Reversing the C Balance of Soil-Free Substrates By Substituting Biochar for Peat.

Soil-free substrates in the greenhouse and nursey industries typically include a significant peat component. However, the increasing expense of peat, its harvest’s impacts on wetland ecosystems, and perceptions of unsustainability have spurred investigation for alternatives. Biochar (BC) is a promising substitute for peat, yet the majority of studies examine additions of BC to peat-based substrates rather than replacing the peat component (substitution), or are limited to low substitution rates. Furthermore, the general alkalinity of BC may impact substrate pH and plant growth at high substitution rates. We evaluated BC substitution for peat and pH adjustment of resulting substrates on plant performance under standard greenhouse conditions (e.g., fertigation). A high pH (9.5) softwood BC was used to substitute for peat in a standard 70:30 (v/v) peat:perlite mixture at 10 %v increments. Substrate pH was either not adjusted or adjusted to pH 6.0 using a BC by-product, pyroligneous acid (PLA). After 9 weeks, BC substitution for peat did not negatively impact marigold biomass or flowering. At low substitution rates (10-30% total substrate volume), marigold biomass and leaf SPAD values were greater than the control peat-perlite mixture (0% BC). Substrate pH management had a significant effect on marigold germination and total biomass. Germination was inhibited in pH-adjusted substrates with high BC substitution (50-70% total substrate volume) likely due to higher dosages of PLA to neutralize high pH. Over 9 weeks, the initial pH gradient (5.6-10.4) that mirrored the BC substitution gradient decreased to 5.6-7.5. This study indicates the strong feasibility of BC as a replacement for peat in soil-free substrates. Crop- and BC-specific considerations and economic potential should be investigated for wider application. Switching from peat to BC offers environmental benefits by maintaining peat ecosystems and their stored C, reducing transportation C emissions, and employing a highly recalcitrant form of C. We estimate a potential reversal of C balance by substituting BC for peat in soil-free mixtures.