**3.2 What is biochar exactly?**

Biochar is produced by thermal treatment at oxygen deficiency e. g. by pyrolysis or gasification, resulting in three products: char, gas and tarry oils. The relative amounts and characteristics of each are controlled by the process conditions such as temperature, residence time, pressure, and feedstock type. Biochar production can be chemically described by water elimination followed by increasing aromatic condensation, which can be expressed as decreasing atomic ratios of O/C and H/C along the combustion continuum (Fig. 4). However, biochar is no clearly defined chemical compound. Instead, it is a class of compounds along the combustion continuum and we need to define thresholds for materials which are claimed to be biochar (Fig. 4). Recently, on the basis of about 100 biochar samples differing in feedstock and production process, the following elemental ratio thresholds were suggested for biochar, O/C < 0.4 and H/C < 0.6 (Schimmelpfennig & Glaser, 2012).

As a consequence, the content of condensed aromatic moieties, known as black carbon, increases being responsible for its stability in the environment. The second important ecological property of biochar is presence of functional groups on the edges of the polyaromatic backbone (Fig. 4) which are formed by partial oxidation (Glaser, 2007). Therefore, biochar is an option for long-term C sequestration while maintaining or increasing soil fertility which was successfully proven by the *terra preta* phenomenon for at least 2,000 years (Glaser, 2007). Due to this fact, *terra preta* could be a model for sustainable resource management in the future not only in the humid tropics but also in temperate and arid regions around the world providing a solution for land degradation due to intensive land use and growing world population. In the following, we will review reported biochar effects on ecosystem services.

Fig. 4. Combustion continuum and biochar window (red rectangle) and model for biochar structure being important for ecological properties.

### **3.3 How long will biochar survive in soil?**

180 Management of Organic Waste

Biochar is produced by thermal treatment at oxygen deficiency e. g. by pyrolysis or gasification, resulting in three products: char, gas and tarry oils. The relative amounts and characteristics of each are controlled by the process conditions such as temperature, residence time, pressure, and feedstock type. Biochar production can be chemically described by water elimination followed by increasing aromatic condensation, which can be expressed as decreasing atomic ratios of O/C and H/C along the combustion continuum (Fig. 4). However, biochar is no clearly defined chemical compound. Instead, it is a class of compounds along the combustion continuum and we need to define thresholds for materials which are claimed to be biochar (Fig. 4). Recently, on the basis of about 100 biochar samples differing in feedstock and production process, the following elemental ratio thresholds were suggested for biochar, O/C < 0.4 and H/C < 0.6 (Schimmelpfennig &

As a consequence, the content of condensed aromatic moieties, known as black carbon, increases being responsible for its stability in the environment. The second important ecological property of biochar is presence of functional groups on the edges of the polyaromatic backbone (Fig. 4) which are formed by partial oxidation (Glaser, 2007). Therefore, biochar is an option for long-term C sequestration while maintaining or increasing soil fertility which was successfully proven by the *terra preta* phenomenon for at least 2,000 years (Glaser, 2007). Due to this fact, *terra preta* could be a model for sustainable resource management in the future not only in the humid tropics but also in temperate and arid regions around the world providing a solution for land degradation due to intensive land use and growing world population. In the following, we will review reported biochar

Fig. 4. Combustion continuum and biochar window (red rectangle) and model for biochar

structure being important for ecological properties.

**3.2 What is biochar exactly?** 

effects on ecosystem services.

Glaser, 2012).

Due to its recalcitrance against microbial degradation, biochar is very stable in soil compared to other OM additions, making its application to soils a suitable approach for the build-up of SOM and thus, for C sequestration. The prevailing scientific understanding of biochar degradation in soil is that some portions of it are quite readily decomposable (labile), while the core structure of the material is highly resistant to degradation (Fig. 4). Biochar in *terra preta* has been dated to 1,000 to 1,500 years (Glaser et al., 2000) and naturally occurring biochar in Australian soils to 1,300 – 2,600 years (Lehmann et al., 2008). As SOM decomposition rates in temperate regions are slower, mean residence time for biochar can be assumed to be higher in European soils. Controlled biochar decomposition experiments revealed a mean residence time in soils between 1,300 to 4,000 years (Cheng et al., 2008; Liang et al., 2008; Kuzyakov et al., 2009). Management practices such as tillage and addition of labile C (e.g. slurry) to soil significantly increased biochar mineralization by a factor of 0.5 to 2, however, only in the short-term (Kuzyakov et al., 2009) so that biochar application can be combined with such agricultural technologies without the disadvantage of additional SOM and biochar degradation.

