Wednesday, October 30, 2013

Structural and chemical composition of biochar

Structural composition

     Thermal degradation of cellulose between 250ºC and 350ºC results in considerable mass loss in the form of volatiles, leaving behind a rigid amorphous C matrix. As the pyrolysis temperature increases, so thus the proportion of aromatic carbon in the biochar, due to the relative increase in the loss of volatile matter (initially water, followed by hydrocarbons, tarry vapors, H2, CO and CO2), and the conversion of alkyl and O-alkyl C to aryl C (Baldock and Smernik, 2002; Demirbas, 2004).

   Around 330ºC, polyaromatic graphene sheets begin to grow laterally, at the expense of the amorphous C phase, and eventually coalesce. Above 600ºC, carbonization becomes the dominant process. Carbonization is marked by the removal of most remaining non-C atoms and consequent relative increase of the C content, which can be up to 90% (by weight) in biochars from woody feedstocks (Antal and Gronli, 2003; Demirbas, 2004).

Each layer consists of hexagonal rings of carbon atoms linked together in a planar lattice, the formula is Cn where n is a very large number BUT a graphene layer is just a single atom in thickness.

    It is commonly accepted that each biochar particle comprises of two main structural fractions: stacked crystalline graphene sheets and randomly ordered amorphous aromatic structures (Figure 1). Hydrogen, O, N, P and S are found predominantly incorporated within the aromatic rings as heteroatoms (Bourke et al., 2007). The presence of heteroatoms is thought to be a great contribution to the highly heterogeneous surface chemistry and reactivity of biochar.

Composition -C/99.45% H/0.55%

Figure 1. Structure of charcoal. A model of a microcrystalline graphitic structure is shown on the left and an aromatic structure containing oxygen and carbon free radicals on the right (Bourke et al., 2007).

Chemical composition and surface chemistry
     Biochar composition is highly heterogeneous, containing both stable and labile components (Sohi et al., 2009). Carbon, volatile matter, mineral matter (ash) and moisture are generally regarded as its major constituents (Antal and Gronli, 2003). Table 1 summarizes their relative proportion ranges in biochar as commonly found for a variety of source materials and pyrolysis conditions (Antal and Gronli, 2003; Brown, 2009).

Table 1. Relative proportion range of the four main components of biochar. Weight percentage as commonly found for a variety of source materials and pyrolysis conditions 

     The relative proportion of biochar components determines the chemical and physical behavior and function of biochar as a whole (Brown, 2009), which in turn determines its suitability for a site-specific application, as well as transport and fate in the environment (Downie et al., 2009). Moisture is another critical component of biochar (Antal and Gronli, 2003), as higher moisture contents increase the costs of biochar production and transportation for unit of biochar produced. 

Biochar for a two hectare block of zucchinis, cucumber and watermelons.

     Keeping the moisture content up to 10% (by weight) appears to be desirable. In order for this to be achieved, pre-drying the biomass feedstock may be a necessity in biochar production. Despite the feasibility of biochar being produced from a wide range of feedstocks under different pyrolysis conditions, its high carbon content and strongly aromatic structure are constant features (Sohi et al., 2009). According to Sohi et al. (2009), these features largely account for its chemical stability. Similarly, pH shows little variability between biochars, and is typically >7 (Table 2). 

Table 2.  Summary of total elemental composition
(C, N, C:N, P, K, available P and mineral N) and pH ranges and means of biochars from a variety of feedstocks (wood, green wastes, crop residues, sewage sludge, litter, nut shells) and pyrolysis conditions (350ºC-500ºC) used in various studies (Chan and Xu, 2009).

     Total carbon content in biochar was found to range between 172 to 905 g kg-1, although OC often accounts for < 500 g kg-1, as reviewed by Chan and Xu (2009) for a variety of source materials. Total N varied between 1.8 and 56.4 g kg-1, depending on the feedstock (Chan and Xu, 2009). Despite seemingly high, biochar total N content may not be necessarily beneficial to crops, since N is mostly present in an unavailable form (mineral N contents < 2 mg k-1; Chan and Xu, 2009).

Effect of temperature on carbon and oxygen contents in bio-char from bio-waste. Particle size: 1.5–2.5 mm.

