**4.1 Effect of process (mixing, composting, fermentation)**

*Terra preta* was most likely formed by mixing of charring residues (biochar) with biogenic wastes from human settlements (excrements and food wastes including bones and ashes) which were microbially converted to a biochar-compost-like substrate (Glaser et al., 2001; Glaser, 2007; Glaser and Birk, 2011). Thus, co-composting of biochar and fresh organic material is likely to have a number of benefits compared to the mere mixing of biochar or compost with soil. Examples are enhanced nutrient use efficiency, biological activation of biochar and better material flow management and a higher and long-term C sequestration potential compared to individual compost and biochar applications (negative priming effect).

Compared to compost and biochar mixing, an increased decomposition of biochar can be expected during composting although biochar is much more stable than other organic materials. As observed by Kuzyakov et al. (2009), biochar decomposition rates increase as long as easily degradable C-rich substrate is available. Additionally, Nguyen et al. (2010) reported that higher temperature increased biochar oxidation and thus decomposition. However, these effects are much lower for biochar than for compost feedstock. On the other hand, surface oxidation will enhance the capacity of biochar to chemisorb nutrients, minerals and dissolved OM. The overall reactivity of biochar surfaces therefore probably increases with composting (Thies & Rillig, 2009).

From the compost point of view, there is evidence that biochar as a bulking agent improves oxygen availability and hence stimulates microbial growth and respiration rates (Steiner et al., 2011). Pyrolysis condensates adsorbed to biochar initially provoked increased respiration rates in soils which most likely occur also during composting (Smith et al., 2010). Biochar in compost provides habitats for microbes, thereby enhancing microbial activity. Steiner et al. (2011) reported increased moisture absorption of biochar-amended composts with beneficial effects on the composting process.

It was often stated in non-scientific literature, that *terra preta* was formed by anaerobic fermentation of biochar with organic wastes using "effective microorganisms ®" (EM ®) which consist mainly of a mix of lactic acid and photosynthetic bacteria, yeasts, actinomycetes as well as other genera and species of beneficial microorganisms (Higa & Wididana, 1991). However, there is no scientific proof for this and from a practical point of view it is most unlikely that pre-Columbian Indians manually moved tremendous amounts of soil and organic wastes for fermentation in closed containers. For the average dimension of *terra preta* being 20 ha wide and one meter deep, 200,000 m3 or 260,000 tons of soil would have being moved by hand twice (forth and back) for terra preta generation which is most unlikely.

to sterilized soil. However, alfalfa shoot weight was increased by a factor of 1.7 and nodule weight by 2.3 times in a treatment receiving biochar, fertilizer and rhizobia compared to a set only treated with fertilizer and rhizobia. According to Nishio (1996), this clearly indicates that the stimulatory effect of adding biochar may appear only when a certain level of indigenous AMF is present. Biochar amendment generally seem to stimulate soil fungi which seems logic as biochar is a complex matrix being degradable only by soil fauna and

*Terra preta* was most likely formed by mixing of charring residues (biochar) with biogenic wastes from human settlements (excrements and food wastes including bones and ashes) which were microbially converted to a biochar-compost-like substrate (Glaser et al., 2001; Glaser, 2007; Glaser and Birk, 2011). Thus, co-composting of biochar and fresh organic material is likely to have a number of benefits compared to the mere mixing of biochar or compost with soil. Examples are enhanced nutrient use efficiency, biological activation of biochar and better material flow management and a higher and long-term C sequestration potential compared to

Compared to compost and biochar mixing, an increased decomposition of biochar can be expected during composting although biochar is much more stable than other organic materials. As observed by Kuzyakov et al. (2009), biochar decomposition rates increase as long as easily degradable C-rich substrate is available. Additionally, Nguyen et al. (2010) reported that higher temperature increased biochar oxidation and thus decomposition. However, these effects are much lower for biochar than for compost feedstock. On the other hand, surface oxidation will enhance the capacity of biochar to chemisorb nutrients, minerals and dissolved OM. The overall reactivity of biochar surfaces therefore probably

From the compost point of view, there is evidence that biochar as a bulking agent improves oxygen availability and hence stimulates microbial growth and respiration rates (Steiner et al., 2011). Pyrolysis condensates adsorbed to biochar initially provoked increased respiration rates in soils which most likely occur also during composting (Smith et al., 2010). Biochar in compost provides habitats for microbes, thereby enhancing microbial activity. Steiner et al. (2011) reported increased moisture absorption of biochar-amended composts with beneficial

