (450°C) initiated positive priming up to 0.15 Mg C ha -1 over 62 days, switching to negative priming after 188 days in the presence of plants (Weng et al., 2015).”

• cites Weng et al., 2015Not checked yet

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Abiotic processes, in particular with biochars containing high Fe and Mn content, can directly catalyze the reduction of N2O to N2.

“Abiotic processes, in particular with biochars containing high Fe and Mn content (see Section 2.2.1), can directly catalyze the reduction of N 2 O to N 2 .”

• AssumptionRests on an assumption: Microbiological N transformations are the main source of N2O emissions from soil, with autotrophic nitrification and heterotrophic denitrification being the main N2O formation pathways.

There is solid evidence that biochar can mitigate soil N2O and CH4 emissions from soil, at least in the short and medium term.

“there is solid evidence that biochar can mitigate soil N 2 O and CH 4 emissions from soil, at least in the short and medium term”

• cites Borchard et al., 2019Not checked yet

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• cites Cayuela et al., 2014Not checked yet

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• cites Cayuela et al., 2015Not checked yet

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• AssumptionRests on an assumption: Microbiological N transformations are the main source of N2O emissions from soil, with autotrophic nitrification and heterotrophic denitrification being the main N2O formation pathways.
• AssumptionRests on an assumption: Nonflooded soils mostly act as a sink of atmospheric CH4, while paddy soils can be a significant source of CH4.

The elemental ratio H/C org has been identified as a simple and reliable parameter for characterizing biochar persistence.

“The elemental ratio of hydrogen to organic C expressed as H/C org has been identified as a simple and reliable parameter for characterizing biochar persistence and recommendations for conservative thresholds have been provided (Budai et al., 2013).”

• cites Budai et al., 2013Not checked yet

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Further heat converts the C matrix to a highly persistent three-dimensional nanographitic structure at around 600°C.

“Further heat converts the C matrix to a highly persistent three-dimensional nanographitic structure at around 600°C (McDonald-Wharry et al., 2016).”

• cites McDonald-Wharry et al., 2016Not checked yet

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In a 6-year field experiment, SOC stocks increased by 14 Mg C ha -1 , twice the quantity of C added in biochar, as a result of negative priming.

“In a 6-year field experiment where woody biochar was applied to corn and bioenergy crops, SOC stocks increased by 14 Mg C ha -1 , twice the quantity of C added in the biochar, as a result of negative priming (Blanco-Canqui et al., 2020).”

• cites Blanco-Canqui et al., 2020Not checked yet

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There is some evidence that biochar persistence decreases as ambient temperature increases.

“There is some evidence that biochar persistence decreases as ambient temperature increases (Fang et al., 2017).”

• cites Fang et al., 2017Not checked yet

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Biochar can facilitate the reduction of N2O to N2 by changing the relative abundance and composition of N2O-reducing microbial communities.

“changing the relative abundance and composition of N 2 O-reducing microbial communities (Harter, Weigold, et al., 2016)”

• cites Harter, Weigold, et al., 2016Not checked yet

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• AssumptionRests on an assumption: Microbiological N transformations are the main source of N2O emissions from soil, with autotrophic nitrification and heterotrophic denitrification being the main N2O formation pathways.

Temperature thresholds identified in transformation processes can indicate persistence.

“Temperature thresholds identified in the transformation processes can indicate persistence.”

Early meta-analyses on N2O emissions showed very high mitigation, around 50% reductions, of N2O with biochar.

“Early meta-analyses on N 2 O emissions showed very high mitigation (around 50% reductions) of N 2 O with biochar”

• cites Cayuela et al., 2014Not checked yet

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• cites Cayuela et al., 2015Not checked yet

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• AssumptionRests on an assumption: Microbiological N transformations are the main source of N2O emissions from soil, with autotrophic nitrification and heterotrophic denitrification being the main N2O formation pathways.

P and K are fully retained in biochar at typical pyrolysis temperatures of 300°-700°.

“Of the main plant nutrients, P and K are fully retained in biochar at typical pyrolysis temperatures (300°-700°) (Bridle & Pritchard, 2004;Buss et al., 2016).”

• cites Bridle & Pritchard, 2004Not checked yet

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• cites Buss et al., 2016Not checked yet

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There is strong evidence that biochar amendment reduces, on average, direct N2O emissions from soil, particularly when compared to other organic amendments.

“there is strong evidence that biochar amendment reduces (on average) direct N 2 O emissions from soil particularly when compared to other organic amendments.”

• AssumptionRests on an assumption: Microbiological N transformations are the main source of N2O emissions from soil, with autotrophic nitrification and heterotrophic denitrification being the main N2O formation pathways.

Qian et al. (2014) found a decrease in N2O and CH4 emissions from paddy soil when a range of biochar-based BCFs was compared with NPK fertilizers.

“Qian et al. (2014) found a decrease in N 2 O and CH 4 emissions from paddy soil when a range of biochar-based BCFs was compared with NPK fertilizers.”

• cites Qian et al., 2014Not checked yet

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• AssumptionRests on an assumption: Nonflooded soils mostly act as a sink of atmospheric CH4, while paddy soils can be a significant source of CH4.

The effect of biochar on N2O emissions might diminish with time as biochar ages in soil.

“The effect of biochar might diminish with time, as biochar ages in soil (Borchard et al., 2019;Fungo et al., 2017;Liu, Zhang, et al., 2018).”

• cites Borchard et al., 2019Not checked yet

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• cites Fungo et al., 2017Not checked yet

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• cites Liu, Zhang, et al., 2018Not checked yet

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• AssumptionRests on an assumption: Microbiological N transformations are the main source of N2O emissions from soil, with autotrophic nitrification and heterotrophic denitrification being the main N2O formation pathways.

