Optimised co-composting of chicken manure with biochar and <i>Tithonia diversifolia</i> for enhanced nutrient recovery and reduced phytotoxicity

The escalating expansion of poultry farming presents significant challenges in chicken manure management. Its direct application to agricultural soils often results in nitrogen volatilisation, nutrient leaching, and phytotoxicity. While traditional composting methods enhance organic matter breakdown, they often fall short in adequate nutrient stabilisation, especially when <i>Tithonia diversifolia</i> is used as an amendment. This study investigates a co-composting approach that integrates biochar and <i>Tithonia diversifolia</i> to improve nutrient retention and facilitate the <i>in-situ</i> formation of struvite, a natural mechanism for stabilising both nitrogen and phosphorus. A 60-day pilot-scale experiment was conducted in polyethylene-lined reactors, with feedstock ratios optimised using Response Surface Methodology to model macronutrient dynamics (N, P, and K). The models showed strong predictive accuracy (R² = 0.971 for N, 0.983 for P, and 0.974 for K; p < 0.01). Co-amended treatments sustained longer thermophilic phases and accelerated organic matter degradation. The optimal formulation (60% <i>Tithonia diversifolia</i>, 5.7% biochar, and 34.3% chicken manure) produced a mature compost with a C/N ratio of 9.8 and a <i>Raphanus sativus</i> germination index of 89.2%. Substantial increases in nutrient concentrations were observed (N: 2.648%, P: 2.306%, K: 2.793%). FTIR spectra confirmed struvite crystallisation as the primary nutrient stabilisation mechanism. The findings demonstrate a viable and environmentally sustainable strategy for producing a nutrient-rich, safe organic fertiliser, particularly within smallholder agricultural systems.