It was often stated in non-scientific literature, that *terra preta* was formed by anaerobic fermentation of biochar with organic wastes using "effective microorganisms ®" (EM ®) which consist mainly of a mix of lactic acid and photosynthetic bacteria, yeasts, actinomycetes as well as other genera and species of beneficial microorganisms (Higa & Wididana, 1991). However, there is no scientific proof for this and from a practical point of view it is most unlikely that pre-Columbian Indians manually moved tremendous amounts of soil and organic wastes for fermentation in closed containers. For the average dimension of *terra preta* being 20 ha wide and one meter deep, 200,000 m3 or 260,000 tons of soil would have being moved by hand twice (forth and back) for terra preta generation which is most

soil fungi (Birk et al., 2009).

**4. Combined compost and biochar** 

increases with composting (Thies & Rillig, 2009).

effects on the composting process.

unlikely.

**4.1 Effect of process (mixing, composting, fermentation)** 

individual compost and biochar applications (negative priming effect).

Nevertheless, fermentation theoretically provides microorganisms to soil which could be beneficial for soil health and ecosystem services. In a composting / fermentation experiment with and without biochar, the overall C loss during fermentation was about 30% lower when compared to composting (Fig. 5). However, when composting the fermented material, overall OC loss was even higher compared to the composted only material (Fig. 5). This indicates that fermented OM is only stable as long as it was kept anaerobic. As soon as piles were turned (after fermentation) and oxygen became available, the intermediate fermentation products were mineralized to an even higher extent than the non-fermented counterparts (Fig. 5). Biochar addition appeared to amplify fermentation-induced stabilization, since compost piles with 50 (DEM50) and 100 (DEM100) kg biochar per ton of organic feedstock material showed reduced OC loss compared to the fermentation control without biochar (DEM0, Fig. 5).

Fig. 5. Relative mass balance of organic carbon (OC) during fermentation (Days 1 – 29) and composting (Days 9 – 85; means ± standard error; significant differences between 'Days1-29' OC losses of D0 and DEM0 (see asterisks), p < 0.05, n=3, tested with a Student's t-test). D0, D50 and D100 are composted materials at 0, 50 and 100 kg biochar addition per ton of composted materials, respectively, For DEM 0, 50 and 100 it was the same approach but during the first 29 days of the experiment, these piles were incubated with effective microorganisms under anaerobic conditions (Erben, 2011).

Fermentation-induced negative priming of fresh OM could indeed be observed, but only as a temporary effect which was reversed during subsequent composting. Thus, fermentation did not result in an enhanced stabilization of compost. A distinct effect of the EM preparation could not be identified (Erben, 2011). Nevertheless, benefits of fermentation in OM treatment and for soil application remain to be assessed.

Synergisms between Compost and Biochar for Sustainable Soil Amelioration 187

Fig. 6. Crop (oats, *Avena sativa*) response of two consecutive harvests on a sandy soil amended with different materials. Treatments comprised control (only water), mineral fertilizer (111.5 kg N ha-1, 111.5 kg P ha-1 and 82.9 kg K ha-1), compost (5% by weight), biochar (5% by weight) and combinations of biochar (5% by weight) plus mineral fertilizer (111.5 kg N ha-1, 111.5 kg P ha-1 and 82.9 kg K ha-1) and biochar (2.5% by weight) plus

> Compost-biochar 3 Compost-biochar 5 Compost-biochar 10 Compost-biochar 0 TPN control

Fig. 7. Crop (oats, *Avena sativa*) response on a sandy (left) and loamy (right) soil with increasing biochar-compost amendments (x axis) at low biochar additions (3, 5 and 10 kg per ton of compost, different symbols) compared to control soil (without amendments) and

a commercial biochar-containing product (TPN) (Schulz and Glaser, unpublished).

**Plant weight (g)**

**Total plant weight in loamy soil**

Compost-biochar 3 Compost-biochar 5 Compost-biochar 10 Compost-biochar 0 TPN control

0 50 100 150 200 250 300 **Biochar-compost application amount (Mg ha-1 20 cm-1)**

compost (2.5% by weight) (Schulz & Glaser, 2011).

**Total plant weight in sandy soil**

0 50 100 150 200 250 300 **Biochar-compost application amount (Mg ha-1 20 cm-1)**

**Plant weight (g)**
