Can biochar conserve water in Oregon agricultural soils?

Abstract

Few studies have evaluated the ability of biochar to improve soil water content under field conditions. Our objective was to assess the potential drought benefits of biochar across a range of soil textures by measuring indicators of hydraulic capacity and water-retention curves for field-amended soils. Two biochars, conifer wood and wheat straw, were incorporated by rotary tillage at amendment rates of 9−36 Mg ha−1 in four locations. All four soils showed a significant increase (p≤0.04) in saturated water content of 0.05–1.7 % volume per Mg ha−1 biochar added, but no increase in water content at field capacity. There were significant (p<0.01) decreases in volumetric water content at wilting point by 0.09 to 0.8 % per Mg ha−1 of biochar added in all soil textures. This translated to small, significant increases in plant-available water capacity of the same magnitude. Although the conifer wood biochar was less hydrophobic and had 14 % more internal pore space than the wheat straw biochar, both biochars had similar impacts on volumetric soil water content (θ). Simulations of soil drying and recharge using moisture retention curves and the Hydrus-1D model indicated that biochar amendment would generally decrease θ in the loam soil, and would have minimal impact on θ (± 1 %) in the more coarse textured soils. Monitoring of in situ θ in two of these soils showed similar results. Biochar sped up soil drying in the loam following irrigation, but had no impact on θ in the loamy sand. These results suggest limited utility for biochar as a drought adaptation tool in the first year following incorporation into these medium- to coarse-textured soils.