Can Substituting Biochar for Peat Maintain Southern Pine Seedling Quality and Reduce Nursery Carbon Emissions?

Abstract Peat extraction for nursery substrates contributes to carbon emissions and peatland destruction, highlighting the need for sustainable alternatives such as biochar. In this study, we evaluated how biochar substitution of peat (0–100% by volume) combined with four fertilizer rates (0, 0.5 × , 1 × (standard), 2 ×) affected seed germination, seedling survival, total seedling biomass, root-to-shoot biomass ratio (RSR), and CO₂ flux rates during containerized seedling production of three primary southern pine species ( Pinus taeda (loblolly pine), Pinus palustris (longleaf pine), and Pinus elliottii (slash pine)). Seed germination differed among species, with slash pine showing the highest rate (85.6% germination), followed by loblolly (82.1%) and longleaf pine (66.1%), while biochar had no detectable effect ( p = 0.0618). Seedling survival followed a similar pattern, with longleaf pine exhibiting the lowest survival (36–44% survival) and loblolly the highest (68–92%). Partial biochar substitution (25–50% biochar levels) supported seedling survival comparable to peat alone, whereas 100% biochar level significantly reduced survival ( p < 0.05). Total seedling biomass responded strongly to fertilizer and high biochar levels. Fertilizer increased biomass in loblolly and slash pine, peaking under the standard rate, whereas high biochar levels (100%) reduced growth, particularly in these species. Longleaf pine seedling biomass increased modestly under the 0.5 × and standard fertilization rates and 25% biochar level. RSR responded to fertilizer, while biochar had no effect. CO₂ flux rate declined only at 100% biochar level and increased with fertilizer ( p < 0.001), remaining lower only under no fertilizer. These findings indicate that partial peat substitution with 25–50% biochar may sustain seedling quality, whereas complete substitution impairs growth. Responses were species-specific, underscoring the need for tailored substrate and nutrient management for sustainable nursery production.