Biochar as a growing media component for containerized production of Douglas-fir

In the inland northwestern United States (US), Douglas-fir artificial regeneration commonly includes growing seedlings in media containing sphagnum peat. Concerns over the sustainability of peat and rising plant production costs are initiating investigation of growing media alternatives. Biochar is a potential media amendment that has positive physical and chemical properties for seedling production, including high water and nutrient retention due to large surface area, which may reduce leaching losses and improve fertilizer use efficiency. We used different amounts of biochar to amend peat-based growing media to determine if seedling growth response to various fertilizer rates differed with biochar amendments. Every 13 weeks for 39 weeks, replicate seedlings were measured for photosynthetic activity, destructively harvested, and analyzed for leaf nitrogen concentration. Biochar did not reduce fertilizer rates required to grow equal-sized seedlings or improve seedling growth. When mixed with peat at rates of 25% or 50% by volume, biochar progressively reduced height and diameter growth rates, seedling biomass, and photosynthetic rate. Biochar increased growing media pH to levels incompatible with conifer seedling requirements and decreased media extractable P concentration, which may have caused decreased photosynthesis. Adjusting pH of the biochar used would be necessary to grow Douglas-fir seedlings for forest regeneration.