**4.3 Synergisms for soil fertility and plant growth**

Combination of biochar with inorganic and organic fertilizers is clearly advantageous over the sole biochar or fertilizer amendments (Fig. 6). Plant growth significantly increased after biochar addition. Although pure compost application showed highest absolute yield during two growth periods, biochar compost mixture revealed highest relative performance. It should be mentioned here that biochar compost mixture received only 50% of pure biochar and 50% of pure compost treatments, thus providing evidence for biochar compost synergism (Fig. 6). In addition, it can be expected that in the long-term, compost will be mineralized more quickly than biochar or compost biochar mixtures. Mineral fertilizer retention was significantly more efficient when biochar was present although biochar did not increase cation exchange capacity at least after the first harvest (Schulz & Glaser, 2011). In comparison to mere mineral fertilizer there were clear advantages of plant growth and soil quality of the biochar-amended soils, especially when combining fertilizer (both inorganic and organic) with biochar. Therefore, optimization of biochar compost systems will be discussed in the following.

In a greenhouse experiment on a sandy soil under temperate climate conditions, plant growth (and thus soil fertility) generally increased with increasing amendment of biocharcompost (Fig. 7). This effect is more pronounced in a (nutrient-poor) sandy soil compared to a loamy soil (Fig. 7). It is interesting to note however, that at individual application rates, a synergistic effect of higher biochar application is obvious in the sandy soil (Fig. 7). This is even more interesting as biochar application rates were generally low with a maximum of 10 kg biochar per ton of compost material.

Biochar could cause a positive priming effect due to its high surface area providing habitat for microorganisms and due to input of partly labile C substrate (condensates). On the other hand, biochar is a stable compound which could stabilize labile compost OM thus providing

Composting of biochar could be successfully conducted over a wide biochar / organic material ratio covering up to 50% biochar by weight. During composting, a relative enrichment of biochar was observed which is obvious as biochar is much more stable than organic waste materials (Erben, 2011). However, biochar caused a significant positive priming effect on non-biochar composting materials at low (up to 1 weight%) biochar concentrations (Erben 2011) while at high (up to 50 weight%) biochar concentrations a significantly negative priming effect could be observed (Erben, 2011; Fig. 5). Therefore, a synergistic benefit for overall C sequestration could be observed when biochar was composted together with organic waste material (Erben, 2011). Further co-benefits might arise for soil microbial biomass and community structure composition and for biochar

Combining biochar addition and fermentation resulted in negative priming (Fig. 5), but the effect was weaker here than that of non-fermented treatments and hence ascribed rather to

Combination of biochar with inorganic and organic fertilizers is clearly advantageous over the sole biochar or fertilizer amendments (Fig. 6). Plant growth significantly increased after biochar addition. Although pure compost application showed highest absolute yield during two growth periods, biochar compost mixture revealed highest relative performance. It should be mentioned here that biochar compost mixture received only 50% of pure biochar and 50% of pure compost treatments, thus providing evidence for biochar compost synergism (Fig. 6). In addition, it can be expected that in the long-term, compost will be mineralized more quickly than biochar or compost biochar mixtures. Mineral fertilizer retention was significantly more efficient when biochar was present although biochar did not increase cation exchange capacity at least after the first harvest (Schulz & Glaser, 2011). In comparison to mere mineral fertilizer there were clear advantages of plant growth and soil quality of the biochar-amended soils, especially when combining fertilizer (both inorganic and organic) with biochar. Therefore, optimization of biochar compost systems

In a greenhouse experiment on a sandy soil under temperate climate conditions, plant growth (and thus soil fertility) generally increased with increasing amendment of biocharcompost (Fig. 7). This effect is more pronounced in a (nutrient-poor) sandy soil compared to a loamy soil (Fig. 7). It is interesting to note however, that at individual application rates, a synergistic effect of higher biochar application is obvious in the sandy soil (Fig. 7). This is even more interesting as biochar application rates were generally low with a maximum of 10

biochar alone than to its reinforcement of fermentation-induced negative priming.

