**2. Compost**

Composting is the biological decompozition and stabilisation of organic matter derived from plants, animals or humans through the action of diverse microorganisms under aerobic conditions (Smith & Collins, 2007). The nal product of this biological process is a humus-like, stable substrate, being free of pathogens and plant seeds which can be beneficially applied to land as an agent for soil amelioration or as an organic fertilizer. Although historical traditions such as those of Ancient Egyptians or Pre-Columbian Indians of Amazonia suggest that composting is an ancient method for soil amelioration, fundamental scientific studies of this biological process were published only in the past four decades. Process engineering and the knowledge about the dependence and interaction of numerous competing forces and factors within a composting matrix have been just recently established (Haug, 1993).

Multiple composting methods and systems have been developed, varying from small, home-made reactors used by individual households, over medium-sized, on-site reactors operated by farmers, to large, high-tech reactors used by professional compost producers. In spite of different process techniques, the fundamental biological, chemical and physical aspects of composting remain always the same. This concerns for example the suitability of different input materials and amendments as well as their appropriate composition, substrate degradability, moisture control, porosity, free air space, energy balance as well as decomposition and stabilization (Haug, 1993; Bidlingmaier et al., 2000).

#### **2.1 What is compost and how is it produced?**

All proper composting processes go through four stages: (1) mesophilic, (2) thermophilic, (3) cooling, finally ending with (4) compost maturation (Fig. 2). The duration of each stage depends on the initial composition of the mixture, its water content, aeration and quantity and composition of microbial populations (Neklyudov et al., 2006; Smith & Collins, 2007).

resources, i.e. fossil fuels, fossil nutrients stocks and arable land, can be recognized. On the other hand, urbanization and growing population interconnected with an increased amount of waste output is responsible for environmental hazards and pollution. Therefore, composting became an efficient means of waste processing, soil amelioration and general

However, up to now reported C sequestration potential due to compost management is limited in terms of C use efficiency and long-term C preservation even combined with organic farming and no till management. Therefore, new concepts for C sequestration combating against further raise of atmospheric CO2 emissions are urgently needed. One promising option is using the "*terra preta* concept" combining biochar and composting technologies. This concept could enhance quality and material properties of compost products leading to a higher added value and to a much better C sequestration potential due

We hypothesize that composting of biochar together with other biogenous materials containing labile organic matter and nutrients can be an appropriate tool to produce a substrate with similar properties as *terra preta* such as enhanced soil fertility and C

Composting is the biological decompozition and stabilisation of organic matter derived from plants, animals or humans through the action of diverse microorganisms under aerobic conditions (Smith & Collins, 2007). The nal product of this biological process is a humus-like, stable substrate, being free of pathogens and plant seeds which can be beneficially applied to land as an agent for soil amelioration or as an organic fertilizer. Although historical traditions such as those of Ancient Egyptians or Pre-Columbian Indians of Amazonia suggest that composting is an ancient method for soil amelioration, fundamental scientific studies of this biological process were published only in the past four decades. Process engineering and the knowledge about the dependence and interaction of numerous competing forces and factors

Multiple composting methods and systems have been developed, varying from small, home-made reactors used by individual households, over medium-sized, on-site reactors operated by farmers, to large, high-tech reactors used by professional compost producers. In spite of different process techniques, the fundamental biological, chemical and physical aspects of composting remain always the same. This concerns for example the suitability of different input materials and amendments as well as their appropriate composition, substrate degradability, moisture control, porosity, free air space, energy balance as well as

All proper composting processes go through four stages: (1) mesophilic, (2) thermophilic, (3) cooling, finally ending with (4) compost maturation (Fig. 2). The duration of each stage depends on the initial composition of the mixture, its water content, aeration and quantity and composition of microbial populations (Neklyudov et al., 2006; Smith & Collins, 2007).

sequestration. Current available literature will be reviewed on these aspects.

within a composting matrix have been just recently established (Haug, 1993).

decomposition and stabilization (Haug, 1993; Bidlingmaier et al., 2000).

**2.1 What is compost and how is it produced?** 

environmental improvement.

to the long-term stability of biochar.

**2. Compost** 

Fig. 2. Different stages during composting as function of time, appearance and succession of compost biota, temperature and further processes (based on Lechner et al., 2005 and Smith & Collins, 2007).