In a range of other biochar incubation experiments, the interactive effects of biochar addition to soil on CO2 evolution (priming) were evaluated by comparing the additive CO2 release expected from separate incubations of soil and biochar with corresponding biochar and soil mixtures. Positive (C mineralization stimulation) or negative (C mineralization suppression) priming effects and magnitude varied with soil and biochar type. In general, C mineralization was higher than expected (positive priming) for soils combined with biochars produced at low temperatures (250 – 400 °C) and from grasses, particularly during the early incubation stage (rst 90 d) and in soils of lower organic C content (Zimmerman et al., 2011). In contrast, C mineralization was generally less than expected (negative priming) for soils combined with biochars produced at high temperatures (525 – 650 °C) and from hard woods, particularly during the later incubation stage (250 – 500 d). Obtained data strongly suggests that biochar soil interaction will enhance C sequestration via SOM sorption and organo-mineral interaction in the long term.

### **3.4 How much biochar can be stored in soils?**

C sequestration with biochar addition to soils could be quite signicant since the technology could potentially be applied in many areas including croplands, grasslands and also a fraction of forestlands. The maximum capacity of carbon sequestration through biochar soil amendment in croplands alone was estimated to be about 428 Gt C for the world (Table 6). This capacity is estimated according to (i) the maximal biochar amount that could be cumulatively placed into soil while still beneficial to soil properties and plant growth; and (ii) the arable land area that the technology could potentially be applied through biochar agricultural practice. If using also grassland soils and 30% of forest soils, a worldwide biochar sequestration potential of 1,126 Gt C would be possible (Table 6).

#### **3.5 Can we solve our climate problem with biochar alone?**

Photosynthesis captures more CO2 from the atmosphere than any other process on Earth. Each year, terrestrial plants photosynthetically fix about 440 Gt CO2 being equivalent to 120

Synergisms between Compost and Biochar for Sustainable Soil Amelioration 183

capacity can also cause a higher nutrient retention because of a reduced percolation of water

However, since biochar has only low nutrients contents in general, plant nutrients must be supplied externally (Woods & Mann, 2000; Glaser & Birk, 2011). With respect to potential nutrient sources, only C and N can be produced in situ via photosynthetic organisms and biological N xation, respectively. All other elements, such as P, K, Ca and Mg must be added for nutrient accumulation (Glaser, 2007) which can be best achieved by adding

According to Ogawa (1994), biochar is generally characterized by a proliferation effect for several symbiosis microorganisms due to its porous structure providing an appropriate habitat for soil microbes. Steiner et al. (2004) observed a significant increase of microbial activity and growth rates by applying biochar to a Ferralsol. Furthermore, an increase of soil microbial biomass and a changed composition of soil microbial community were also

While microbial reproduction rates after glucose addition in soils amended with biochar increased, soil respiration rates were not higher (Steiner et al., 2004). This difference between low soil respiration and high microbial population growth potential is one of the characteristics of *terra preta*. These results indicate that a low biodegradable SOM together with a sufficient soil nutrient content are able to support microbial population growth. According to Birk et al. (2009), these effects can be ascribed to different habitat properties in the porous structure of biochar. The following factors might be the reason in decreasing

high surface area and porous structure of biochar suitable for several kinds of microbes

 preserving character against decay probably resulting in the (partial) inhibition of certain 'destructive' and pathogenous organisms while simultaneously supporting

