Phosphorus Fraction Transformation and Soil Acidity Mitigation through Water Hyacinth Biochar Integration with Organic and Synthetic Phosphorus Sources in Acidic Soil

Mekuanint Lewoyehu^1*Yudai Kohira²Solomon Addisu¹Ashenafei Gezahegn³Tassapak Wutisirirattanachai²Desalew Fentie⁴Shinjiro Sato²

• ¹Bahir Dar University, Bahir Dar, Ethiopia
• ²Soka Daigaku, Hachioji, Japan
• ³Debark University, Debark, Ethiopia
• ⁴Injibara University, Injibara, Ethiopia

Phosphorus (P) deficiency is a major problem in acidic and tropical soils, including Ethiopia, because over 80% of synthetic fertilizer P becomes fixed and unavailable. Lime's short-lived effect and limited fertilizer addition due to rising costs cannot solve the problem, leading to significant declines in crop productivity. This study examined how integrating water hyacinth biochar with organic and synthetic P sources affects the transformation of soil P fractions and soil acidity amelioration in Ethiopian soil. We set up a 90-day incubation experiment using P fertilizer, poultry manure, and their pairwise combinations with 1% and 2% biochar and lime, along with a negative control. Soil samples were taken on days 0, 3, 7, 15, 30, 45, 60, and 90 for pH, exchangeable acidity, exchangeable Al3+, and available P analysis, while samples taken on days 3, 30, and 90 were used to examine transformations of P fractions. Results showed that biochar– fertilizer and biochar–poultry manure integrations outperformed fertilizer and poultry manure alone, and their combinations with lime, significantly increasing soil pH by 0.99– 1.79 units, available P by 611–839%, labile P fractions by 1180–1795%, and calcium-associated P by 372–611%. In contrast, they decreased exchangeable acidity by 80.0– 95.7%, exchangeable Al3+ by 92.5–100%, Al/Fe-bound P fractions by 35–60%, and recalcitrant P by 18–20% compared with the control after incubation. More importantly, co-applying 2% biochar with poultry and fertilizer eliminated exchangeable Al3+. Overall, biochar–poultry manure combinations topped biochar–fertilizer counterparts. Therefore, it can be concluded that valorizing invasive water hyacinth into biochar and co-applying it with organic and synthetic P fertilizers can be a cohesive strategy for repurposing waste to address soil acidity and P deficiency, enhance soil fertility, and reduce fertilizer reliance, thereby supporting sustainable agriculture and effective waste management.