## 1 Introduction  
Sandy soils are considered marginal for agriculture due to coarse texture, high saturated hydraulic conductivity, low soil organic carbon, and poor aggregate stability and water retention capacity. Farming in sandy soils of drylands relies on the use of external inputs such as organic and inorganic fertilizers. However, organic amendments in sandy soils face the challenge of constant turnover, as their decomposition rate is high and the added organics are usually mineralized within only small cropping seasons. Thus, organic amendments must be applied frequently to sustain productivity. A suitable option is the use of more stable organic materials. In this respect, biochar could be a suitable choice to replace easily decomposable organic manures [1]. Biochar (BC) is produced by pyrolysis of organic residues under limited oxygen conditions. It is resistant to soil microorganisms' decomposition and acts as a sink of carbon, leading to a decrease in the emissions of greenhouse gases into the environment and an increase in crop production [2]. BC is being increasingly discussed as a means to sequestrate carbon in the soil, and in this way reduce the CO\({}_{2}\) emissions into the environment or atmosphere and mitigate climate change [3]. In their meta-analysis, [PERSON] et al. [4] presented that BC addition increased the porosity, connectivity of pore space, number of pores, available water content, field capacity, and permanent wilting point of soils, and reduced the bulk density and saturated hydraulic conductivity of sandy soils. The effect of BC was dependent on its application rate, particle size, specific surface area, porosity, and pyrolysis temperature. In addition to the above-mentioned improvement of hydrophysical properties of sandy soil, the conversion of biomass into biochar with subsequent soil storage is assumed as a prospective strategy for reducing atmospheric CO\({}_{2}\) concentrations and mitigating climate change by enhancing the short-term carbon sequestration [5]. Biochar can increase sandy soil porosity directly by reducing bulk density and through its inherent high porosity. This has a direct effect on soil water storage and mobility. The increased number of pores (especially mesopores) and total soil porosity lead to an increase in soil moisture retention. [PERSON] et al. [6] presented a small decrease in bulk density (from 1.60 to 1.56 g cm\({}^{-3}\)) and a corresponding small increase in porosity (from 4.5 to 4.7%) after the addition of 1% of hardwood (_Eucalyptus polychratea_) BC (pyrolyzed at 550 \({}^{\circ}\)C and sieved to 0.25-2 mm) to sandy soil. [PERSON] et al. [7] used the BC from date palm tree residues pyrolyzed at 300, 400, 500, and 600 \({}^{\circ}\)C, and found that the porosity of sandy desert soil increased significantly (from 38 to 46%) after the addition of BC pyrolyzed at 600 \({}^{\circ}\)C at the rate of 8 Mg ha\({}^{-1}\). [PERSON] and [PERSON] [8] found that amending the pre-graded (0.13-0.36 mm) fine sand with 20 g kg\({}^{-1}\) of woodchip and forest residue BC did not significantly increase the total porosity. However, the total porosity of fine sand was significantly increased when the amendment rate was increased to 50 and 100 g kg\({}^{-1}\). [PERSON] et al. [9], [PERSON] et al. [10], and [PERSON] et al. [11] showed that BC application increases soil water retention, especially in sandy soils, which has generally been attributed to an increase in soil micro-porosity and the high porous structure of BC. [PERSON] et al. [3] found that the maize BC addition resulted in a 16.3% increase in available water content (AWC) in the lab, but only an AWC increase of 4.3% in the field. [PERSON] et al. [6] presented a small increase in AWC (from 5 to 6%) after the addition of 1% of hardwood (_Eucalyptus polychratea_) BC (pyrolyzed at 550 \({}^{\circ}\)C and sieved to 0.25-2 mm) to sandy soil.