    Nuclear magnetic resonance (NMR) spectroscopy has shown that aromatic and heterocyclic N-containing structures in biochar occur as a result of biomass heating, converting labile structures into more recalcitrant forms (Almendros et al., 2003). C:N (carbon to nitrogen) ratio in biochar has been found to vary widely between 7 and 500 (Chan and Xu, 2009), with implications for nutrient retention in soils. C:N ratio has been commonly used as an indicator of the capacity of organic substrates to release inorganic N when incorporated into soils. Total P and total K in biochar were found to range broadly according to feedstock, with values between 2.7 - 480 and 1.0 - 58.0 g kg-1, respectively (Chan and Xu, 2009).

Cross-sectional SEM-EDS elemental maps of biochar particles with P and K contents. 

     Interestingly, total ranges of N, P and K in biochar are wider than those reported in the literature for typical organic fertilizers. Most minerals within the ash fraction of biochar are thought to occur as discrete associations independent of the carbon matrix, with the exception of K and Ca (Amonette and Joseph, 2009). Typically, each mineral association comprises more than one type of mineral. Joseph et al. (2009) emphasize that our current understanding of the role of high-mineral ash biochars is yet limited, as we face the lack of available data on their long-term effect on soil properties.

     The review of relevant literature has indicated that the full knowledge on the composition of biochar as a soil amendment, and the way it is influenced by those parameters, as well as the implications for soil functioning, is still scarce. Partially, this can be explained by the fact that most characterization work has involved charcoals with high carbon and low ash content, as required by the increasingly demanding market for activated carbon. Another factor is the wide variety of processing conditions and feedstocks available. Nevertheless, the current sparsity of biochar standards is largely reflected on the poor understanding of the link between biochar composition and its behavior and function in soil.

Biochar function in soil and atmosphere. Source:

Saturday, October 26, 2013

Bamboo for biochar production

     Compared to timber forests in the same growing conditions, bamboo can yield up to 25 times the amount of timber because it is ready to harvest so quickly. Some studies have found that bamboo can sequester four times more carbon and timber forests alone and at the same time releases 35% more oxygen than the timber forests, so there are many ecological benefits to bamboo growth (Brenner, 2008).

     Since bamboo can be used as a substitute of timber, it will also help decrease deforestation. Moreover, bamboo is highly sustainable as it can be regenerated within two to three years while timber could take longer than 25 years (FAO- NWFP-Digest-L, 2012). Biochar may be considered as a potential alternative to bamboo products as a durable carbon stock.


     Through a process of pyrolysis, up to 50% of the carbon can be transferred from plant tissue to the biochar, with the remaining 50% used to produce energy and fuels (Lehmann, 2007). Bamboo charcoal (BC) is one kind of manufactured biochar, a plentiful residual byproduct of the bamboo processing industry. BC has a highly micro-porous physical structure.


     The porosity is about five times greater and the absorption efficiency ten times higher than that of wood charcoal (Zhang, 2001). Bamboo charcoal may be an ideal amendment for nutrient conservation and heavy metal stabilization due to its excellent adsorption capability.


     Recent research found that biochar could act as soil fertilizers or conditioners to increase crop yield and plant growth by supplying and retaining nutrients (Glaser et al., 2000; Major et al., 2005; Steiner et al., 2007). Hua et al., (2009) found that bamboo biochar is an effective fertilizer when incorporated with sludge composing thereby effectively reducing nitrogen loses in the soil.

Tomato plants with no biochar application (-) and with biochar application (+)

     The positive effect was related to the high adsorption capacity of biochar particles during composting (Dias et al., 2007). In a similar research made by (Asada et al., 2002), it was found that bamboo biochar was effective in absorbing ammonia in soils. This was attributed to acidic functional groups formed as a result of thermolysis of cellulose and lignin at temperatures of 400°C and 500°C (Lehmann and Joseph 2009).


     Furthermore bamboo biochar has been used in studies in combination with municipal solid waste bottom ash as soil modifiers where the content of polyphenols released by the carbon matrix was measured, as well has been tested is combination with the same type of bottom ash as agronomic materials (Milla and Huang, 2013; Milla, Wang and Huang, 2013).

