S Joseph, P Taylor, F Rezende, K Draper, A Cowie

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The Properties of Fresh and Aged Biochar


Overview: The physical and chemical properties of biochar affect its performance as a soil amendment. The properties of biochar change as biochar interacts with microbes, soil organic and mineral matter, and plant roots. This module describes the properties of biochar that influence its effects as a soil amendment, and the processes by which biochar “ages” in soil.It should be read after reading the guide Basic Principles and Practice of Biochar Production.


  • Introduction
  • Properties of fresh biochar
  • Impacts of biochar in soil
  • Biochar ageing in soil: Processes and implications

The context

  • There are many kinds of biochar with different properties.
  • The properties of biochar:
    • affect its performance as a soil amendment.
    • can be modified by conditioning including post treating the biochar with minerals, nutrients and/or microorganisms.
    • measured in the lab, made from clean biomass, may be different than biochar made from field residue or stored biomass that has been in contact with soil, manures or fertilizers.
    • may change as biochar interacts with microbes, soil organic and mineral matter, and plant roots.  Biochar ages –  like wine.
  • The goal is to select/design biochars to meet plant nutrient requirements & soil constraints.
  • In some jurisdictions there are regulatory constraints on biochar production, sale and application, which maychange as the industry grows.
  • Customers would like assurance that their purchase will meet their needs and not do harm.

Biomass used to make biochar

Most biomass can be used for making biochar, as long as it has not been contaminated with toxic substances (e.g. heavy metals, PCBs).

Biomass feedstocks for biochar can be broadly classified into:

  1. Woods
  2. Grasses, seaweed, straw and other plant residues
  3. Husks and shells
  4. Manures and sludges
  5. Municipal Solid Waste including food waste
  6. Other industrial wastes, such as papermill waste.
  • The type of biomass can have a big impact on biochar properties.
  • Biomass generally consists of mineral matter, cellulose, hemicellulose, lignin and other organic compounds such as proteins, lipids and other complex organic compounds, in different proportions depending on biomass type.

Transformation of biomass into biochar

  • During pyrolysis most mineral matter is retained and most of the organic compounds break down, separating into volatile components that are released as hot gas and aerosols, and residual solid components referred to as biochar.
  • The biochar contains many different organic matter forms that mineralise at different rates, depending on the environment and the microbial community.
  • The organic components can be classified into two main fractions:
    • Persistent carbon that is resistant to breakdown for decades to millennia
    • Mineralizable (sometimes referred to as labile) carbon in the form of organic compounds that are potentially available to microbes and plants.
  • Condensates of the liberated gases, called wood vinegar or smoke water, contain over 300 chemicals, many beneficial to plants.

The physical and chemical properties of fresh biochars

Important physical properties of fresh (and aged) biochars include:

  • Bulk density and particle density, particle size, macro and microporosity, surface area, water holding capacity.

The chemical, electrical and magnetic properties include:

  • Macro nutrient content (N, P, K, S, Ca, Mg) and micro-nutrient content (Fe, Mn, Cu, Zn, Mo, B, Cl, Si, Na, Ni ). Measuring their solubility at different soil pH will provide an indication of their plant availability
  • Soluble organic compounds (soluble in water or in other solvents)
  • pH (proton activity), Eh (electron activity), EC (electrical conductivity), liming value, cation and anion exchange capacity (CEC and AEC), reducing and oxidising potential, magnetic susceptibility/paramagnetism
  • Adsorptivity of heavy metals and organic pollutants.

