Effect of biochar addition on hydraulic functions of two textural soils

Land degradation reduces soil productivity and exacerbates the problem of food security. Thus, there is an increasing need to look for sustainable farming practices aimed to water and soil conservation. In this regards, conditioning the soil with biochar has been identified as a possible means on improving water conservation and soil biochemical, physical, i.e. mechanical and hydraulic characteristics. Many studies demonstrate these effects of biochar. However, the extended number of variables influencing its impact require further measurements. The objective of this study is to determine the effect of wood-based biochar on two different textured soils and to evaluate their influence on some hydrological properties: water retention, shrinkage behavior and the effect of wetting and drying periods on the hydraulic conductivity. Test samples were prepared by adding 2.5 and 5% (by dry mass) of pyrolyzed mango-wood biochar to a sand and a sandy loam. Soil water retention and soil shrinkage curves were measured. Additionally, extra core samples were exposed to four intense cycles of wetting and drying, by drying the samples at 30 °C for three consecutive days. Biochar amendments increased water retention in the coarse textured sand compared to the unamended soil. Pore size distribution was significantly altered in the sandy substrate, reducing the fraction of wide coarse pores and increasing meso porosity. Repeated wetting and drying cycles enhanced the structural stability of the pore system increasing the saturated hydraulic conductivity. Soil rigidity, especially in the sandy mixtures, was enhanced by the addition of biochar, forming an internal pore structure able to resist better hydraulic deformation due to drying. These results confirm the suitability of biochar to overcome extreme hydrological conditions.