     However, there has been no research to date on the effects of pyrolyzation temperatures of bamboo biochar in seed germination and plant growth. In a recent study presented by Solaiman et al. (2012), biochars made from ‘Oil Mallee’, Rice Husks’, ‘New Jarrah’, ‘Old Jarrah’ and ‘Wheat Chaff’ the authors concluded that biochar type and application rate influenced wheat seed germination and seedling growth in a similar manner in the soil-less Petri dish and soil-based bioassays that were performed. Germination and early root growth of mung bean and subterranean clover differed from that of wheat in response to the five biochars.

Photograph of effect of biochar (new jarrah, NJ) at different rates 0, 0.5, 1.0, 2.5, 5.0 g/Petri dish (equivalent to 0, 10, 20, 50,100 t/ha based on 10 cm field depth) on seed germination (%) of wheat conducted in the soil-less Petri dish bioassay. 

     According to Rajkovich et al. (2012), the effects of biochar properties on crop growth are vaguely understood. For their study, biochar was produced from eight feedstocks and pyrolyzed at four temperatures (300°C, 400°C, 500°C, 600°C) using slow pyrolysis. In their results, animal manure biochars increased biomass by up to 43% and corn stover biochar by up to 30%, while food waste biochar decreased biomass by up to 92% in relation to similarly fertilized controls. Increasing the pyrolysis temperature from 300°C to 600°C decreased the negative effect of food waste as well as paper sludge biochars.


     On average, plant growth was the highest with additions of biochar produced at a pyrolysis temperature of 500°C, but feedstock type caused eight times more variation in growth than pyrolysis temperature. Biochar application rates above 2.0% (w/w) (equivalent to 26t ha−1) did generally not improve corn growth and rather decreased growth when biochars produced from dairy manure, paper sludge, or food waste were applied.  In a similar study Free et al. (2010), used biochars made from biosolids, corn stover, eucalyptus, fresh pine or willow pyrolyzed at 550ºC  and incorporated into sandy loam at rates from 0 to 10 t/ha.
 The results showed that any of the biochars affected significantly the germination or early growth (root and coleoptile length, and dry weight) of maize seeds. There were no interactions between type and rate of biochar with soil type. Their results suggest that biochar incorporation prior to a maize crop should be investigated as a method of increasing stable soil carbon with the potential for mitigating carbon emissions.

At temperatures above 400°C the lignin decomposes. This decomposition stage produces some bio-oil but the majority of the lignin remains as a solid and contributes heavily to the mass of the biochar product. Source:

     Previous studies realized by the authors on water spinach growth where application of rice husk biochar and wood biochar at temperatures between 250ºC and 350ºC proved that the application of rice husk biochar improves biomass production. The wood biochar added soil increased the plant weight of water spinach by increasing the root size and leaf width; while the rice husk biochar supplemented soil increased the plant weight of water spinach by increasing the stem size and lead length. In addition, the stem size of water spinach is proportional to the water holding capacity/silt ratio, while the root size of water spinach is proportional to organic matter/organic carbon ratio of soil.

     We also proposed that the working mechanism of wood biochar and rice husk biochar in soil would be such that the decomposition of organic carbon in biochar to soil organic matter resulted in the increased water holding capacity and decreased silt of biochar-added soil (Milla et al., 2013).


    The main objectives of our study were to investigate the potential capability of bamboo biochar to affect germination and growth of edible crops. We hypothesize that the results of this study will provide practical information about which temperatures are the best to use in biochar production for future agricultural applications.


Monday, October 21, 2013

Silicon content in rice husk biochar (RHB)

     Silicon oxide forms the main component (90-97%) of the rice husk ash with trace amounts of CaO, MgO, K2O and Na2O.  The melting point of SiO2 is 1410-1610°C, while that of K2O and Na2O is 350 and 1275°C respectively. It has been suggested that at higher temperatures, the low-melting oxides fuse with silica on the surface of the rice husk char and form glassy or amorphous phases, preventing the completion of reaction. (Anshu et al., 2004).

Silicon Oxide Molecule

     We realized an investigation on the effects of rice husk biochar and its silicon content on corn (Zea mays L.) growth; the analysis of the fresh rice husk used to obtain biochar showed high levels of Si, Ca and Mg (Milla et al., 2013). After pyrolysis the same elements were found to increase in the rice husk biochar. Wood biochar was found to have a higher content of Ca when compared with RHB, having a low content of Si and Mg.

