Impact of Biochar on Soil Water Retention Characteristics Is Limited: An Application of Neutron Imaging Technology.

The ability of soil to retain water under drought and other extreme hydrological events is critical to the survival and viability of both crops and microbes in soil. We investigated the impact of biochar on water retention properties of two typical California agricultural soils using pressure plate apparatus and also a neutron imaging method to study spatial and temporal distribution of biochar amended sample. The impact of biochar type (a walnut shell biochar and a softwood biochar), application rate (0, 0.5, 1%, wt.), and particle diameter (0-0.25, 0.25-0.5, 0.5-1, 1-2 mm) were also investigated. Our results indicate that high surface area biochar (walnut shell biochar) can increase field capacity of sandy soil, while low surface area biochar (wood biochar) had no significant impact. Neither biochar had an impact on the field capacity of the high clay content soil. The data, for the sandy soil, also revealed a positive correlation with increasing biochar particle diameter and field capacity. No impact of biochar on soil wilting point was observed across treatments. Thus, biochar’s potential to increase plant available water in soil is limited to increasing field capacity in a coarse textured soil. Our results also indicate that biochar can hold more water than the surrounding material due to its pore structure and can release water to the surrounding material during drying. Moreover, the biochar moisture content remained higher than the surrounding soil. Generally, the moisture distribution in the biochar amended sample was greatest for biochar interiors, followed by biochar exterior surfaces, and finally the surrounding sand. The impact of biochar’s influence on surrounding soil moisture content was dependent on its own moisture content and reduced with time. This data demonstrates that biochar has limited potential for use to increase water holding capacity of soils under conditions of drought, with modest benefits only observed in a course textured soil. It is important to note that this current study does not consider biochar impacts on soil aggregation and organic matter storage, which are also important for soil-water relations.