Key Points

  1. The physical and chemical properties of fresh biochar depend mainly on the feedstock and the temperature at which the biochar is produced.
  2. As the temperature increases the persistence of the carbon matrix increases but there can be less available nitrogen and mineralizable organic compounds.
  3. In high mineral ash biochars, carbon content is reduced significantly.
  4. Other factors that can affect biochar properties are type of pyrolyser (e.g. direct vs.indirect heating), residence time and heating rate.
  5. Research has shown that straw, plant residues and manure biochars commonly give the greatest increase in plant yield response, particularly in infertile and acidic soils. This in part is due to their greater liming effect and greater nutrient content.
  6. Some biochars (such as from wood and made at high temperature) can result in a decrease in plant yield, at least in the first season, due to the fresh biochar strongly adsorbing nutrients that would normally be available for plants. However, woody biochars can also be very beneficial in aerating soils, and can prevent runoff and leaching of chemical fertiliser.
  7. Nutrient enhancement and activation, either through pretreatment of biomass before pyrolysis, or of the hot or cold biochar after pyrolysis, can result in higher plant responses in terms of increase in yield, product quality or disease resistance. These enhancements can also increase the abundance of beneficial micro-organisms.

Physical properties of biochar

Physical properties of biochar: General

The physical properties of biochar impact its:

  • mobility in the environment
  • interaction with soil water, minerals and nutrients
  • suitability as an ecological niche for soil microbes and mycorrhizal fungi by providing surfaces, growing space and shelter from predators.

The basic physical properties of bulk density, particle density, particle size, porosity and surface area are all interrelated numerically, in measurement, and in action.

  • Particle density and surface area depend on the porosity.
  • A high porosity, low density biochar may have higher water holding capacity, but it may be more easily removed by wind or water.

Physical properties of biochar: Density

These terms can apply to biochars, soils or biomass feedstocks.

Bulk density:  Mass of a unit volume of a collection of particles or pieces. It is not an intrinsic property of the material, but depends on size, shape and compaction of the particles. It is important in materials handling, production yield and application considerations.  Biochars: 0.06 – 0.7 g/cm3

Particle density:  The mass per unit volume displaced by the envelope of the particle, including internal pores.

Grass biochars: 0.25-0.3 g/cm3

Wood biochars: 0.47 to 0.6 g/cm3

Particle density affects the ease of mobility and loss of biochar in wind and water.

Pore volume: The volume of pores per unit weight of material, expressed as cm3/g.

Source: Kathleen Draper

A low bulk density biochar is useful for amending compacted soils, or using in roof and wall gardens

Physical properties of biochar: Particle size

Particle size:  Diameter of particles is typically measured by fraction passing through a series of sieves of different sizes.

Particle size:

  • depends on feedstock and its preprocessing, and the production technique (screw augers, rotating drums etc.) and temperature.
  • can be reduced by grinding biochar, and sieving can ensure a more uniform particle size
  • can be increased by pelletizing or granulating
  • may decrease after applying to soil due to freeze-thaw cycles, ingesting and excretion by worms, and compaction from equipment.

Particle size influences:

  • the pore volume between particles and the bulk density
  • the water holding capacity and hydraulic conductivity
  • the effectiveness in compaction remediation
  • the mobility and loss of biochar in wind and water
  • the ease of transport and ingestion by soil flora and fauna
  • the speed of oxidation and aging
  • materials handling including dust production.

Physical properties of biochar: Macro and micro-porosity

Porosity is a measure of the percentage of empty space in a material.

Pore sizes can range over 6 orders of magnitude, classified as macro-, meso- and micro-pores, with different relevance to physiochemical phenomena for biochar interactions with the environment.

Physical properties of biochar: Porosity, surface area

Images from Lehmann and Joseph 2009

Physical properties of biochar: Porosity, surface area, bulk density

Specific Surface Area (SSA): the total surface area of a material per unit mass, including the micro-pores.

Rajkovich et al., 2012

Surface Area and bulk density of biochars made at 4 temperatures

Brewer et al., 2014

Pycnometer porosity for biochars made by slow pyrolysis at 350oC to 700oC from Miscanthus grass and Mesquite wood

Physical properties of biochar: Hydrophobicity

Zornoza et al., 2016. Chemosphere

  • Hydrophobicity affects the water uptake by biochar, and therefore water holding capacity of biochar, and microbial interactions.
  • Hydrophobicity is caused by tars (aliphatic compounds) condensing on the biochar surface during pyrolysis.
  • Low temperature biochars are strongly hydrophobic, but longer pyrolysis time or washing biochar can reduce hydrophobicity.
  • As biochar reacts in soil, hydrophobicity may decrease.

