Effects of Pine Nut Shell Biochar Activation State, Particle Size, and Dose on Peat Substrate Properties and Arugula Microgreen Performance

Background : Renewable peat‐reducing components are needed for horticultural substrates; however, the combined effects of biochar activation state, particle size, and dose on substrate function remain unresolved. Aim : This study evaluated the effects of pine nut shell biochar on peat substrate properties and arugula microgreen performance and identified screening‐level criteria for peat‐reduced substrate design. Methods : Peat was partially replaced by nonactivated or steam‐activated biochar at 10% or 20% (v/v) using fine (<30 µm) or large (2–10 mm) fractions. After 28 days, the substrate hydrophysical and chemical properties, extractable nutrients, mobile trace elements, germination, morphometric traits, and biomass were determined. Results : Steam activation significantly increased the biochar pH; electrical conductivity; surface area; and extractable K, Mg, Ca, and P contents. In the substrate, fine fractions improved the water‐holding capacity and total porosity, whereas activated large fractions reduced these properties. Nonactivated biochar lowered the substrate electrical conductivity relative to the peat control, whereas activated biochar markedly increased it. Potassium increased in all amended substrates, but phosphorus usually declined, except for a slight increase in the 10% fine activated treatment. The best plant response was obtained with 10% fine biochar. Fresh biomass was approximately 2.0–2.8‐fold higher than that in the peat control, whereas the 20% rate provided no additional benefit, and some activated‐biochar treatments reduced germination and growth. Mobile heavy metal concentrations remained low across all treatments. Conclusions : Under the tested conditions, 10% fine nonactivated pine nut shell biochar provided the best overall compromise between peat replacement, substrate performance, and manageable electrical conductivity.