**3.1 How can biochar improve soils?**

In central Amazonia, up to 350 ha wide patches of a pre-Columbian black earth-like anthropogenic soil exist, very well known as *terra preta (de Indio*) characterized by a sustainable enhanced fertility due to high levels of SOM and nutrients such as N, P and Ca (Glaser et al., 2001; Glaser, 2007; Glaser & Birk, 2011). However, the key for *terra preta* formation is the tremendous input of charred organic materials, known as biochar comprising up to 35% of SOM and on average 50 Mg ha-1 (Glaser et al., 2001). Biochar acts as a stable C compound being degraded only slowly with a mean residence time in the millennial time scale. Biochar has a high specific surface area (400 – 800 m2 g-1), it provides a habitat for soil microorganisms which can degrade more labile SOM. In addition, higher microbial activity accelerates soil stabilization as outlined in the previous section. Furthermore, higher mineralization of labile SOM and biochar itself provided important nutrients for plant growth. The general recipe of *terra preta* generation and the principal function of biochar are shown in Fig. 3.

Fig. 3. Principles of terra preta formation and soil biochar interaction.

Synergisms between Compost and Biochar for Sustainable Soil Amelioration 181

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

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

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

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

biochar sequestration potential of 1,126 Gt C would be possible (Table 6).

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

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

SOM and biochar degradation.

sorption and organo-mineral interaction in the long term.

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