**4.2 C sequestration and priming as function of biochar amount** 

surface oxidation which still has to be proven scientifically.

**4.3 Synergisms for soil fertility and plant growth** 

will be discussed in the following.

kg biochar per ton of compost material.

a negative priming effect.

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

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).

Synergisms between Compost and Biochar for Sustainable Soil Amelioration 189

Fig. 9. Sustainable management of natural resources by combining biochar with organic and

Based on the model of *terra preta* genesis (Glaser & Birk, 2011) various organic and inorganic feedstocks are mixed for composting providing different nutrients resources. Ideally, their physico-chemical properties should complete each other promoting an appreciable C/N ratio, water content, aeration, nutrient composition etc. of the initial compost pile. Besides their nutrient level, the used organic input materials can be characterized by their biological degradability and their contribution to different carbon pools. N-rich feedstocks such as grass clippings are easily decomposable particularly contributing to the labile OM pool which is used as an easy available food source of microorganisms and thus providing optimum conditions for a rapid rotting process. In contrast, ligneous materials are characterized by a lower degradability due to their higher lignin content partially contributing to the stable OM pool which has beneficial long-term effects for soil amelioration, carbon sequestration (Fig. 1) as well as humus reproduction (Table 1). The most recalcitrant material towards biological degradation is represented by biochar contributing at most to the stable OM pool of substrate mixtures. During subsequent aerobic decomposition OM getting stabilized resulting in an increase of stable C content. According to Yoshizawa et al. (2005) biochar promotes this rotting process due to its functions as a matrix for the involved aerobic microorganisms probably increasing decomposition speed. An co-composting experiment with poultry litter and biochar applied by Steiner et al. (2010)

inorganic wastes in compost processing (based on Glaser & Birk, 2011).

When looking at high biochar amounts, crop (oats, *Avena sativa*) yield significantly increased with increasing amounts of biochar and compost amendments, both for sandy (Fig. 8 left) and loamy soils (Fig. 8 right). However, in both cases, plant growth response was higher for biochar than for compost (sand: plant weight = 2.490 + 0.00676 compost + 0.0400 biochar, loam: plant weight = 4.088 + 0.0144 compost + 0.0349 biochar).

Fig. 8. Crop (oats, *Avena sativa*) response on a sandy (left) and loamy (right) soil with increasing biochar-compost amendments at high biochar additions (Schulz & Glaser, unpbulished).

#### **4.4 Can combined biochar compost processing contribute to optimized material flow management?**

By taking into account that *terra preta* formation was originally induced by human activity relying on the combined incorporation and biological transformation of charred stable OM on the one hand and nutrient-rich, organic feedstocks on the other hand (Fig. 3), it seems obvious that *terra preta* genesis can be understood as a sustainable and optimized management of natural resources. However, *terra preta* soils do not normally occur under conditions in which just compost or mulching material have been applied. Therefore, the addition of biochar can be recognized as a key factor for the reproduction of Terra preta similar substrates (chapter 3.1). However, the sole addition of charred biomass does also not result into the formation of *terra preta* soils. Thus, nutrient incorporation and microbial activity can be specified as further key factors.

In this respect, it seems to be a promising approach to combine the existing scientific knowledge about ancient *terra preta* genesis with modern composting technology to promote positive, synergestic effects for an efficient and optimized management of natural resources including 'organic wastes' to create humus and nutrient-rich substrates with beneficial effects for soil amelioration, carbon sequestration and sustainable land use systems. Fig. 9 gives a synthesis of the information about composting and biochar application and their beneficial effects hitherto presented in this review to show options for a sustainable material flow management.

When looking at high biochar amounts, crop (oats, *Avena sativa*) yield significantly increased with increasing amounts of biochar and compost amendments, both for sandy (Fig. 8 left) and loamy soils (Fig. 8 right). However, in both cases, plant growth response was higher for biochar than for compost (sand: plant weight = 2.490 + 0.00676 compost + 0.0400 biochar,

loam: plant weight = 4.088 + 0.0144 compost + 0.0349 biochar).

0

0

activity can be specified as further key factors.