Properties of biochar: Grindability


Weber and Quicker, 2018. Fuel.

The Hardgrove grindability index (HGI) of raw wood, torrefied wood and charcoal (woody biochar) as a function of the volatile matter content. Typical ranges for fossil coal are also included in the figure. A low HGI indicates poor grindability, whereas a high value of HGI means that the material is easily grindable. For charcoal (woody biochar) with a volatile matter content of less than 20%, typically produced at temperatures around 600°C, HGI of 80 – 120 can be reached, classifying charcoal as easily grindable.

Chemical properties of biochar

Proximate vs. Ultimate Analysis

Proximate Analysis

  • Determined by thermal decomposition in an oxygen-limited environment
  • Moisture content
  • Volatile matter (gases released when fuel is heated).
  • Fixed matter (solid fuel left after the volatile matter is driven off, excluding the ash).
  • Ash (the residue after the fuel is burned, consisting of silica, iron, alumina, and other incombustible matter).

Ultimate Analysis

  • Determines the elemental amount of carbon, hydrogen, oxygen, nitrogen, and sulfur.
  • The elemental carbon is from both the volatile and fixed matter, not differentiated.
  • Sample is combusted, and measured using spectroscopy.

Examples: Proximate and Ultimate Analyses

Relationship between Fixed Carbon, Ash and Volatile Carbon

General locations on composition diagram of biochars made from various biomass feedstock groups at various temperatures.

  • Original feedstocks are high in volatiles.
  • Poultry manure is high in ash.
  • Pyrolysis removes some volatiles and increases proportions of fixed C and ash.
  • Arrows indicate trend in composition with increasing temperatures.
  • Volatile matter is the organic compounds that are given off when heated to 450-6000C.

Relationship between C, H, O for fresh biochar

There is a proportional relationship between H/C and O/C ratios for biochars produced at different temperatures, and from different feedstocks, excluding manure biochars. Arrows show direction of temperature increase and depolymerization. The slope of the arrows for both the woody and non- woody biochars is 1:2, indicating close to two oxygen are released for every hydrogen.

During pyrolysis volatiles carry off about 50% of the C, but more of the O and H.

Biomass → Biochar

C6H8.7O4 →  C3H1.4O0.4

There is a proportional relationship between H/C and O/C ratios for biochars produced at different temperatures, and from different feedstocks, excluding manure biochars. Arrows show direction of temperature increase and depolymerization. The slope of the arrows for both the woody and non- woody biochars is 1:2, indicating close to two oxygen are released for every hydrogen.

Weber, 2018

Properties of fresh biochars: Persistent carbon

Persistent carbon consists mainly of ring structures of carbon, with some replacement of carbon with nitrogen and oxygen.  The size of these ring structures depends on the temperature at which the biochar was produced.

The aromatic carbon rings can have a variety of functional groups consisting of nitrogen (N) oxygen (O) and sulphur (S).

At low temperature there are no crystals of graphite. At higher temperatures > 600oC graphite/graphene structures are observed.

These structures tend to exist in three dimensions.

The Composition of Biochars

Persistent Carbon

Persistence is indicated by ratio of Hydrogen to Organic Carbon

  • The persistence or longevity of the carbon in biochar can be approximately indicated by the ratio of hydrogen to organic carbon (H/Corg).
  • Organic carbon is calculated by subtracting the C in mineral carbonates from the total C content.
  • Hydrogen content of biochars varies between 1-5%, with hydrogen content increasing at lower temperatures.
  • With H/Corg ratio of 0.7, approximately 65% of the carbon in the biochar will remain after 100 years.
  • With H/Corg ratio of 0.4, 80% of the carbon will remain.
  • IBI and the EBC (European Biochar Certificate) set 0.7 as the maximum H/Corg ratio for a carbonised product to be considered as biochar.

