Optimizing Nitrogen Management to Enhance Growth and Minimize Pollution Risk in <i>Pennisetum hydridum</i> Cultivation

Nitrogen fertilization plays a crucial role in optimizing plant growth, but excessive application can lead to nutrient leaching, environmental pollution, and soil degradation. This study investigates the impact of nitrogen application rates (0–400 kg·ha⁻¹) on the growth, biomass allocation, and carbon sequestration capacity of <i>Pennisetum hydridum</i> (Imperial Bamboo, <i>PHY</i>), a fast-growing tropical grass increasingly used for forage and bioenergy production in subtropical regions. Despite its agronomic potential, nutrient management strategies for <i>P. hydridum</i> remain poorly understood. We hypothesized that moderate nitrogen application (100–200 kg·ha⁻¹) would enhance growth and nutrient use efficiency, while maintaining environmental sustainability. Results show that moderate nitrogen levels (100–200 kg·ha⁻¹) significantly enhanced biomass production, with the highest aboveground biomass observed at 180 days under T2 (100 kg·ha⁻¹) and T3 (200 kg·ha⁻¹), reaching 166.5 g/plant and 140.6 g/plant, respectively. In contrast, excessive nitrogen application (400 kg·ha⁻¹) led to a decline in biomass (T4, 76.8 g/plant) and impaired carbon sequestration efficiency. In addition, it was found that nitrogen uptake increased with moderate fertilization, with T2 and T3 showing optimal nitrogen use efficiency. Soil analysis revealed that soil organic matter and total nitrogen content were positively correlated with root biomass, with significant linear relationships between soil nitrogen, carbon/nitrogen ratios, and <i>PHY</i> biomass. Specifically, the total nitrogen content in rhizomes and fibrous roots showed coefficients of determination (R²) of 0.65 and 0.67, indicating a strong correlation with soil nitrogen levels. Furthermore, nitrogen application increased soil nitrate (NO₃⁻-N) and ammonium (NH₄⁺-N) concentrations, with T4 showing the highest levels at 90 days (41.35 mg/kg for NO₃⁻-N and 15.6 mg/kg for NH₄⁺-N), signaling potential nutrient loss to the environment. These findings underscore the importance of sustainable nitrogen management for maximizing the growth potential of <i>P. hydridum</i>, while minimizing environmental risks in subtropical agricultural systems.