Based on these possible stimulating factors, biochar promotes the propagation of useful microorganisms such as free-living nitrogen fixing bacteria (Tryon, 1948; Ogawa, 1994; Nishio, 1996; Rondon et al., 2007). Further reports from Japanese scientists prove increased yields by the stimulation of indigenous arbuscular mycorrhizal fungi (AMF) via the application of biochar (Ogawa, 1994; Nishio, 1996; Saito & Marumoto, 2002): e. g. improved yields for soybeans because of enhanced nodule formation by means of biochar addition (Ogawa et al., 1983). Results from Nishio (1996) obtained with alfalfa (*Medicago sativa*) in pot experiments indicate that biochar was ineffective in stimulating alfalfa growth when added

 enhanced ability to retain water and nutrients resulting in a stimulation of microbes; formation of 'active' surfaces covered by water film, dissolved nutrients and substances providing an optimal habitat for microorganisms; these specific surfaces serve as interaction matrix for storage and exchange processes of water and substances between

soil fauna, microorganisms and root hairs (Amlinger et al., 2007);

and the herein dissolved nutrients (Glaser et al., 2002).

observed after biochar amendments (Birk et al., 2009).

order of currently available evidence supporting them:

**3.7 The role of soil organisms** 

as habitat and retreats;

weak alkalinity (Ogawa, 1994);

beneficial microbes.

organic fertilizers such as manure or compost (Schulz & Glaser, 2011).


Table 6. Potential C sequestration with biochar in soils of the world. Estimated storage capacity is based on a maximum of 10 weight% biochar addition to the upper 30 cm and a soil bulk density of 1.3 Mg m-3 and 70% stable carbon in biochar (Lee et al., 2010).

Gt C per year from the atmosphere into biomass (Smith & Collins, 2007). This corresponds to about one-seventh of the CO2 stock in the atmosphere (820 Gt C). However, biomass is not a stable form of carbon material with nearly all returning to the atmosphere in a relatively short time as CO2 because of respiration and biomass decomposition. As a result, using biomass for carbon sequestration is no good option. Any technology that could significantly prolong the lifetime of biomass materials would be helpful to global carbon sequestration. A conversion of only about 7% of the annual terrestrial gross photosynthetic products into a stable biomass carbon material such as biochar would be sufficient to offset the entire amount (nearly 9.7 Gt C a-1) of CO2 emitted into the atmosphere annually from the use of fossil fuels (www.iwr.de). More realistic estimates are that annual net CO2, CH4 and N2O emissions could be reduced by a maximum of 1.8 Pg C a-1 without endangering world food security and soil fertility (Woolf et al., 2010) corresponding to 16% of current anthropogenic CO2 emissions. Therefore, biochar can significantly contribute to climate change mitigation but additional technologies are required to quantitatively offset fossil fuel-derived CO2 emissions. The substitution of fossil fuels by developing and extending renewable energies is another essential key factor for greenhouse gas emission reduction or avoidance while still meeting the basic requirements of electrical or thermic energy consumption demands of society. An impressive example for such a concept is the integrative combination of innovative technologies: a PYREG pyrolysis reactor unit (Gerber 2010), for instance, locally produces biochars from organic wastes in an environmentally friendly way, while also generating heat and electricity from renewable, carbon neutral resources.

Thus, a decentralized application of this technology represents a promising and sustainable strategy for the future.

#### **3.6 Can biochar increase soil fertility and thus crop performance?**

Biochar application to soil influences various soil physico-chemical properties. Due to the high specific surface area of biochar and because of direct nutrient additions via ash or organic fertilizer amendments, nutrient retention and nutrient availability were reported being enhanced after biochar application (Glaser et al., 2002; Pietikäinen et al., 2000). Higher nutrient retention ability, in turn, improves fertilizer use efficiency and reduces leaching (Steiner et al., 2008; Roberts et al., 2010). Most benefits for soil fertility were obtained in highly weathered tropical soils but also higher crop yields of about 30% were obtained upon biochar addition in temperate soils (Verheijen, 2009). Furthermore, enhanced water-holding capacity can also cause a higher nutrient retention because of a reduced percolation of water and the herein dissolved nutrients (Glaser et al., 2002).

However, since biochar has only low nutrients contents in general, plant nutrients must be supplied externally (Woods & Mann, 2000; Glaser & Birk, 2011). With respect to potential nutrient sources, only C and N can be produced in situ via photosynthetic organisms and biological N xation, respectively. All other elements, such as P, K, Ca and Mg must be added for nutrient accumulation (Glaser, 2007) which can be best achieved by adding organic fertilizers such as manure or compost (Schulz & Glaser, 2011).