H/C ratio – examples

  • H, C, N (and sometimes S) are measured in ultimate analyses as percentage by weight. O is obtained by difference.
  • To convert to molar fractions (comparing numbers of molecules) the weight percentages are divided by the molecular weights:  H = 1.008, C = 12.011

Soluble and Mineralisable carbon

  • Biochars have water soluble and mineralisable compounds, which can be food for microbes and stimulate seeds and plants.  This is known as labile C.
  • Most of these compounds are derived from the volatile compounds when the biomass breaks down (depolymerises) when heated in an oxygen-starved environment.
  • These can be divided into categories that include:
    • low molecular weight neutral compounds such as alcohols, aldehydes, ketones, sugars and amino acids)
    • low molecular weight acids
    • biopolymers (polysaccharides, proteins and amino sugars)
    • building blocks (polyphenolics/polyaromatic acids)
    • large macro molecules similar to humic acids
  • Water and solvent extractable organic molecules include carboxylic and benzoic acids, alcohols, lactones, alkanes, urea, complex sugars, butenolide, glycerol, polyphenols, aldehydes, ketones, quinones, benzene and polycyclic aromatic hydrocarbons.

Mineralisable carbon

  • Example: Variation in types of water soluble organic compounds in biochar produced from Acacia saligna, eucalyptus sawdust and jarrah at different temperatures.
  • The water-soluble compounds in the high temperature biochars were mainly low molecular weight (LMW) acids.
  • Neutral LMW compounds, “Building blocks” and “humics” dominate the extracts at low temperatures.
  • Low temperature biochars have much higher concentrations of water-extractable organics.

“Humics” refers to large macromolecules that have a structure similar to a humic acid standard

Building blocks: Molecular chains of polyphenolics/polyaromatic acids

Yun Lin presentation  2012

Polycyclic Aromatic Hydrocarbon (PAH)

There are maximum tolerable limits to total and bioavailable PAH. Naphthalene is the most abundant PAH (approximately 90% of total PAH) in biochar.

Gerard Cornelissen

Polycyclic Aromatic Hydrocarbon (PAH) content

Hale, S. E et al., Quantifying the total and bioavailable polycyclic aromatic hydrocarbons and dioxins in biochars. Environ. Sci. Technol. 2012.

Total PAH concentration (upper graph), and bioavailable PAHs (lower), for the same biochars produced by slow pyrolysis in the lab at various temperatures (shown in the label), or in the field.

  • Strong solvents (toluene in this study) are used to extract PAHs. Total concentrations (µg/g) may not reflect bioavailability.
  • To measure bioavailability the concentration   (ng/L) in a water leachate was measured.
  • All of the total and bioavailable PAHs and dioxins (not shown) in the slow pyrolysis biochars were below environmental guidelines for the acceptable levels of the toxins in soils.
  • Available PAHs were higher in some lower temperature biochars.
  • The concentrations for some biochars produced using gasification were above acceptable levels.

Properties of fresh biochars: Mineralisable carbon, Key points

  • Many biochars produced at temperatures between 350°C-500°C have been found to contain mineralisable organic compounds that have a beneficial effect on plants and soil.
  • Low doses of phenols, butenolide (an active ingredient in smoke), carboxylic and fatty acids, and even PAH, can stimulate plant growth, while having an inhibitory or toxic effect at high doses, a behavior known as hormesis.
  • Some of the available organic compounds in biochar can:
    • help plants resist disease
    • help germinate seeds
    • trigger growth of beneficial fungi
    • enhance or promote mineral weathering
    • act as signal molecules to either facilitate or discourage interactions with other organisms
    • have growth regulatory activities.

Properties of fresh Biochars: Ash (Inorganic Compounds)

Examples of elemental analysis of ash (excluding C,H,N,O) in different biochars.

  • Ash, the inorganic compounds in biochar, include metals and non-metals. Some have crystalline structure and others are amorphous and have dimension < 1 micron.
  • Amorphous and crystalline  compounds can include nitrates, chlorides, oxides, sulphates, sulphides, carbonates and phosphates.
  • Crystalline minerals can include rock phosphate (CaPO4), salt (NaCl), sylvite (KCl), struvite (NH4MgPO4. 6H2O), calcite (CaCO3), dolomite (CaMg(CO3)2, anatase (TiO2), SiO2, clays (Al2O3.SiO2.H2O), FeS/Fe2O3/Fe3O4.
  • They can be soluble (e.g. sodium chloride) or insoluble (e.g. calcium sulphate).
  • The solubility of the compound is a function of the pH and temperature of the liquid on the surfaces of the biochar.

