Enhancement of nitrogen use efficiency using Tithonia diversifolia green manure in lime-amended Ultisol: A ¹⁵N isotope tracing study for sustainable chili Production

This research investigated the potential of Tithonia diversifolia as a sustainable nitrogen source for chili (Capsicum annuum L.) production in lime -amended Ultisol using ¹⁵N isotope tracing technology. A factorial pot experiment was conducted with two factors: lime application (0 and 2 ton ha⁻¹) and nitrogen substitution levels (0%, 25%, 50%, 75%, and 100% replacement of synthetic N fertilizer with fresh Tithonia biomass). A pot experiment was conducted to evaluate the potential of Tithonia diversifolia green manure as a partial substitute for synthetic nitrogen fertili zer in chili ( Capsicum annuum L.) production on acidic Ultisol soil. The study employed a factorial design with varying N substitution levels (0%, 25%, 50%, 75%, and 100%) and lime treatments (with and without lime application). Tithonia biomass was incorporated into the soil and incu bated for 21 days before chili transplantation. Soil chemical properties, plant growth parameters, yield components, nutrient uptake, and ¹⁵N isotope analysis were measured throughout the 12- week growing period. The Ultisol soi l exhibited typical constraints with pH 5.27, high Al saturation (36.85%), and low fertility status. Results demonstrated that Tithonia application significantly improved soil chemical properties by reducing exchangeable Al from 2.00 to 0.53 cmol kg⁻¹ and Al saturation from 36.85% to 7.48% without lime treatment. Soil organic carbon increased linearly with Tithonia application, reaching 2.47% at the highest substitution level with lime. Lime application enhanced soil pH and further reduced Al toxicity, creating synergistic effects with Tithonia amendments. Optimal chili performance was achieved with 25 -30% N substitution, producing maximum straw dry mass (74.95 g pot⁻¹), fruit fresh weight (397.53 g pot⁻¹), and nutrient uptake efficiency. The ¹⁵N isotope ana lysis revealed that Tithonia contributed 1.78% to 28.40% of total plant nitrogen uptake, with higher substitution levels showing increased N contribution but reduced overall plant performance. Complete substitution (100%) significantly reduced plant growth and yield, indicating incomplete nutrient mineralization within the 21 - day incubation period. The study concludes that Tithonia diversifolia can effectively substitute up to 50% of synthetic nitrogen fertilizer in chili production systems while maintaining optimal growth and yield parameters. This integrated approach offers a sustainable solution for reducing synthetic fertilizer dependency in tropical agriculture, particularly in acidic soils where lime application enhances the effectiveness of organic amendments.