Sustainable Intensification of Napier Grass: Intercropping with Legumes and Reduced Nitrogen Fertilization for Yield Maintenance and Carbon Sequestration

Driven by global climate change and Taiwan’s Net-Zero initiatives, high-input livestock farming faces urgent pressures to transform. Pennisetum purpureum, a primary forage and bioenergy crop, traditionally relies on heavy chemical nitrogen inputs, thereby exacerbating its carbon footprint. This study evaluated a sustainable cultivation model integrating legume intercropping with reduced fertilization. A field trial in central Taiwan utilizing a Completely Randomized Design (CRD) compared four treatments: conventional full nitrogen (800 kg N/ha), sunn hemp (Crotalaria juncea) intercropping with half nitrogen, sunn hemp intercropping only, and organic compost. We monitored biomass, forage chemistry, and soil organic carbon (SOC) dynamics. The sunn hemp plus half-nitrogen treatment achieved dry matter yields (30–35 Mg/ha) and crude protein levels (7.1–7.4%) statistically equivalent to the full-nitrogen control, significantly surpassing other groups. While short-term SOC stocks (72–81 Mg C/ha) showed no significant differences, trends indicated carbon accumulation in deep soil (30–50 cm). Consequently, substituting 50% of chemical nitrogen through biological fixation proves a viable strategy to maintain yield while reducing inputs. This model sustains productivity and mitigates greenhouse gas emissions, offering an economically feasible, potential carbon-negative solution for tropical forage systems.