(Source S Joseph UNSW)

Mineral distribution

Image (left) and elemental map (right) of a fresh greenwaste biochar showing the distribution of elements on the surface at the micron level (Each map is about 40 x 30 micron (0.04 x 0.03mm).

  • Many minerals and inorganic compounds have dimensions less than a micron (0.001mm), and some < 10nm (0.00001mm).

Properties of fresh Biochars : Heavy metals

Example: Some of the heavy metals in wood, straw and Terra Preta particles. Other heavy metals that were below the level of detection include Arsenic (As), Cadmium (Cd), Chromium (Cr), Cobalt (Co), Lead (Pb), Mercury (Hg), Nickel (Ni) and Selenium (Se).

Total & Available nutrients

  • Low ash woody biochars have low mineral content, ranging 5mg/kg (P in Oak) to 5g/kg of total elements. (Scale is 10x in graph)
  • Available amounts range from 24mg/kg (Mg in oak and pine) to 1.6g/kg (K in Hazelnut) (4% to 37%).
  • Biochars made from ash-rich biomass have ~10-100 times more total nutrients, from 1-70g/kg.
  • Except for Na in corn, and K in paperwaste, the available fraction in high ash biochars tends to be higher, ranging from 15% to 98% (~1g to 50g/kg).

The Availability of Macro Nutrients in Biochars: Fertiliser Value

Example: Plant nutrients in biochar from biosolids mixed with eucalyptus wood, pyrolysed at 5500C.

Source: Camps-Arbestain, M. et al., 2015 A biochar classification system and associated test methods.

  • Although nitrogen content of most manure and straw biochars is high, very little is readily available either in the form of nitrates or ammonium for uptake by plants when it is added to the soil.
  • Other than N, the concentration of many other elements is higher in biochar than in the biomass.
  • Availability of phosphorus (P) & potassium (K) is high (36% & 54% of the total P & K), whereas the available N is low (<5% of the total N).
  • Over time some of the less available nitrogen may become available at approximately the rate that C is mineralised.

Matching fertiliser value to crop requirements

Example: Nutrient requirement of corn crop to meet desired yield 13t/ha of grain:

Nutrients in biochar from biosolids mixed with eucalyptus wood, pyrolysed at 5500C at application rates from 1 to 10 t/ha

  • Applying 3 t/ha provides enough P
  • Applying at 9 t/ha fulfills the need for Mg (but gives 3.9 times the P requirement).
  • The balance of nutrients comes from other nutrient amendments and available nutrients in the soil.

Source: Marta Camps-Arbestain. 2016; Biochar Classification System for Biochars Applied to Soil; Korean Asian Pacific Biochar Conference

pH and Liming value

Two methods of measuring pH:

  1. A crushed air-dry biochar sample is mixed with five times its weight of distilled water, shaken for 1 hour, and the pH is measured using an electrode  àpH(H20)or pHw.
  2. A dilute concentration (0.01M) of calcium chloride (CaCl2) can be used in place of water. The results are expressed as pH(CaCl2) or pHCa

The pH of most woody biochars is around pH 6 at 350°C, increases to about 8 at 450°C, and continues to increases more slowly with temperatures above 450°.

Higher ash content biochars tend to have higher pH.

The liming value of biochars determines their capacity to lower soil acidity expressed as calcium carbonate (CaCO3) equivalent, comparing effectiveness with adding the equivalent amount of pure CaCO3(lime).

The soil lime buffer capacity (LBC) is a property of soil defined as the weight of pure lime (CaCO3), in milligrams, needed to raise the soil pH of one kilogram of soil by one unit.