Xia et al. (2018) found an average increase of 22% in N2O emissions when straw was applied.

“Xia et al. (2018) found an average increase of 22% in N 2 O emissions when straw was applied.”

• cites Xia et al., 2018Not checked yet

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• AssumptionRests on an assumption: Microbiological N transformations are the main source of N2O emissions from soil, with autotrophic nitrification and heterotrophic denitrification being the main N2O formation pathways.

A meta-analysis focusing on vegetable fields in China found that biochar application decreased yield-scaled N2O emissions by an average of 35%.

“A meta-analysis focusing on vegetable fields in China also found that biochar application decreased yieldscaled N 2 O emissions by an average of 35% (Gu et al., 2020).”

• cites Gu et al., 2020Not checked yet

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Most organic contaminants are destroyed with high efficiency during pyrolysis, by thermal degradation and volatilization followed by destruction during vapor combustion.

“most organic contaminants are destroyed with high efficiency during pyrolysis, by thermal degradation and volatilization followed by destruction during vapor combustion.”

• cites Zielińska & Oleszczuk, 2015Not checked yet

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• cites Bridle et al., 1990Not checked yet

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• cites Kundu et al., 2021Not checked yet

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Some sewage sludge biochars contain as much as 6%-20% total P.

“some sewage sludge biochars contain as much as 6%-20% total P (Faria et al., 2018;Roberts et al., 2017;Shepherd et al., 2016;Zhang et al., 2015).”

• cites Faria et al., 2018Not checked yet

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• cites Roberts et al., 2017Not checked yet

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• cites Shepherd et al., 2016Not checked yet

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A dose of 10-20 Mg ha-1 has been found to significantly reduce N2O emissions.

“A dose of 10-20 Mg ha -1 has been found to significantly reduce N 2 O emissions (Borchard et al., 2019;Cayuela et al., 2014).”

• cites Borchard et al., 2019Not checked yet

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• cites Cayuela et al., 2014Not checked yet

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• AssumptionRests on an assumption: Microbiological N transformations are the main source of N2O emissions from soil, with autotrophic nitrification and heterotrophic denitrification being the main N2O formation pathways.

Biochar can support the development of a circular economy at regional and farm scale by improving nutrient recovery and nutrient use efficiency.

“Biochar can support the development of a circular economy at regional and farm scale by improving nutrient recovery and nutrient use efficiency.”

• AssumptionRests on an assumption: The economic case for biochar production is strongest for biochar made from residue materials, especially when the residues contain high concentrations of nutrients, such as animal manures and sewage sludge.

A meta-analysis on manure application to soil found an average increase of 33% in N2O emissions compared to synthetic fertilizer.

“a meta-analysis on manure application to soil found an average increase of 33% in N 2 O emissions compared to synthetic fertilizer (Zhou, Zhu, et al., 2017).”

• cites Zhou, Zhu, et al., 2017Not checked yet

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• AssumptionRests on an assumption: Microbiological N transformations are the main source of N2O emissions from soil, with autotrophic nitrification and heterotrophic denitrification being the main N2O formation pathways.

Biochars produced by slow pyrolysis, with high degree of carbonization, high pH, and high surface area, are most effective in suppressing N2O emissions.

“Biochars produced by slow pyrolysis, with high degree of carbonization, high pH, and high surface area, are most effective in suppressing N 2 O emissions (Borchard et al., 2019;Cayuela et al., 2015;Weldon et al., 2019).”

• cites Borchard et al., 2019Not checked yet

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• cites Cayuela et al., 2015Not checked yet

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• cites Weldon et al., 2019Not checked yet

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• AssumptionRests on an assumption: Microbiological N transformations are the main source of N2O emissions from soil, with autotrophic nitrification and heterotrophic denitrification being the main N2O formation pathways.

Biochar systems contribute to building a circular economy.

“Biochar systems (Figure 5) thereby contribute to building a circular economy.”

• AssumptionRests on an assumption: The economic case for biochar production is strongest for biochar made from residue materials, especially when the residues contain high concentrations of nutrients, such as animal manures and sewage sludge.
• AssumptionRests on an assumption: Pyrolysis gases can be captured for use as a renewable energy product.

Soil and plant responses to addition of biochar can be negative, positive, or neutral, depending on many variables, including feedstock and pyrolysis temperature, application rate and method, and application context.

“Soil and plant responses to addition of biochar can be negative, positive, or neutral, depending on many variables, including feedstock and pyrolysis temperature, application rate and method, and application context (crop, soil type, and environmental and biological stresses).”

Significant yield increases occur where site-specific soil constraints, nutrient and water limitations are addressed by appropriate biochar formulations applied at an optimal application rate.

“Significant yield increases occur where site-specific soil constraints, nutrient and water limitations are addressed by appropriate biochar formulations applied at an optimal application rate.”

Controlled release biochar-fertilizer combinations can be effective at low application rates when applied in a band near the seed/plant.

“and these can be effective at low application rates when applied in a band near the seed/plant (Qian et al., 2014;Schmidt et al., 2015;Yao et al., 2015;Zheng et al., 2017).”

• cites Qian et al., 2014Not checked yet

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• cites Schmidt et al., 2015Not checked yet

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• cites Yao et al., 2015Not checked yet

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The microscale processes on the biochar surface and in the rhizosphere mediate the macro responses of plants to biochar.

“The microscale processes on the biochar surface and in the rhizosphere mediate the macro responses of plants to biochar.”