## 1 Introduction  
Animal wastes such as manure, a low-value byproduct of livestock feedlots, are readily available in large quantities in most countries around the world, including those of South Europe. In Greece, where animal breeding activity is highly developed, about 41 Mm\({}^{3}\) of animal wastes is produced annually [1] and stocked outside animal processing industries or in rural areas, causing environmental pollution. Thus, traditional land application is no longer a viable solution. EU directives 1991/31/EC and 1991/676/EC imposed a reduction in the quantity of biodegradable wastes going to landfill. The use of animal manure as a fertilizer reduces the volume of wastes and ameliorates the physical, chemical and biological properties of soil, providing various nutrients for plant growth [2; 3]. However, the process of composting can change the properties of manure and, if not carefully managed, can cause eutrophication or contamination of soil and water sources [3; 4; 5]. The limitations imposed by the composting process have turned scientific attention to investigate the blending of composts with other wastes, to potentially improving the availability of nutrients and leaching of toxic heavy metals in soils [6; 7; 8; 9; 10]. Such wastes may be agricultural residues, which are particularly abundant in Mediterranean countries, and contain significant concentrations of Ca, Mg and K [5], in addition to urban bio-solids, which are rich in N, Ca, P and Fe [11]. All of these elements are important for improving soil fertility. In Greece, about 3.8 M dry tons of agricultural residues are produced annually [12], whereas about 30 kg/capita/year of dry bio-solids [11] are produced from wastewater treatment plants. As an alternative to soil application, animal manures can be converted to added-value products through thermal treatment. Pyrolysis destroys pathogens from these wastes [5], and generates bio-oil and gas, which provide useful energy [13; 14], and a carbon rich and porous material, biochar, which is of high interest. In addition to its potential to be used as fuel, as a carbon sink in soils for thousands of years, or as soil fertilizer [15; 16; 17; 18], biochar may exhibit similar properties as activated carbons, adsorbing various contaminants from soil and waste waters, such as heavy metals [19; 20; 21] and organic pollutants [17; 22; 23]. These properties, such as specific surface area, pore structure and organic functional groups, can be enhanced by physical or chemical activation [17; 24], providing a bio-carbon of higher reactivity and lack of tar in the product gas [25]. Physical activation without using chemical reagents, such as alkali or acids, is more environmentally friendly and has a lower cost. When steam is used, the calorific value of the product gas is enhanced, whereas when carbon dioxide from residual flue gases is the activation agent, the process offers a potential solution to the greenhouse gas problem, avoiding its capture and sequestration [24; 25]. Therefore, physical activation by steam or carbon dioxide is both environmentally and economically more advantageous. The final properties of biochar depend on the raw material and the conditions of the thermal process [5; 13; 14]. A large number of studies have been undertaken relevant to soil amendment using composts [3; 4; 26; 27] or biochars [7; 9; 10; 13; 26; 27; 28; 29] from various biomass materials (agricultural wastes, sewage sludge). Co-application of composts from animal manure and woody biochars has been less investigated (pig slurry/wood chips compost and rice huski biochar [2], chicken manure/straw compost and empty palm fruit biochar [6], pig manure/straw compost and maize straw biochar [23], horse and rabbit manure/grass, fruits, seaweed compost and oak wood biochar [30]). Reported results include reduction in heavy metals [2] and nutrients mobility [6] and bio-availability of organic pollutants [23] from contaminated soils, in addition to suppression of plant-parasitic nematodes [6].