During the mesophilic phase, labile C-rich substrates are rapidly metabolized by a mixture of bacteria, actinomycetes and fungi preferring moderate temperature typically between 15 and 40 °C. Due to this aerobic metabolism, heat is generated. Turning the material leading to aeration temporarily decreases temperature, resulting in a rapid decomposition of further available material and thus, temperature increases again (Fig. 2). During the termophilic phase, temperature rises above 40 °C, favouring mainly actinomycetes and thermophilic bacteria such as *Bacillus*. When labile C compounds of the feed substrates decline, a gradual decrease in temperature occurs leading to the cooling phase (Fig. 2). Especially fungi have a preference for the remaining and more complex and thus degradation-resistant lignin and cellulose compounds. In addition, actinomycetes have a major importance when humic materials are formed from decomposition and condensation reactions Smith & Collins, 2007). The final maturing phase is characterized by even lower temperature below 25 °C and reduced oxygen uptake rates of aerobic microorganisms. During this stage, degradation of the more refractory organic compounds continues and soil meso and macro fauna enters. Organisms of this stage have beneficial influence on compost maturation as well as plant diseases suppression as they are able to metabolize phytotoxic compounds (Gottschall, 1984; Haug, 1993). Thus, compost quality increases especially during the last phase. Compared to the starting feed mixture, the final compost is attributed by a lower C/N ratio of 15 – 20 and

Synergisms between Compost and Biochar for Sustainable Soil Amelioration 171

majority of studies of different authors have unambiguously proven a better humus

According to Amlinger et al. (2007), the average SOM demand of agriculturally used soils can be met by applying 7 – 10 Mg (dry matter) compost ha-1 a-1. Therefore, for a long-term

**Organic fertilizer Organic matter (dm) Organic C (dm) Humus reproduction TOC increase [Mg ha-1**

Table 1. Organic matter and SOM reproduction of different organic fertilizers when applied

**Reduction of Bulk Density**: Compost application generally influences soil structure in a beneficial way by lowering soil density due to the admixture of low density OM into the mineral soil fraction. This positive effect has been detected in most cases and it is typically associated with an increase in porosity because of the interactions between organic and

**Increase of aggregate stability**: In general, soil structure is defined by size and spatial distributions of particles, aggregates and pores in soils. The volume of solid soil particles and the pore volume influences air balance and root penetration ability. As a general fact the more soil structure is compacted, the more unfavorable are the soil conditions for plant growth. By incorporation of compost into the soil, aggregate stability increases most effectively in clayey and sandy soils. Positive effects can be expected by well humified (promoting micro-aggregates), as well as fresh, low-molecular OM (promoting macroaggregates). Macro-aggregates are mainly stabilized by fungal hyphen, fine roots, root hair and microorganisms with a high portion of easily degradable polysaccharides (Amlinger et al., 2007). Subsequently, the importance of stronger degraded organic compounds for the stabilization of smaller aggregates increases with time of transformation periods ranging from some few to several thousand years (Kong et al., 2005; Lützow et al., 2008; Marschner & Flessa, 2006). In this respect, the aromatic structure of inert organic compounds seems to play an important role in stabilizing micro-aggregates in the range of 2 – 20 µm as well as 20 – 250 µm by polyvalent cation bridges with clay minerals (Tisdall & Oades, 1982). Besides clay minerals and oxides, fine roots, hyphen networks as well as glue-like polysaccharides originated from root and microbial exudates significantly contribute to the formation of

Furthermore, aggregate and pore properties of soils are associated with specific "active" surface area influencing several storage and exchange processes in soil. The higher the specific surface area, the more intensive interactions can occur between soil fauna, microorganisms and root hairs under optimum conditions (e.g. sufficient humidity). As a result, a high specific surface area can create the prerequisite for an optimal soil formation

**Improvement of pore volume and hydraulic conductivity**: Hydraulic conductivity is the percolation rate in soils per area and time unit depending on (i) actual soil moisture tension

**]**

increase of SOM, however, more than 10 Mg dry matter compost ha-1 a-1 is required.

Green manure, leaves 90% 52% 15% 0,4 Slurry, straw, liquid manure 75% 44% 20-30% 0,5 Fresh compost, stable manure, solid digestate 50% 50% 35-45% 1,0 Mature compost 36% 50% >50% 1,3

reproduction for composted materials (Amlinger et al., 2007).

at 10 Mg ha-1 a-1 (Data from BGK & FAL, 2006).

inorganic fractions (Amlinger et al., 2007).

micro-aggregates.

(Amlinger et al., 2007).

**2.2.2 How are physical soil properties influenced?** 

a higher pH value (Smith & Collins, 2007). It can contain considerable amount of plantavailable NO3 - while NO4+ content is generally decreasing (Fig. 2). Furthermore, odour potential from compost is significantly reduced (Haug, 1993). But of utmost importance seems the fact that the organic matter has been stabilized, thus containing fairly resistant C compounds (Smith & Collins, 2007). Its application to the land influences several biological, chemical and physical soil properties in a positive and sustainable way which is outlined in Fig. 1 and which will be discussed in the following in more detail.
