2.1 Composting
Composting is a controlled aerobic biodegradation process for the stabilization and sanita-tion of solid organic wastes. Microbial degradasanita-tion processes transform the organic sub-stance to minerals and stabilized OM under the release of CO2, water and heat. Due to the production of heat energy by the microbial activity, the composting process undergoes a thermophilic phase. In this phase, temperatures of maximum 80 °C can be reached, and san-itation of the composted material through pathogen elimination takes place. (Insam and de Bertoldi, 2007)
Composting is a well-established process for a wide range of organic wastes. It is one method for the stabilization and sanitation of human excreta which is successfully applied (Vin-nerås et al., 2003; Ogwang et al., 2012). Compost from faecal matter has already been tested as a soil amendment and was effectively improving crop productivity (Krause et al., 2016).
However, research on the potentials of faecal composts in crop production is still scarce.
Comparison to other urban organic fertilizers can help to estimate the potential of faecal composts for the integration into a more sustainable and regional nutrient cycling economy.
2.2 Vermicomposting
2.2.1 Process of vermicomposting
Vermicomposting is a managed stabilization process for organic wastes using earthworms (Dominguez, 2004). The worms increase the surface area of the original material by ingesting and fragmenting it, simultaneously aerating and mixing it by moving through the substrate.
Hence, they optimally prepare it for decomposition by microorganisms, thus enhancing mi-crobial decomposition rate of the waste (Lim et al., 2016). Since a large share of the waste passes through the digestive tract of the earthworms during the process of vermicomposting, the resulting VC is a fine-textured product (Arancon et al., 2004).
2.2.2 Microbiology of vermicomposting and Potential for AMF supply
Part of the degradation process of organic material is carried out by endosymbiotic bacteria in the intestines of the worms. Enzymes produced by these endosymbiotic bacteria, readily available OM and mucus are released together by the worms as vermicast and serve as cat-alysts for decay of OM or nutrients for the growth of microorganisms. At the same time, the
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earthworms feed on microorganisms colonizing the waste material, thus having a strong in-fluence on the composition of the microbial community in the material. Overall, the total biomass of bacteria and fungi is reduced during vermicomposting. Bacterial growth rates are decreased, whereas fungal growth rates are not affected. (Dominguez, 2011)
Brown et al., (2000) showed, that earthworms are able to digest a range of microorganisms, such as bacteria, fungi, but also protozoa and nematodes. Fungal spores, on the other hand, can only partly be degraded, depending on the spore characteristics and the earthworm spe-cies (Reddell and Spain, 1991; Brown, 1995). It was examined for several fungal spespe-cies (e.g.
fusarium lateritium, a plant parasite), that their spores were not able to germinate after pas-sage through earthworm intestines (Moody et al., 1996). In a study about earthworms as vectors for mycorrhiza in the soil, on the other hand, it was found, that the majority of AMF spores remained intact and able to germinate after the passage through earthworms of dif-ferent species (Reddell and Spain, 1991). In the same study, sorghum (sorghum bicolour) was successfully infected with AMF by the examined earthworm casts (ibid.).
2.2.3 Sanitation of vermicomposts and removal of human pathogens
Vermicomposting is a mesophilic process, usually taking place in a temperature range of 10 - 35 °C, with an optimum of 15 - 25 °C, depending on the species (Dominguez, 2004). Out-side this range of tolerable temperatures, the feeding and reproduction activity of the earth-worms decreases or even ceases completely (Edwards and Bohlen, 1996). The movement of the earthworms through the waste material aerates and mixes the substrate, thus preventing the process of self-heating, which is induced by microbial activity in thermophilic composting systems. Therefore, in contrast to thermophilic composting, there is no thermophilic phase, in which temperatures of minimum 55 - 70 °C are reached over a period of at least three days (Edwards and Subler, 2011).
Studies on the reduction of human pathogens during vermicomposting showed contradicting results. In many cases, however, a considerable reduction of various microbes, such as E. coli and Salmonella (Edwards and Subler, 2011), or Salmonella and Pseudomonas (Finola et al., 1995) was achieved. Riggle (1996) found that after two months the levels of Salmonella, enteric viruses and viable helminth eggs fell below the detection limit, while faecal bacteria were still found, yet in notably reduced numbers. Thus, input material like human excreta should undergo further treatment steps, to ensure elimination of human pathogens, before used as a soil amendment in food crops (Lalander et al., 2013).
