The potential impact of biochar: Soil hydraulics and responses of maize under soil drying cycles

Biochar (BC) has been shown to positively impact soil hydraulic properties. However, its effect on water flow in unsaturated soil and root access to water have been poorly understood. This study was conducted to investigate the potential impact of BC application on soil water accessibility and plant responses under soil drying cycles. A grapevine wood-based BC was mixed with a sandy loam soil at the rates of 0, 20, and 40 g kg⁻¹ and preincubated in the PVC pots for two months. Then, maize plants were grown in preincubated pots and regularly irrigated during their vegetative phase (64 days) to maintain soil moisture at optimum condition (0.30 cm³ cm⁻³). Thereafter, plants were subjected to repeated soil drying (via plant transpiration) and wetting cycles (via irrigation from the top) with soil water content varying between 0.18 and 0.30 cm³ cm⁻³, respectively. Some selected plant growth parameters and plant physiological responses to soil drying cycles were determined. In parallel, soil water retention and hydraulic conductivity curves of different treatments were measured by a combination of the sandbox, pressure plate apparatus, and Hyprop. The results showed that plant dry shoot biomass was slightly increased by the BC application (9% on average). In contrast, the leaf area and root volume did not show any significant difference. As soil dried during a soil drying cycle, the transpiration rate decreased among all treatments. However, the drop in transpiration rate was smaller in the BC-amended soils than in the control. At soil water content of 018 cm³ cm⁻³, plants grown in BC-amended soils had on average 83% and 164% higher transpiration and photosynthesis rate than plants grown in control soil, respectively. These comparisons suggest that BC improved water accessibility to plants during soil drying, resulting in 76 and 58% increase in grain yield and grain water productivity. The application of BC had little effect on soil water retention in the wet range and had no impacts in the drier range, while the unsaturated hydraulic conductivity (K_h) and calculated matric flux potential (φ) were positively impacted in a wide range of soil water content. The findings of this study revealed that BC mixed with sandy loam soil could prevent a big drop in K_h as soil dries, thereby improving root access to water and transpiration rate under drought stress.