Rajkovich 2012

pH increases with higher ash content and higher pyrolysis temperature

Enders 2012

Camps-Arbestain, M. 2016; Biochar Classification System for Biochars applied to soil

Electrical Conductivity

When biochar is placed in water some of its salts will be dissolved and the water will be able to conduct electricity. Electrical conductivity is a measure of total dissolved salts (TDS), and gives an indication of the availability of nutrients, or presence of excess ash/salt.

Electrical conductivity can be measured, like pH, by making a slurry of the ground biochar in distilled water and measuring with an EC or TDS meter.

Electrical conductivity (EC) is commonly expressed in units of deciSiemens per meter or milliSiemens per meter (1 dS/m = 100 mS/m).

Rajkovich 2012

  • The EC of most softwood biochars is very low, 19-4mS/m.
  • Hardwood biochars are up to three times greater.
  • Corn stalks are around 200mS/m, poultry manure up to 500mS/m.
  • Electrical conductivity of common soils is a few hundred mS/m
  • BC added to soil can increase soil EC

Cation Exchange Capacity

Available CEC in mmolc/kgfor biochars at different temperatures

Rajkovich 2012

Cation exchange capacity (CEC) of biochars measures the ability to hold exchangeable cations such as Calcium (Ca2+) and potassium (K+).

The CEC of biochar is mainly due to  oxygen functional groups on the surface.

Low temperature biochars usually have higher CEC, but high temperature biochars can adsorb more nutrients and OM.

Most biochars have a lower CEC than fertile soils but can have higher CEC than sandy and low OM soils.

CEC of biochar can increase as it ages in soils (especially those with a low pH).

  • CEC is a function biomass type, and often highest around 400oC.
  • Highest value was found for biochar produced from straw at a temperature of 400oC
  • In measurements on biochars from 20 different feedstocks, other research found highest CEC for switch grass made at 400oC.

Anion Exchange Capacity (AEC)

The ability of biochars to retain anions such as PO43-(phosphate) and NO3(nitrate) via Anion Exchange may be important to help retain nutrients and prevent leaching and associated water pollution.  On exposure to soil, this property declines.

During an experiment to test whether this related to oxidation of the biochars it was found that:

  • The biochars made at 700oC had 3 to 7 times higher AEC than biochars made at 500oC
  • AEC in biochars declined with oxidation with a mean decrease of 54% after 4 months treatment
  • The 700°C HTT biochars had greater resistance than the 500°C to loss of AEC during oxidation.

Other methods of retaining anions:

  • Anions such as phosphate may be retained on biochar via precipitation with e.g. Mg, Fe, Ca and Al
  • Retention by trapping in pores is more important for anions that do not precipitate (e.g., nitrate).

Lawrinenko, 2014. Thesis

Overview of trends of properties with temperature and residence time

Weber and Quicker, 2018. Fuel.

Weber and Quicker, 2018. Fuel.

The impact of biochars in soil and composting

Example: 0%, 1% and 5% oil mallee biochar in Vertisol soil in 30mm diameter tubes.

Biochar characteristics:

Porosity 75%. Organic C 6-%, Sieve 0.25 to 2mm

Results: 5% OM biochar increased porosity, pore size and connectivity, and reduced volumetric water content in the two clay rich soils. It decreased water loss through draining and evaporation in all 3 soils, and increased the available water content in the Arenosol.

X-ray computed tomography ( µ-CT) show biochars can increase micro-porosity in clay soils

Quin et al, 2014

Plant Available Water

Review of the literature indicates that grasses and straw biochars provide the greatest increase in plant available water

Selecting the kind of biochar (feedstock, treatment temperature, conditioning, application) that works best in specific soils is important to get the desired result.

High amounts of biochar in the topsoil layer may bring best results, if the biochar is covered to prevent evaporation of retained water.

Rajkovich et al., 2012

Available water capacity (AWC) of fine-loamy soil amended at 2% with various biochars.AWC is the difference between water retained at permanent wilting point and at field capacity.

Horizontal bars show average for each feedstock over all temperatures.

Average for each temperature over all feedstocks is shown in red bars.

