Water Repellency of Two Forest Soils after Biochar Addition

Abstract. Practical application of black carbon (biochar) to improve forest soil may be limited because biochar is hydrophobic. In a laboratory, we tested the water repellency of biochar application (mixed or surface applied) to two forest soils of varying texture (a granitic coarse-textured Inceptisol and an ash cap fine-textured Andisol) at four different application rates (0, 1, 5, and 10 Mg ha^-1) and five soil moisture contents (0%, 25%, 50%, 75%, and 100% of saturation). To address the impact of biochar on water infiltration into the soil, we measured soil water repellency using three methods (tension infiltrometer, water drop penetration, and molarity of ethanol). Generally, all three infiltration methods gave similar results. Compared to the unamended coarse-textured Inceptisol at 0% saturation (oven dry), biochar mixed into the soil at the rate of 5 Mg ha^-1 did not result in a significant change (p ≤ 0.05) in infiltration rate. The fine-textured Andisol soil at 0% saturation did not show a significant change in infiltration at the application rate of 1 Mg ha^-1 when biochar was mixed into the soil. Surface applications of biochar on both soil textures resulted in less water infiltration than the mixing treatments. Our results suggest that biochar decreases infiltration rates less on coarse-textured forest soils as compared to finer-textured soils, and 1 to 5 Mg ha^-1 will likely not detrimentally alter water infiltration rates.