Impacts of Biochar Pyrolysis Temperature, Particle Size, and Application Rate on Water Retention of Loess in the Semiarid Region

To explore the effect of corn straw biochar on soil water retention, the characterization of corn straw biochar and its application in semiarid loess were investigated. For the corn straw biochars with different preparation conditions, significant differences were observed in elemental composition, specific surface area, pores distribution, surface functional groups, water absorption, and retention performance. The findings demonstrated that while the pyrolysis temperature (300 °C, 500 °C, and 700 °C) had no significant effect, the water absorption performance of biochar increased steadily as particle size increased (<0.25 mm, 0.25–1 mm, and >1 mm). Further, a greenhouse pot experiment with cucumber seedlings was performed using different proportions of biochar application (0.25%, 0.5%, 1%). Compared with no biochar application, the cucumber seedling fresh weight displayed significant improvement (8.89–95.56%), followed by capillary porosity (3.28–30.04%), total porosity (7.91–21.04%), and field water capacity (1.59–11.96%). Conversely, soil bulk density decreased by 3.50–14.69% after the treatments of biochar. Among all the prepared biochars, CSBC700 (particle size > 1 mm, 1% application rate) exhibited the maximum values in both field water capacity (38.78%) and saturated water content (42.25%). Based on the findings of the correlation analysis, the following characteristics may be used to rank the effect of corn straw biochar on soil water retention: application rate, O/C, pH, Ash%, C%, specific surface area, pore volume, and pore width. Biochar with larger particle sizes and abundant hydrophilic functional groups (hydroxyl and carboxyl groups) can greatly improve soil water retention performance. These results provide new insight and support for the utilization of straw and the improvement of soil water retention in semiarid regions.