Burrell L. D. et al. 2016

Example: In a sandy acidic soil (Planosol) straw had the highest impact on plant available water, and woodchip BC had no impact.

Electrochemical Properties Of Biochars

  • Most biochars are semiconductors and can store electrons and give out electrons.
  • High temperature biochars (>600oC) are conductors of electricity.
  • Some carbon in the biochar can react with oxygen to produce CO2, H2O and electrons. These electrons can be accepted by oxygen, Fe3+, nitrates or some bacteria.
  • Thus biochar can assist in making nutrients more available

The voltage difference between soil and biochar. C+= cations, A- = anions

Electron activity, Redox effects

Biochar makes oxidised soils more reducing. It lowers the Eh (electron activity) and increases the pH. This helps plants and microbes resist stress e.g. increase in temperature.

Olivier Husson 2016

The voltage difference between soil and biochar. C+= cations, A- = anions

Optimal conditions for plant growth and increase in abundance of beneficial micro organisms

A graph of Eh versus pH for rice straw biochar produced at two temperatures, added to a sandy soil at 0.5, 1, 2 and 5%.

Joseph et al., 2015 Agronomy

Impacts of biochar in soil: Heavy metal availability

  • CH350, CH500, CH650, and CH800:  Biochar from cotton seed hulls pyrolysed from 200oC to 800oC.
  • PS800: from Pecan at 800oC activated with phosphoric acid.
  • BL: biochar from poultry litter pyrolysed at 700oC.

Concentrations of Cu, Ni, Cd, and Pb ions remaining in solution after 48h equilibration with Norfolk soil amended with 10% biochar (g/g).

Uchimiya et al., 2011 Journal of Hazardous Materials

Biochars can adsorb heavy metals from soil, reducing their availability to plants and microbes.

  • Adsorption is influenced by biomass type, pyrolysis temperature and post-processing of biochars.
  • Low temperature high mineral ash biochars adsorbed best overall, and adsorbed all of the Cu & Cd.
  • High temperature high-ash or activated biochars also adsorbed well, with 700BL adsorbing all the Pb & Cu.
  • Superior adsorption correlated with higher volatile matter and oxygen content, not with surface area, fixed C or pH of biochars, suggesting that surface functional groups are key for adsorption in this acid sandy soil.
  • Other mechanisms for binding heavy metals include precipitation, redox reactions and complexation with organic molecules.

Adsorption of heavy metals by biochar can be increased (decreasing their bioavailability) by modifying the biochar surface structure

Biochar mixed with sulphur and heated at 550oC for 2hours, and applied to soil with 1g/kg of Hg, reduced available Hg in a leachate test by up to 99.3%

O’Connor, et al (2018).
Science of The Total Environment.

Impacts of biochar on GHG emissions from soil

A meta-analysis of 242 studies found that biochar application resulted in a mean decrease of 54±6%  in emissions of N2O from soil.

Van Zwieten, et al., 2015. Chapter 17, in “Biochar for Environmental Management: Science and Technology II” eds Lehmann J and Joseph S. Earthscan (489-520).

Biochar can reduce methane, nitrous oxide and ammonia emissions from compost and animal pens and also improve quality of the fresh and composted manure as a soil amendment

Agyarko-Mintah et al., 2017 Waste Management

Greenwaste biochar  and poultry litter biochar added to poultry litter compost reduced mass loss during composting.

Poultry litter biochar reduced N loss.

Biochar ageing in soil: Processes and implications

Biochar ageing: Effects on physical and chemical properties, Key Points

  1. The physical and chemical properties of biochar that has been aged in soil or compost, or passed through the gut of animals, are different from fresh biochar.
  2. The properties of aged biochars depend also on the type and properties of soil, microbial dynamics, tillage methods, type of plantsgrown, type of fertiliser added and other environmental factors (e.g. rainfall, temperature).
  3. Most aged biochars have a coating of porous clusters composed mainly of small (sub 100nm) minerals that are bonded together with organic compounds (micro-agglomerates). These clusters can, in turn, bond together to form porous micro-aggregates. Root hairs and micro-organisms can live on and in these micro-aggregates.