20 40 60 80

50

100

Compost [Mg

150 200

ha-1]

Fig. 8. Crop (oats, *Avena sativa*) response on a sandy (left) and loamy (right) soil with increasing biochar-compost amendments at high biochar additions (Schulz & Glaser,

**4.4 Can combined biochar compost processing contribute to optimized material flow** 

By taking into account that *terra preta* formation was originally induced by human activity relying on the combined incorporation and biological transformation of charred stable OM on the one hand and nutrient-rich, organic feedstocks on the other hand (Fig. 3), it seems obvious that *terra preta* genesis can be understood as a sustainable and optimized management of natural resources. However, *terra preta* soils do not normally occur under conditions in which just compost or mulching material have been applied. Therefore, the addition of biochar can be recognized as a key factor for the reproduction of Terra preta similar substrates (chapter 3.1). However, the sole addition of charred biomass does also not result into the formation of *terra preta* soils. Thus, nutrient incorporation and microbial

In this respect, it seems to be a promising approach to combine the existing scientific knowledge about ancient *terra preta* genesis with modern composting technology to promote positive, synergestic effects for an efficient and optimized management of natural resources including 'organic wastes' to create humus and nutrient-rich substrates with beneficial effects for soil amelioration, carbon sequestration and sustainable land use systems. Fig. 9 gives a synthesis of the information about composting and biochar application and their beneficial effects hitherto presented in this review to show options for a sustainable material

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0

> Biochar [Mg ha-1]

P al nt we gi ht M[ g ha-1]

0

0

20 40 60 80

50

100

Compost [Mg ha-1]

150 200

1.8 2.1 2.4 2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8 5.1 5.4 5.7 6.0 6.3 6.6 6.9 7.2

B

unpbulished).

**management?** 

flow management.

iochar [Mg ha-1]

P al nt we gi ht M[ g ha-1]

Fig. 9. Sustainable management of natural resources by combining biochar with organic and inorganic wastes in compost processing (based on Glaser & Birk, 2011).

Based on the model of *terra preta* genesis (Glaser & Birk, 2011) various organic and inorganic feedstocks are mixed for composting providing different nutrients resources. Ideally, their physico-chemical properties should complete each other promoting an appreciable C/N ratio, water content, aeration, nutrient composition etc. of the initial compost pile. Besides their nutrient level, the used organic input materials can be characterized by their biological degradability and their contribution to different carbon pools. N-rich feedstocks such as grass clippings are easily decomposable particularly contributing to the labile OM pool which is used as an easy available food source of microorganisms and thus providing optimum conditions for a rapid rotting process. In contrast, ligneous materials are characterized by a lower degradability due to their higher lignin content partially contributing to the stable OM pool which has beneficial long-term effects for soil amelioration, carbon sequestration (Fig. 1) as well as humus reproduction (Table 1). The most recalcitrant material towards biological degradation is represented by biochar contributing at most to the stable OM pool of substrate mixtures. During subsequent aerobic decomposition OM getting stabilized resulting in an increase of stable C content. According to Yoshizawa et al. (2005) biochar promotes this rotting process due to its functions as a matrix for the involved aerobic microorganisms probably increasing decomposition speed. An co-composting experiment with poultry litter and biochar applied by Steiner et al. (2010)

Synergisms between Compost and Biochar for Sustainable Soil Amelioration 191

the integration of biochar into management of natural resources. Recent studies provide optimism for synergistic effects of compost and biochar technologies for ecosystem services

Ahmad, Z.; Yamamoto, S. & Honna, T. (2008). Leachability and Phytoavailability of

Alluvione, F.; Bertora, C.; Zavattaro, L. & Grignani, C. (2010). Nitrous Oxide and Carbon

Amlinger, F.; Peyr, S.; Geszti, J.; Dreher, P.; Karlheinz, W. & Nortcliff, S. (2007). *Beneficial* 

Annabi, M.; Houot, S.; Francou, C.; Poitrenaud, M. & Le Bissonnais, Y. (2007). Soil

Arroyo-Kalin M.; Neves E. G. & Woods W. I. (2009). Anthropogenic Dark Earths of the

