## 4 Discussion  
A statistically significant increase in porosity was registered for 4 of 8 treatments (namely for those with a large-sized (125 \(\upmu\)m-2 mm) BC, produced at both temperatures and applied at both rates). It means that the hypothesis that biochar addition will increase porosity was only partially fulfilled. Our findings are consistent with those of [PERSON] et al. [6], who presented a small increase in porosity (from 4.5 to 4.7%) after the addition of 1% of hardwood (_Eucalyptus polyphracta_) BC (pyrolyzed at 550 \({}^{\circ}\)C and sieved to 250 \(\upmu\)m-2 mm) to sandy soil. [PERSON] et al. [35] observed a higher increase in porosity (18%) after the addition of 1% of biochar to sandy soil. In our study, porosity in the soil after BC addition was significantly affected by pyrolysis temperature, particle size, and application rate. Similar results were observed by [PERSON] et al. [36], who found that porosity in sandy soil was affected by biochar rates and particle size but not by BC feedstock type. The changes in the physical properties of soil, such as porosity, are attributed to the mixing of soil with less dense material. This phenomenon is evident immediately after application in compacted, coarse-textured soils, as observed by [PERSON] [37]. Our results confirmed the effects of the pyrolysis temperature and application rate interaction and particle size and application rate interaction on \(p\) values. This fact is consistent with [PERSON] et al. [38], who stated that the BC's effects on \(P\) depend on the BC's internal porosity, which is influenced by production conditions such as the gasification, pyrolysis temperature, and particle size of the applied BC. On the contrary, [PERSON] et al. [36] did not confirm the significant effect of particle size and application rate interaction or the BC feedstock and application interaction on \(P\) in sandy soil. The BC has been found to enhance soil porosity through various mechanisms. These include the creation of new pores directly [39], a decrease in bulk density, and an increase in soil aggregation [38]. Moreover, the formation of accommodating pores between BC particles and soil aggregates [40], as well as the reduction in soil packing and restructuring of pore distribution, contribute to the increase in soil porosity. A statistically significant increase in the available water content for plants (AWC) was registered in all but one BC treatment (an increase in AWC for the treatment with large-sized BC pyrolyzed at 520 \({}^{\circ}\)C and applied at a rate of 20 Mg ha\({}^{-1}\) was not significant) compared to sandy soil. It means that the hypothesis that biochar addition will increase the AWC was fulfilled. Our findings are consistent with those of [PERSON] et al. [41], who registered an increase in the AWC for all the biochar size fractions (<50, 50-100, 100-250, <2000 \(\upmu\)m) after the addition of 0.95% of biochar to sandy soil. Also, the study by [PERSON] et al. [42] observed the highest increase of AWC after the addition of BC with a particle size of 0.5-1 mm in comparison to sandy soil without BC. Changes in the AWC after BC application in our study depended on all three examined factors (pyrolysis temperature,particle size, and application rate), similar to the study by [PERSON] et al. [36], who found that the AWC was affected by BC rates, particle size, and BC feedstock type. [PERSON] et al. [43], [PERSON] et al. [44], and [PERSON] et al. [45] found that higher pyrolysis temperatures and BC rates resulted in a greater increase in AWC after BC addition, which is consistent with our findings. Our study confirmed the effect of the interaction between pyrolysis temperature and particle size on AWC. The particle size of the BC can significantly affect AWC, influencing surface area and exposure of internal pore space [46].