     Silicon (Si) is not yet classed as an essential nutrient but it exists in all plants grown in soil and is recognized as a functional nutrient. The benefits of silicon include increasing pest and pathogen resistance, drought and heavy metal tolerance, and improved quality and yield of agricultural crops. Si is taken up at levels equal or greater than essential nutrients such as Nitrogen and Potassium in certain plants such as rice and sugarcane, for which it is considered agronomically essential for sustainable crop production (Savant et al., 1999). Si exists in all plants grown in soil (Takahashi, 1995) and its content in plant tissue ranges from 0.1 to 10% (Epstein, 1999).

     Si is considered as a nutrient of agronomic essentiality in that its absence causes imbalances of other nutrients resulting in poor growth, if not death of the plant (Savant et al., 1997). Numerous laboratory, greenhouse and field experiments have shown the benefits of silicon fertilizers for agricultural crops and the importance of silicon fertilizers as a component in sustainable agriculture (Matichenkov and Calvert, 1999).


There are two different effects on plants due to silicon fertilizers:
1. An indirect influence through soil fertility, and
2. A direct influence on the plants

The benefits of Si on plants include (Ma and Yamaji, 2006; Savant et al., 1999):
- Increased growth and fruit yields in some species.
- Tolerance to abiotic stress: frost, drought and salinity, toxicity by Al, Mn, heavy metals.
- Tolerance to biotic stress: insects and infection.
- Resistance to lodging.

Si also controls the chemical and biological properties of soil with the following benefits:
- Reduced leaching of phosphorous (P) and potassium (K) (Sadgrove, 2006).
- Reduced Aluminium (Al), Iron (Fe), Manganese (Mn) and heavy metal mobility (Maichenkov and         Calvert, 2002).
- Improved microbial activity (Matichenkov and Calvert, 2002).
- Increased stability of soil organic matter.
- Improved soil texture (Sadgrove, 2006).
- Improved water-holding capacity (Sadgrove, 2006).
- Increased stability against soil erosion (Sadgrove, 2006).
- Increased cationic exchange capacity (CEC) (Camberato, 2001).

      Therefore, even if a plant is a low Si-accumulator, it will benefit from the improved soil properties resulting from the application of Si. Silicon deficiency in crops has been recognized since the 1970’s. The optimization of silicon nutrition has been shown to have positive effects on plants. In particular, substantial research on rice and sugarcane has shown that silicon application can significantly enhance insect pest and disease resistance with consequent yield increases.
Development of leaf blast symptoms at 96 h after inoculation with Magnaporthe grisea in rice plants nonamended (-Si) or amended with (+Si) with silicon.

      Plants differ in their ability to accumulate Si (Ma and Yamaji, 2006), but in order for any plant to benefit from Si it must be able to acquire this element in high concentrations. Several reports in the literature suggest that Si nutrition has a definite role in certain type of crop cultivation, especially on weathered tropical soils such as Oxisols, Ultisols, Entisols and Histosols (Savant et al., 1999). In our tests, Si from rice husk biochar  played a significative roll in the water spinach nutrition, boosting the mass production of the plant, showing a significative difference when compared with the mass production of water spinach where wood biochar was applied.                    

                           RICE HUSK BIOCHAR                                      WOOD BIOCHAR

      Rice husks are unique within nature. They contain approximately 20% opaline silica in combination with a large amount of the phenyl propanoid structural polymer called lignin (Oliver, 2004). In our test, the Si properties of the rice husk were increased after pyrolysis.

      In a study made by Hossain et al. (2011), about the influence of pyrolysis temperature on production and nutrient properties of biochar, the researchers concluded that: pyrolysis temperature has significant effect on the chemical properties of the biochar produced, with important implications regarding their suitability as a soil amendment. In addition, the study confirmed that the yield of biochar decreases with increasing pyrolysis temperature.

     The study also shows that biochar produced at low temperatures (300°C, 400°C) is acidic. Biochars produced at lower temperatures might be suitable for alkaline soils to correct for alkalinity problems.

