# Agricultural Waste-Based Biochar for Agronomic Applications
## 6 Biochar in Composting  
The mixed use of biochar and compost to soils may significantly affect soil quality and crop productivity [221]. The addition of biochar combined with compost can be performed in two ways: one is that biochar is mixed with compost after the composting process is finished, and the other is that biochar is added to compost at the early stage of the composting process [222]. The first method was reported as enhancing soil nutrient content and crops productivity [89; 223], while other authors showed that biochar-based compost is more beneficial for soil and plants. [PERSON] et al. [224] found that the application of biochar-based compost improves the water retention capacity of sandy soils to a greater extent than compost without biochar. In another study, [PERSON] et al. [225] described that the application of compost, which is based on biochar, to soils protects nitrate content in biochar from leaching due to ion-water bonding in biochar micro- and nanopores. The addition of biochar during composting allows for improving the properties of the final product due to the biochar porous structure, which increases pile porosity and then enhances O\({}_{2}\) supply and avoids anaerobic fermentation [226]. [PERSON] et al. [227] showed that biochar adjusts the pile density of the compost and prevents the formation of agglomerates larger than 70 mm. [PERSON] et al. [228] reported that the use of biochars with alkaline pHs increases compost pH value. Contrary to this, [PERSON] et al. [229] showed that biochar addition to wheat straw and pig manure compostos promoted the decomposition of bulk organic matter in the mixture and the release of inorganic and organic acids, which decrease the pH of the final compostos. Biochar addition can also promote the creation of micropores for lactic-acid bacteria, which leads to a decrease in the pH of the compost [230]. Biochar has been reported to accelerate the humification process during composting [231; 232]. Its addition promotes the C transformation from raw materials to humic substances, mainly humicacids, fulvic acids and humans [233]. A compost rich in humic substances can provide a variety of agronomic benefices to soil, including water and nutrients, which enhances plant growth [234]. The time required to enter the thermophilic phase is also reduced after biochar addition and the temperature of composting is increased, which improves the humification efficiency [235]. Moreover, biochar reduces the leaching of organic C, P and N and promotes nutrient retention during the composting process. [PERSON] and [PERSON] [236] observed an increase in nutrient content during green waste composting after the addition of 20% biochar combined to 35% mushroom compost (w/w). Similarly, [PERSON] et al. [229] recorded an increase in water-soluble nutrients (PO\({}_{4}\)\({}^{3-}\), K\({}^{+}\) and Ca\({}^{2+}\)) of the compost product after addition of 10-15% (_w/w_) of wheat straw derived-biochar. In the study performed by [PERSON] et al. [235], the total nitrogen of the final compost increased by about 45% after the addition of biochar at a rate of 5% (_w/w_) as compared to the control. The ratio of C/N (_w/w_) is one of the most important parameters controlling the composting efficiency of organic wastes, and its optimal value should be about 25:1 [237]. As biochars contain high content of C, its addition could increase the C/N ratio of raw feedstocks rich in N. Simultaneously, composting can increase the biochar O/C ratio following the acceleration of the biochar surface oxidation [238]. Addition of biochar combined with compost can bring the same benefits to soils as chemical fertilizers. Biochar recycles P and C, and compost recycles P, N, C, and K; a combined application of both materials promoted the recycling of macro and micronutrients necessary for plant growth [239]. The biochar application rate significantly influences the composting process. A high loading rate could induce desiccation in composting due to excessive aeration. [PERSON] et al.