How do the properties of biochar change when applied to soil?

  • Long term trials in different soils indicate that properties of the aged biochars are dependent on the composition and concentration of the organo-mineral layer that forms on the surface of the biochar.
  • This in turn is a function of the soil properties, environmental conditions and agronomic practices (e.g. use of chemical or organic fertilisers, no-till or conventional tillage).
  • Clay (content and type) plays a crucial role in the persistence of biochars. Low clay soils leave the biochar surfaces exposed to oxidation.
  • Micro- and macro-organisms can play a major role in changing the properties of the biochars. For example, ingestion by earthworms can result in the biochar fracturing and new surfaces being formed that have similar properties to the fresh biochar.
  • Properties of aged biochar  are also a function of where and how often biochar is applied (i.e. every year, once every ten years, under the seed, next to the trees etc.)

How do the Properties of Biochar Change When in the Soil: Field Evidence

  • Plant response to biochar will depend on biochar properties and specific soil constraints.
  • The few long term studies that have been reported in the literature have found that manure, mixed feedstock and grass biochars have greater long term effect on pasture productivity than woody biochars (Slavich et al., 2013, Rajiq et al., 2018).
  • Some studies have shown a drop in yield as biochar ages, as the liming effect decreases (Cornelissen et al 2018), but other studies have shown increases in yields over time after a single application.
  • Many studies report a build up of soil carbon after the addition of biochar over at least a three year period. After 9 years there was a distinct negative priming effect with soil carbon increase much greater than the C  added from the biochar (Weng et al., 2017).
  • Soil aggregate stability has been reported to increase (D. Burrell, et al., 2016 Geoderma).
  • Ability to adsorb some heavy metals appears to decline after 1-3 years,  for some biochars (Bian et al., 2015).
  • A study where woody biochar was continually applied to land via feeding to cattle showed a large improvement in soil health and pasture productivity (Joseph et al., 2015)

Field evaluation of Greenwaste (GW) and Feedlot manure (FM) biochars: A long-term study

  • 69kg P added per ha in biochar
  • 60-90kg exported as forage in the first 2 years

Slavich, Van Zwieten et al (2013) Plant and Soil

Lukas Van Zwieten

Yield for winter rye grass,
7 years after biochar application (2012/13)

Lukas Van Zwieten

An Overview of The ageing Process

A Simplified Schematic of a Process in One set of Pores

Images of Plant-Soil-Microbe-Biochar Interaction

The Importance of Mineral Nanoparticles in Biochar, besides Nutrient Value

There is a considerable body of knowledge indicating that mineral nanoparticles (especially Fe/O, SiO2, clay) and probably TiO2 on the surfaces of biochar may catalyse a range of reactions, including:

  • increase nutrient availability and reduction of N2O through the redox wheel process
  • increase concentration of beneficial micro-organisms
  • assist in the breakdown of toxic compounds especially conversion of Cr6+ to Cr3+
  • microbial fixation of CO2
  • trap phosphates and nitrates within pores of the biochars
  • bind heavy metals
  • accelerate reactions between metals, non-metals and organic compounds in soils to form large macro-molecules.

Li, Yu, Strong & Wang (2012) Redox Wheel J Soils Sediments

Jun Ye et al (2017) ISME

  • Fe and Mn are released into the soil solution, and gases form.
  • Cl anions are adsorbed into an alumina phase, solubilising Al.
  • Electrons flow to the surface of the biochar then react with O2 to form OH- anions.
  • This results in formation of different Fe and Al oxide phases on the surface of the pore.
  • Organic nitrogen compounds react with iron oxides to precipitate on the iron phases.
  • Reduction and Oxidation of nitrate and ammonium can occur.

Joseph et al., 2015 Carbon Management

Postscript:  Profitability, yield of saleable product and other plant responses can depend on the properties of the fresh and aged biochar. The optimal rate of biochar depends on the interaction between yield response, other plant responses, and biochar costs

Jaiswal et al., 2015 Plant and Soil