Low-Temperature Biochar Replacing Peat Enables Halving Nutrient Solution in Dwarf Tomato Soilless Culture

Horticultural reliance on non-renewable peat faces critical sustainability challenges. Low-temperature biochar (LTB) presents a promising alternative, offering higher biochar yields and lower energy inputs compared to conventional high-temperature biochar. However, LTB’s distinct physicochemical properties necessitate empirical validation of its efficacy as a peat substitute. This study investigated rice straw-derived LTB (pyrolyzed at 350 °C for 10 or 30 min) as a peat substitute at different ratios (10%, 20%, 40%), combined with three Hoagland nutrient solution concentrations (25%, 50%, 100%), on the growth, substrate properties, and fruit quality of dwarf tomato. The results show that a 10–20% LTB substitution improved substrate physical properties (reduced bulk density, increased porosity) and promoted plant growth (biomass, height). Conversely, a 40% LTB substitution inhibited growth, primarily attributed to osmotic stress caused by excessively high substrate electrical conductivity (EC). At the optimal 10–20% rates, tomato yield and fruit quality (soluble solids, lycopene, vitamin C) were significantly enhanced. Most importantly, a comprehensive evaluation revealed that 10–20% LTB substitution allowed for a 50% reduction in nutrient solution concentration while achieving a comprehensive performance comparable to the full-strength nutrient control. This study indicates that LTB could effectively replace a portion of peat, potentially enhancing dwarf tomato yield and quality while reducing chemical fertilizer dependency by up to 50%. These findings point toward a possible pathway for more resource-efficient horticultural practices.