# Agricultural Waste-Based Biochar for Agronomic Applications
## 1 Introduction  
The high increase in world population has resulted in a rise in agricultural activities of more than three-fold in the last five years [1]. Technological advancements and the expansion of agricultural soils to meet the increased worldwide demand for food will inevitably result in a continuous growth of agricultural activities [2, 3]. Agricultural activities worldwide generate significant amounts of greenhouse gases; agriculture contributes, respectively, by 52 and 84% of the global anthropogenic emissions of methane and nitrous oxide [4]. Agriculture is often accompanied by negative effects on the environment and ecosystems. New practices are thus required to ensure the sustainability of the agricultural sector, including the sustainable management of agricultural residual materials [5]. This is consistent with the idea of the circular economy (grow-make-use-restore) [6], which, in contrast to the linear economy (take-make-use-dispose) [7], aims to eliminate wastes from a system by converting them into high added-value products [8]. In other words, the concept of the circular economy seeks to recover resources instead of importing new resources from outside of the system [9]. The worldwide challenges related to climate change, biodiversity loss, increased demand for raw materials, waste production and pollution are pushing organizations to shift their activities from a linear to a circular economy. In this context, several applications in many areas such as dairy products [10], agri-food [11], the use of fertilizers [12, 13] and biochar [14, 15] have been presented. Biochar has been gaining increasing attention recently as an organic material that can be effectively involved in agriculture to enhance soil fertility, stimulate plant growth, and increase agronomic productivity [16, 17]. Biochar is a carbon-based solid generally produced by pyrolysis of organic wastes under complete or partial exclusion of oxygen. Agricultural residues generated on farms as by-products of crop production are widely used for biochar preparation. The recovery of biomass harvested from agriculture is more efficient in developed countries as compared to developing countries, where the insufficient government policies and the lack of investment in waste management technologies still limit the appropriate management of these residues. The conversion of the large volume of agricultural residues produced annually to biochar could be a promising alternative to exploit their pronounced and manifold potential [18; 19; 20]. Biochar has many different aspects of application in agriculture, the main examples of which aim to increase soil fertility, increase nutrient uptake, and improve the water retention capacity of the soil. This will simultaneously improve crop productivity, mitigate plant stresses related to both salinity [16; 21] and toxic contaminants [22; 23], and reduce dependence on fertilizers [24; 25]. Biochar application directly affects soil physicochemical characteristics such as pH, nutrient availability, electrical conductivity, porosity, surface area, bulk density, and cation exchange capacity (CEC) [26; 27], and influences soil microbial structure by providing additional habitats to the microbial populations [28]. Biochar was also reportedly able to remediate organic and inorganic pollution in the soil by complexation or through sorption [29; 30]. Biochar has stable aromatic organic carbon. This carbon is hardly degraded and therefore does not return to the atmosphere [31; 32]. This will, in fact, minimize CH\({}_{4}\) and N\({}_{2}\)O emissions to the atmosphere and consequently reduce climate change [33]. The features of biochar essentially depend on both the type of feedstock and the pyrolysis conditions. Consequently, many biochar types with many different properties may be produced from different types of biomasses and at a broad range of pyrolysis temperatures [34].