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2.2.4 Vermicompost for fertilizer application
For the production of VC, a broad variety of organic wastes is utilized, such as different animal manures, food and paper wastes, biogas slurry and crop residues. The type of input material has an effect on the fertilizing qualities of the VC product (Aynehband et al., 2017). However, independent from the type of organic waste used, 84 % of the studies on VC effects on plant growth published until 2018, showed yield increasing effects on plants (Hussain and Ab-basi, 2018). Reasons stated for the growth stimulation of plants are an improved nutrient supply (Tognetti et al., 2005), as well as the presence of growth promoting substances in the vermicast, e.g auxin and cytokinin (Krishnamoorthy and Vajranabhaiah, 1986) or humic acids (Canellas et al. 2002). Hernandez et al. (2015) found that application of humates extracted from VC reduced the time until harvest maturity in lettuce by two to three weeks and Arancon et al. (2004) related the stimulating effect of VC on plant growth to plant growth regulators and enhancement of microbial activity in the soil. Furthermore, VC was found to suppress the populations of plant pests (Arancon et al., 2005; Simsek-Ersahin, 2010), an ef-fect also known from other organic fertilizers (Culliney and Pimentel, 1986). In addition to its plant growth promoting qualities, VC can improve soil properties, e.g. by increasing soil po-rosity and water holding capacity (Goswami et al., 2017).
The application of very high quantities of VC as an amendment for soils or substrates, can on the other hand reduce the growth of plants (Ali et al., 2007) or even lead to their death (Lazcano and Dominguez, 2010). Non-degraded phytotoxic compounds from the organic waste and an increased salinity can be reasons for this effect (Singh et al., 2013).
Compared to thermophilic composts, VC from the same input material were found to achieve a finer and more homogeneous product (Hanc and Dreslova, 2016) and to have a higher con-centration of nutrients (Pattnaik and Reddy, 2010). Thermophilic compost, which was post-treated by vermicomposting was found to have higher nutrient concentrations and improved fertilizing properties compared with the original TC (Tognetti et al., 2005).
2.3 Mycorrhiza
The term mycorrhiza (from Greek ‘mycos’ = fungus and ‘rhiza’ = root) refers to a mutualistic symbiosis between fungi and plant roots, which can be found in the majority of terrestrial plant species (Smith and Read, 2008). Mycorrhiza comprises, among others, the two im-portant types i) ectomycorrhiza, which grows mainly on tree roots and whose hyphal network
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is formed outside and in the intercellular spaces of the outer root layers, and ii) the endomy-corrhizal fungi, which grow between and in the cells of the root epidermis of their host plant (Parniske, 2008).
Endomycorrhiza are the arbuscular mycorrhizal fungi (AMF), which are named after tree-shaped structures (arbuscules), that many of the AMF (Division Glomeromycota) form within the plant root cells. A key function of AMF for the host plant is the supply with a number of nutrients, most of all P, and also N, Zn and Cu, through uptake by fungal hyphae. The hyphal network spreads and thus extends the range of nutrient access for the plant beyond the rhi-zosphere. Furthermore, fungal hyphae are thinner than plant roots and are therefore able to penetrate smaller pores, which are not accessible to the plant roots themselves. Hence, at the same root length, the efficacy of AMF colonized roots in nutrient uptake is higher than in non-colonized roots. (Smith and Read, 2008)
In return, the Glomeromycota fungi are supplied with organic C assimilated by the plant. The fungi are dependent on the C supply of the associated plant to complete their life cycle and are thus obligate biotroph (Parniske, 2008). The supply of C to the fungi can make up 4 - 20 % of the net assimilated C of the plant (Smith and Read, 2008). In addition to the nutrient ex-change between the symbiotic partners, AMF increase the resistance of the plants to root pathogens and improve their drought stress tolerance (Marschner, 2012).
2.4 Lettuce
Lettuce (lactuca sativa var. capitata L.) is an important vegetable plant belonging to the fam-ily of Asteraceae. It can be cultivated in many different soils and prefers temperatures of 10 - 20 °C and a pH of 6 - 8 (Rubatzky and Yamaguchi, 1997). Lettuce is sensitive to acidic pH values and can be inhibited in growth and head formation at temperatures over 30 °C (ibid.).
Germination temperature should be controlled, as temperatures under 20 °C stimulate and accelerate germination and improve germination rates (Borthwick and Robbins, 1928). Let-tuce plants have a comparably shallow root system and are therefore prone to moisture stress (Rubatzky and Yamaguchi, 1997).
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