Is biochar applied as surface mulch beneficial for grassland restoration?

Abstract

Biochar produced from low-value juniper was evaluated for its utility in improving restoration of native bunchgrasses. We hypothesized that biochar could expand the window of favorable conditions for seedling establishment by slowing soil drying. In a greenhouse study, applications of biochar on the soil surface were compared to incorporation of biochar, and improvements in water content, germination, and seedling survival were evaluated. Underlying hydraulics were investigated with measurements of soil and biochar water retention and Hydrus-1D simulations. The greenhouse study showed that applying any depth of biochar on top of soil was not effective at improving soil water content, and furthermore inhibited germination of bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] Á. Löve). In contrast, mixing biochar into soil significantly slowed initial soil drying and increased seedling biomass by 70% compared to unamended soil. However, moisture benefits of biochar incorporation were temporary and, as the soil dried, the treatment with incorporated biochar lost more water than soil that was unamended, because biochar increased unsaturated hydraulic conductivity. Model simulations of the experimental conditions similarly showed that incorporating biochar improved early soil water content, but also suggested that this result was specific to the drainage conditions of pots and may not translate to a deeper soil. Model simulations also supported the idea that biochar applied as a mulch could have an increasing benefit over time by reducing evaporation. This study suggested that both incorporation and mulch application of biochar may benefit seedling establishment, but further investigations in a naturally-draining soil are needed to validate these findings.