Monday, October 14, 2013

Applications of rice husk biochar (RHB) into soil

     The application of biochar has been shown to improve soil chemical properties, and especially rice husk biochar as stated by Sovu et al. (2012) that has an advantage over inorganic fertilizers in the subsequent growth of planted seedlings and soil fertility. Biochar is made from a pyrolysis process that occurs spontaneously at extreme high temperatures that can go above 300°C. At its most extreme state, pyrolysis leaves only a carbon residue, which is called carbonization. The high temperatures used in pyrolysis induce polymerization of the molecules within the feedstocks, producing larger molecules and thermal decomposition of some feedstock components into smaller molecules. The remaining solid component following pyrolysis is charcoal, referred to as biochar, when produced with the intention of adding it to soil to improve it (Schmidt et al., 1999; Preston and Schmidt., 2006; Hussain et al., 2008). Basically, biochar is known as a pyrolized carbon from solid waste used in agriculture application since 1998.  

      In fact, using rice straw and rice husk in rice growing has been practiced for some time (Ponamperuma, 1982; Eagle et al., 2001; Singh et al., 2008; Kaewpradit et al., 2009). Williams et al. (1972) discussed the advantages and drawbacks of burning versus incorporating rice straw in rice growing. Karmakar et al. (2009) and Mahvi et al. (2005) reported the mixed effects of fly ash and rice husk ash on improving soil properties to decrease soil bulk density and to increase soil pH, organic carbon, available nutrients, and crop yield. The increase in crop yield with biochar application has also been reported for crops such as cowpeas (Yamato et al., 2006), soybeans (Tagoe et al., 2008), and maize (Yamato et al., 2006; Rodríguez et al., 2009).

Christoph Steiner (Autor)

      According to Haefele et al. (2009), the total crop residues produced each year in rice-based systems of Asia are roughly estimated at 560 million tons of rice straw and 112 million tons of rice husks (based on 2005 production, a harvest index of 0.5, and a husk/paddy ratio of 0.2). In such consideration, rice residues could be used to produce biochar to improve, maintain, and recycle nutrients in order to enhance soil fertility. Despite their high nutrients properties, biochars stability depends on the temperature used in the pyrolysis process; this stability could help to reduce greenhouse gas emission to some extent. These residues constitute a valuable resource, but actual residue management practices do not use their potential adequately and often cause negative environmental consequences. Increasing opportunity costs of organic fertilizer use and shortened fallow periods because of cropping intensification have caused a continuous decline in recycling crop residues in the past decade (Pandey, 1999).


     Residue burning is widely practiced and causes air pollution, human health problems, and considerable nutrient loss. 
     A publication made in the Farmers of Forty Centuries or Permanent Agriculture in China, Korea, and Japan published in 1911, details the comments of a USDA soil scientist F.H. King. He wrote of how he witnessed farmers in some sides of Asia, regularly composting and recycling all types of organic waste materials as well as ashes to use as soil amendments on their fields. This was a method farmers used to maintain soil fertility and improve crop production through centuries. Actually, this was an original form of sustainable agriculture and was an opening for the organic farming movement (Heckman, 2012). In similarity to this traditional agricultural system, modern agriculture use of commercial nitrogen-phosphorus-potassium fertilizers has mostly replaced compost and other organic amendments. Furthermore, the majority of commercial fertilizers contain small amounts or no content of silicon.

     For the reason that uptake of silicon is relatively large for many crops, a failure to return organic waste materials to farmland contributes to reduction of plant-available silicon from soil. An enrichment of biological activity in compost-amended soils may also have a role in mobilizing silicon for plant uptake. As an outcome of these significant changes in soil fertility management, there are good reasons to pay attention on the role silicon in soil fertility (Heckman, 2012). 

      Findings from over a decade of field trials conducted on the silicon research plots at Rutgers University, New Jersey Agriculture Experiment Station show that calcium silicate slag is an effective liming material and silicon fertilizer. Plants grown on calcium silicate slag amended soil exhibited increased silicon uptake.
      Rice-husk biochar has high silica (SiO2) contents and silicon (Si) is a beneficial element for plant growth that helps plants overcome multiple stresses including biotic and abiotic stresses. Silicon plays an important role in increasing plant resistance to pathogens such as blast on rice (Datnoff et al., 1997) and powdery mildew on cucumbers (Miyake and Takahashi 1982a). Silicon is effective in preventing rice lodging by increasing culm wall thickness and vascular bundle size (Shimoyama, 1958), thereby enhancing stem strength. Silicon alleviates the effects biotic and abiotic stresses including salt stress, metal toxicity, drought stress, radiation damage, nutrient imbalance, high temperature, and freezing (Epstein, 1999; Ma and Tahakashi, 2002) and has various beneficial effects on plant growth and productivity (Ma and Tahakashi, 2002). Maize takes up Si actively from the roots (Liang et al., 2006).

