## 1 Introduction  
Soils of arid and semi-arid areas are characterized by low contents of organic matter, scarce water storage, imbalanced mineral nutrients, and coarse texture that make them low fertile environments [1]. The progressive land degradation process of arid, semi-arid, and dry sub-humal areas by human activities and climatic variations is known as desertification. This phenomenon is responsible for the gradual loss of fertility, leading to non-fertile and desertified lands [2]. According to the IPCC Fifth Assessment Report, 46.2% of the global land area is occupied by drylands, also called arid lands (i.e., hyper-arid, arid, semi-arid, and dry sub-humal areas) [3], and by the end of this century, a further 10% could be added to the previous value if a high greenhouse emission scenario continues [4]. Some of the limiting conditions of arid zone soils could be reversed with the use of bioproducts such as biofertilizers and biochar. The term biofertilizer is too broad, as it comprises microorganisms, anaerobic digestates, green fertilizers, and manures, as well as plant and algae extracts. Therefore, in general terms, biofertilizers are products of biological origin that contain nutrients and beneficial microorganisms to grow plants [5]. In turn,biochar is a porous, rich-carbon material obtained from biomass pyrolysis with limited or no oxygen [6; 7]. Biofertilizers are characterized by providing nutrients to the soil in forms that are either directly absorbed or rapidly transformed into assimilable forms by plants [5]. In addition, microorganisms present in biofertilizers can perform the following functions: (a) biofertilizers, increase the supply of nutrients by fixing atmospheric N, solubilizing minerals, and mineralizing organic compounds [5]; (b) biosimulateprocesses in seeds and plants by the production of phytohormones and vitamins, among others [8]; (c) improve soil by forming stable aggregates [9]; (d) act as biocontrol agents against pathogens [10]; (e) perform bioremediation by degrading xenobiotic or toxic compounds [11]; (f) act as eco-physiological improvers, by increasing the plant resistance to biotic and abiotic stress [12]. According to the above functions, biofertilizers represent an attractive option to improve soil quality, not only in drylands but also in humid ecosystems [13]. However, it is important to keep in mind that there are various factors involved in the interaction of microorganisms, plants, and soil. The crop, the nature of the biofertilizer, the type and current state of the soil, and soil and crop management practices will influence the outcome of biofertilizer application [14]. Currently, biofertilizers have gained importance due to different reasons. On the one hand, for countries that import most chemical fertilizers, biofertilizers represent a more accessible option and tailorable to local needs [14]. On the other hand, in the European Union, the goal for 2030 is to replace 20% of chemical fertilizers with biofertilizers to recover soils, to take care of the health of the population, and to reduce the environmental impact caused by the production and use of chemical fertilizers [15]. Due to its porous carbonaceous structure, high surface area, functional groups, and liming content, biochar is a multifunctional soil product [16]. It has fertilizer potential, depending on the composition of the feedstock and pyrolysis conditions used for its production [17; 18; 19]. Additionally, it can act as a soil conditioner, improving physical characteristics such as bulk density (BD), porosity, and water-holding capacity (WHC) [7]. Besides, there is evidence about biochar as an amendment to ameliorate chemical soil properties, such as pH, CEC, and mineral nutrient content [16; 20]. Likewise, some studies have found positive effects of biochar on the composition and activity of soil microorganisms. [21; 22; 23]. Additionally, biochar has been proved to be an environmentally friendly and cost-effective adsorbent to treat soils contaminated with heavy metals [24; 25].