# Agricultural Waste-Based Biochar for Agronomic Applications
## 6 Biochar in Composting
[240] reported that 10% to 15% loading rates could be considered as optimal, while other values of 20% [240] and 27% [241] have also been reported. Since biochar starting feedstock and pyrolysis temperature are proved to influence biochar properties, these parameters can also influence the quality of the final composting product. Most of the biochars used in composting are produced from woody biomass (e.g.,oak wood, beech wood, bamboo) and crop residues (e.g., wheat straw) by pyrolysis at temperatures ranging from 400 to 700 \({}^{\circ}\)C [229]. The joint application of biochar and compost and their interaction during composing can improve the effect of both materials [237]. Composting promotes the formation of carboxylic and phenolic groups on the biochar surface, which increases its reactivity [238], while biochar can affect the microbiological structure of the compost by affecting moisture, aeration, pH, and nutrient contents of the compost [242, 243]. The biochar porosity and its high surface area enhance qualitatively and quantitatively the composition of the microbial communities [244]. [PERSON] et al. [245] found an increase in microbial growth by 15% CFU/g after addition of biochar to compost. Biochar addition was reportedly able to increase the abundance of N\({}_{2}\)O-consuming bacteria, which reduces N\({}_{2}\)O emissions from biochar-amended compost during the maturation phase [246] and to reduce the abundance of pathogenic microorganisms during composite of organic wastes [247]. Furthermore, some studies reported the positive effect of biochar application on the enzyme's activity during composting. [PERSON] et al. [248] found that urease, phosphatase, and polyphenol oxidase activities increased by 30-40% in the biochar-amended compost. Similarly, [PERSON] et al. [249] reported that biochar addition to sewage sludge compost increased the activity of enzymes such asaryl-sulfatase, b-glucosidase, phosphatase, and dehydrogenase. Biochar addition during composting also reacts with heavy metals and contributes to their immobilization, which accordingly reduces the risks of applying compost to soils. In the study performed by [PERSON] et al. [250], passivation rates of 95%, 66% and 69% were reached for Cu, Pb and Cd, respectively, when biochar and commercial humic acid were added to pig manure compost. Similar results were found by [PERSON] et al. [251], observing a reduction in the bioavailability of heavy metals, particularlyAs and Pb, in compost based on sewage sludge. [PERSON] et al. [252] reported that the addition of biochar mixed with zeolite induces an increase in the binding ability and affinity of humic acid and Cu and Zn forming humic acid-complexed Cu and humic acid-complexed Zn, which enhanced the immobilization of heavy metals. The addition of biochar during the composting process has been also proved to control the GHG emissions from compost [232, 253, 254]. The release of NH\({}_{3}\) and NH\({}_{4}\)\({}^{+}\) during the composting process as a result of organic matter mineralization under increased temperature, pH and aeration, could be reduced after the addition of biochar due to its porous structure and its high adsorption capacity [253]. According to [PERSON] et al. [254], the addition of 10% tobacco stalk biochar combined with 10% zeolite and 2% wood vinegar (_w/w_) to pig manure compost allows for a reduction in NH\({}_{3}\) loss by about 64.45%, and a decrease in CO\({}_{2}\), CH\({}_{4}\) and N\({}_{2}\)O emissions by 33.90%, 50.39% and 79.51%, respectively. Opposite results have been found, however, such as by [PERSON] et al. [255], who observed an increase in CO\({}_{2}\) emissions due to the increase in pile temperature after biochar application.