Silicon vrs. Control
Powdery mildew lesions on wheat foliage were 44% less and yields were 10% greater in silicon amended plots. Source:

     However, agronomists and farmers are not always aware that they could be able to improve crop production with increased stress and disease resistance by adding up a source of available silicon to the soil.

Applications of rice husk biochar to crops

      The benefits of silicon in crop production may be manifested as healthier plants and higher yield with fewer applications of pesticides and other chemical products (Heckman, 2012). Still, reports on the Si effect of rice husk biochar on plant seed germination are scant.

Thursday, October 10, 2013

Rice husk as feedstock for biochar production.

Rice husk and its transformation into biochar
     Rice husks, wood remains, nutshells, manure and crop residues are regarded as agricultural waste, but recently such solid wastes have been transformed into biochar for the purpose of carbon sequestration. Biochar is commonly defined as charred organic matter, produced with the intent of being deliberately added to soil to improve its agronomic properties. On average, one ton of dry biomass can create 400 kg of biochar containing 80 to 90% pure carbon (Lehmann et al., 2009) at 300ºC to 700ºC, under low (preferably zero) oxygen concentrations. 

     Rice husk contains a high content of silicon and potassium, nutrients which have great potential for amending soil, while those with a relatively higher carbon content (e.g. wood or nut shells) are currently used for the production of activated carbon.  The use of rice straw and rice husks in the field has been practiced for some time (Ponamperuma, 1982). Research has shown that incorporation of rice straw and rice husks can significantly improve soil properties by decreasing soil bulk density, enhancing soil pH, adding organic carbon, increasing available nutrients and removing heavy metals from the system, ultimately increasing crop yields (Williams et al., 1972).

     Similar studies on cowpea, soybean, and maize (Yamato et al., 2006) have also supported the application of biochar as a way to increase crop yields. Asia, a principal rice-growing region, has abundant rice residues, estimated at about 560 million tons of rice straw and 112 million tons of rice husks, respectively. These residues could be a valuable resource for the production of biochar to increase soil fertility. Carbonized rice husks consist of a very light material with a micro-porous structure and a bulk density of about 0.150g cm−3 (Haefele et al., 2009). The carbonization process also improves the water-holding capacity of the rice husks (Oshio et al., 1981). Additionally, the widespread old practice of burning rice straw in the field indicates that black carbon from incompletely burned (i.e. carbonized) rice residues could be an important source of organic matter in rice soils, as has been previously shown for a range of other soil types (Schmidt and Noack, 2000).

      The effects of the addition of biochar may vary from soil to soil. However, the following effects have been seen in experiments: a) the rice husk charcoal increases the soil pH, thereby increasing available phosphorus (P), b) improved aeration in the crop root zone, c) improved soil water - holding capacity and d) increased levels of exchangeable potassium (K) and magnesium (Mg) (FFTC, 2001). There is a need to highlight the agronomic properties and the effects of rice husks biochar on the growth of crops, to promote biochar use in the field by small landholders.

      With several Asian countries applying the carbonized rice residues, their real outcome has not been clarified. More field work is required to indicate the relationship between the amount of biochar applied and the growth rate of crops. In our study, we examined the effects of rice husk biochar application and compared these results to wood biochar applied to increase the growth of water spinach (Ipomoea aquatica) in field conditions. Water spinach is a fast growing plant with strong system development. It originates from mainland China and is now widely grown worldwide.

    Our study assumed that rice husk biochar could act as a soil conditioner, enhancing water spinach growth by supplying and retaining nutrients and thus improving the soil’s physical and biological properties. Our aim was to explore whether rice husk biochar (RHB) and wood biochar (WB), in combination with fertilizers, could increase the biomass yield of water spinach. Soil analysis, shared use of a scanning electron microscope, and heavy metal analysis were used to identify the properties of rice husk biochar. We hope that the results of our work may help to determine which of the biochars is more beneficial in boosting the production of water spinach.

Rice husk biochar added to soil.

