Effects of biochar addition on evaporation in the five typical Loess Plateau soils

Soil evaporation is the main route of soil moisture loss and often exceeds precipitation in the arid and semi-arid regions of the Loess Plateau. This study was conducted to determine whether biochar addition could reduce soil evaporation in drylands. We measured the evaporative loss in five typical topsoils (0-20 cm) from the Loess Plateau, Shaanxi, China, that differed in texture (Eum-Orthic Anthrosol, Isohumisol, Loess, Sandy loess, and Aeolian sand) with five different biochar addition amounts (0, 10, 50, 100, and 150 g-biochar/kg soil) and three biochar particle sizes (2-1 mm, 1-0.25 mm, and < 0.25 mm). The results showed that biochar addition generally increased the soil average water content (by 35.6% in biochar treatments and 33.5% in control treatments) and effectively reduced soil cumulative evaporation (by 322.64 g in biochar treatments and 326.68 g in control treatments). In addition, the inhibition of evaporation was enhanced with increases in biochar particle size and addition amount. Biochar addition had contrasting effects in the two evaporation stages: Biochar decreased evaporation through capillary flow during the first stage of evaporation but increased evaporation during the second, diffusion-limited vapour transport stage, particularly in the Aeolian sandy soil. When expressed on a mass basis, the effect of biochar addition amount on the cumulative evaporation (CE) was dependent on biochar particle size. In the larger sized (2-1 mm and 1-0.25 mm) biochar treatments, the final CE decreased as the addition amount increased, but for < 0.25 mm particles, increasing the biochar addition amount increased the final CE due to the creation of micropores. However, biochar addition decreased the ratio of evaporative loss in all soils proportional to the biochar addition amount. Soil texture and biochar particle size were the main factors affecting soil evaporation. Biochar application has the potential to improve soil water availability in semi-arid lands, but the results will depend on the biochar particle size and addition amount.