Atkinson, C.J.; Fitzgerald, J.D. & Hipps, N.A. (2010). Potential mechanisms for achieving

Badr EL-Din, S.M.S.; Attia, M. & Abo-Sedera, S. A. (2000). Field assessment of composts

Bar-Tal, A.; Yermiyahu, U.; Beraud, J.; Keinan, M.; Rosenberg, R.; Zohar, D.; Rosen, V. &

Becker, J.; Hartmann, R. & Hubrich, J. (1995). *Das Modell des standortgerechten Kompostes.* 

Beck-Friis, B.; Smårs, S.; Jönsson, H. & Kirchmann, H. (2001). Gaseous emissions of carbon

Beraud, J.; Fine, P.; Yermiyahu, U.; Keinan, M.; Rosenberg, R.; Hadas, A. & Bar-Tal, A.

*Umlandes*, Univ.-Buchh., ISBN 978-3-88722-338-0, Bremen

Nitrogen, Phosphorus, and Potassium from Different Bio-composts under Chloride- and Sulfate-Dominated Irrigation Water, *J. Environ. Qual.*, Vol. 37, pp.

Dioxide Emissions Following Green Manure and Compost Fertilization in Corn,

*effects of compost application on fertility and productivity of soils. Literature Study*, Federal Ministry for Agriculture and Forestry, Environment and Water

Aggregate Stability Improvement with Urban Composts of Different Maturities,

Central Amazon region: remarks on their evolution and polygenetic composition, In: *Amazonian Dark Earths: Wim Sombroek's Vision*, Woods et al. (Eds.),

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produced by highly effective cellulolytic microorganisms, *Biology and Fertility of* 

Fine, P. (2004). Nitrogen, phosphorus, and potassium uptake by wheat and their distribution in soil following successive, annual compost applications, *J. Environ.* 

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**6. References** 

1288–1298.

seems to confirm the accuracy of this assumption since changes in pH and moisture content with greater peak temperatures and greater CO2 respiration suggest that composting process was more rapid if poultry litter was amended with biochar. In the same study the authors detected a reduction of ammonia emissions by up to 64 % and a decrease of total N losses by up to 52% if poultry litter was mixed with biochar. These observations support the hypothesis of higher nutrient retention ability induced by biochar amendment previously mentioned in this review.

Furthermore by the proliferation of microorganisms on the biochar backbone as well as between its pores, Yoshizawa et al. (2005) suggest that biochar properties are influenced by biological processes. Especially slow oxidation of biochar over time has been suggested to produce carboxylic groups on the edges of the aromatic backbone, increasing the CEC (Glaser et al., 2000). Due to higher temperature during compost processing, especially during thermophilic stage, biological activity as well as chemical reaction rate is increased, probably accelerating the partial oxidation and formation of functional groups of the amended biochar material but also interaction with labile OM and with minerals is favoured.

Besides the importance of biochar incorporation, additional amendments like clay minerals can add further value to the final compost product, e.g. by promoting an enhanced CEC or WHC due to their high adsorption or swelling capacity. Furthermore, their incorporation into organic substrates promotes the formation of organo-mineral complexes initiated by the biological activity of soil fauna after subsequent soil application. This aspect seems important since SOM in *terra preta* is stabilized by interaction with soil minerals (Glaser et al., 2003).

Other amendments like ash, excrements or urine contribute to the nutrients stock of the final composting product and can enhance microbial activity by their nutrient supply (Glaser & Birk 2011). According to Arroyo-Kalin et al. (2009) and Woods (2003), ash may have been a significant input material into *terra preta*, too. Furthermore for providing adequate moisture conditions during composting urine can be added instead of water for preventing the dehydration of composting piles while adding nutrients at the same time.

After compost maturation, the final compost substrate can be beneficially applied to soils. In this respect, the soil biota contribute to a further transformation of the applied material and provide essential ecological services, for instance by promoting aggregation and further OM stabilization. By enhancing the specific biological, physical and chemical properties of soils amended with the biochar composting substrates, plant growth is generally promoted.