Monday, October 7, 2013


     Feedstock is the term conventionally used for the type of biomass that is pyrolyzed and turned into biochar. In principle, any organic feedstock can be pyrolyzed, although the yield of solid residue (char) respective to liquid and gas yield varies greatly along with physicochemical properties of the resulting biochar. Feedstock is, along with pyrolysis conditions, the most important factor controlling the properties of the resulting biochar. 

     Firstly, the chemical and structural composition of the biomass feedstock relates to the chemical and structural composition of the resulting biochar and, therefore, is reflected in its behavior, function and fate in soils. 
     Secondly, the extents of the physical and chemical alterations undergone by the biomass during pyrolysis (e.g. attrition, cracking, microstructural rearrangements) are dependent on the processing conditions (mainly temperature and residence times). 

     Table 1 provides a summary of some of the key components in representative biochar feedstock. Cellulose and lignin undergo thermal degradation at temperatures ranging between 240-350ºC and 280-500ºC, respectively (Demirbas, 2004). 

Table 1.  Summary of key components (by weight) in biochar feedstock

     The relative proportion of each component will, therefore, determine the extent to which the biomass structure is retained during pyrolysis, at any given temperature. For example, pyrolysis of wood-based feedstock generates coarser and more resistant biochars with carbon contents of up to 80%, as the rigid ligninolytic nature of the source material is retained in the biochar residue (Winsley, 2007). 

Biomass with high lignin contents (e.g. olive husks) have shown to produce some of the highest biochar yields, given the stability of lignin to thermal degradation, as demonstrated by Demirbas (2004). Therefore, for comparable temperatures and residence times, lignin loss is typically less than half of cellulose loss (Demirbas, 2004).

      The mineral content of the feedstock is largely retained in the resulting biochar, where it concentrates due to the gradual loss of C, hydrogen (H) and oxygen (O) during processing (Demirbas, 2004). 
The mineral ash content of the feedstock can vary widely and evidence seems to suggest a relationship between that and biochar yield (Amonette and Joseph, 2009). Table 2 provides an example of the elemental composition of representative feedstock.

Table 2.  Examples of the proportions of nutrients (g kg-1) in feedstock

     In the plant, Ca occurs mainly within cell walls, where it is bound to organic acids, while Mg and P are bound to complex organic compounds within the cell Potassium which is the most abundant cation in higher plants and is involved in plant nutrition, growth and osmoregulation Nitrogen, Mn and Fe also occur associated to a number of organic and inorganic forms. 
     During thermal degradation of the biomass, potassium (K), chlorine (Cl) and N vaporize at relatively low temperatures, while calcium (Ca), magnesium (Mg), phosphorus (P) and sulphur (S), due to increased stability, vaporise at temperatures that are considerably higher (Amonette and Joseph, 2009). 

The Thermal Degradation Spectrum

Graph Representation of the Thermal Degradation Spectrum 

     Other relevant minerals can occur in the biomass, such as silicon (Si), which occurs in the cell walls, mostly in the form of silica (SiO2).  Many different materials have been proposed as biomass feedstocks for biochar, including wood, grain husks, nut shells, manure and crop residues, while those with the highest carbon contents (e.g. wood, nut shells), abundance and lower associated costs are currently used for the production of activated carbon (Lua et al., 2004; Martinez et al., 2006; Gonzaléz et al., 2009).

     Regarding the characteristics of some plant feedstocks, even within a biomass feedstock type, different composition may arise from distinct growing environmental conditions (e.g. soil type, temperature and moisture content) and those relating to the time of harvest. In corroboration, (Chan and Xu, 2009) have shown that the adsorbing properties of a charcoal for copper ions can be improved 3-fold by carefully selecting the growth conditions of the plant biomass (in this case, stinging nettles). Even within the same plant material, compositional heterogeneity has also been found to occur among different parts of the same plant (e.g. maize cob and maize stalk, Table 2).

Nature of feedstock
     In addition to plant biomass, an entire range of organic materials, including waste materials such as poultry litter and sewage sludge can be converted to biochars using pyrolysis. Recently, conversion of these other materials to biochars has been promoted as an alternative way of managing a range of organic wastes (Bridle and Pritchard, 2004; Shinogi, 2004; Hospido et al., 2005; Lima and Marshall, 2005). Given the vast differences in the properties of the potential feedstocks, biochars can have very different nutrient contents and availability, as discussed earlier.