Introduction:

Water scarcity, declining soil organic matter, and deterioration of soil structure are important constraints to agricultural production in dry and semi-arid regions. Improving soil water retention and physical quality is therefore essential for increasing water-use efficiency and sustaining crop production. Biochar, a carbon-rich and porous material produced through pyrolysis of biomass under limited oxygen, has been proposed as a soil amendment capable of modifying soil structure, pore characteristics, and water retention. However, its effects depend on feedstock type, pyrolysis temperature, application rate, soil texture, and duration of application. Moreover, limited information is available on the effects of biochar produced from locally available poplar pruning residues and wheat straw on the simultaneous physical and hydraulic properties of soils in northwestern Iran. Therefore, this study evaluated the effects of different application rates of a 1:1 mixture of poplar wood and wheat straw biochars on selected physical and hydraulic properties of a loamy sand soil under greenhouse conditions. The study particularly focused on changes in bulk density, porosity, water retention, total available water, electrical conductivity, and saturated hydraulic conductivity.



Materials and Methods:

The experiment was conducted in the greenhouse and soil laboratory of the Faculty of Agriculture, University of Tabriz, using a completely randomized design with three replications. Surface soil was collected from the 0–30 cm layer of an agricultural field at the university research station. The soil had a loamy sand texture, containing 62% sand, 24% silt, and 14% clay, with a pH of 7.65 and organic carbon content of 0.48%. Biochars were produced separately from poplar pruning wood and wheat straw under oxygen-limited conditions. Wheat straw and poplar wood were pyrolyzed at 350 and 400 °C, respectively, using a heating rate of 15 °C min−1 and a residence time of 120 min at the final temperature. The resulting biochars were mixed at a 1:1 mass ratio. Four application rates were evaluated: 0% (control), 1%, 3%, and 5% biochar by soil mass. After complete mixing, treated soils were transferred to plastic pots and maintained under greenhouse conditions for four months. Soil moisture was regularly adjusted to approximately field capacity using distilled water to maintain comparable moisture conditions among treatments. After incubation, electrical conductivity (EC), bulk density, total porosity, saturated water content, field capacity (FC), permanent wilting point (PWP), total available water (TAW), and saturated hydraulic conductivity (Ks) were measured using standard laboratory methods. FC and PWP were determined at matric potentials of 33 and 1500 kPa, respectively, using pressure plates. Bulk density was measured using sampling cylinders, total porosity using a gas pycnometer, and Ks using the constant-head method. Data normality and homogeneity of variances were assessed using the Shapiro–Wilk and Levene tests, respectively. Following confirmation of the assumptions of parametric analysis, one-way analysis of variance was performed using SPSS version 26. When treatment effects were significant, means were separated using Duncan’s multiple range test at the 5% probability level.



Results and Discussions

The statistical analyses showed that biochar application significantly affected physical and hydraulic properties of the soil. The treatment effect was highly significant for bulk density, field capacity, and total available water (P<0.01), while electrical conductivity, saturated water content, and total porosity were affected significantly at P<0.05. In contrast, the effects on permanent wilting point and saturated hydraulic conductivity were not statistically significant. Bulk density decreased progressively with increasing biochar rate, from 1.53 g cm−3 in the control to 1.44, 1.33, and 1.18 g cm−3 at 1%, 3%, and 5% biochar, respectively. Thus, the 5% treatment reduced bulk density by 22.80% relative to the control. Total porosity increased from 44.00% in the control to 44.61%, 48.68%, and 55.90%, corresponding to increases of 1.38%, 10.64%, and 27.04%, respectively. Saturated water content also increased markedly, from 26.43% in the control to 29.96%, 34.69%, and 41.02% at the three biochar levels. The increase at 5% reached 55.20% relative to the control. Field capacity increased significantly from 14.72% in the control to 16.95%, 18.68%, and 21.22%, representing increases of 15.15%, 33.69%, and 44.16%, respectively. Total available water increased from 12.31 mm to 15.13, 16.83, and 18.70 mm, with the 5% treatment showing a 51.90% increase compared with the control. Permanent wilting point increased numerically from 6.51% to 6.87%, 7.46%, and 8.75% as biochar rate increased, but these differences were not significant. Saturated hydraulic conductivity similarly increased from 11.45 cm h−1 in the control to 12.28, 13.34, and 15.87 cm h−1, respectively, but the variation was not statistically significant. Electrical conductivity increased significantly from 1.11 dS m−1 in the control to 1.36, 1.42, and 1.40 dS m−1 under 1%, 3%, and 5% biochar, respectively; however, the three biochar treatments were statistically similar. The increases in porosity, saturated water content, and field capacity can be attributed to the low density and porous structure of biochar, together with changes in soil pore volume and arrangement. The stronger response of field capacity than permanent wilting point suggests that biochar preferentially increased water retention within pores contributing to plant-available water. Consequently, the increase in total available water resulted mainly from the substantial improvement in field capacity rather than from changes in the lower water-retention limit. Although saturated hydraulic conductivity showed an increasing trend, the lack of statistical significance indicates that greater total porosity did not necessarily produce proportionally greater water flow. This may reflect the importance of pore size distribution, continuity, and connectivity in controlling saturated hydraulic conductivity. The increase in electrical conductivity was likely related to the release of soluble salts and basic cations from biochar, although the measured values remained within a non-saline range. Overall, the 5% treatment produced the greatest improvement in most measured soil properties, particularly bulk density, water retention, and total available water.



Conclusion:

Application of the mixed poplar pruning and wheat straw biochar improved the physical condition and water-retention capacity of the loamy sand soil, with the 5% rate producing the greatest overall improvement. This treatment reduced bulk density and increased porosity, saturated water content, field capacity, and total available water. The results indicate that biochar can enhance soil water storage and potentially support more efficient irrigation management in dry and semi-arid environments. However, because the experiment was conducted under greenhouse conditions for only four months, the results should not be interpreted as establishing a universal optimum application rate. Long-term field studies are required to evaluate persistence, crop response, and economic feasibility.