Feedstock-dependent nutrient retention and release mechanisms of manure-derived biochars: Implications for mitigating...

Nutrient loss from livestock manure-derived fertilizers poses a persistent risk to soil and water quality, undermining the environmental sustainability of intensive agriculture. Transforming manure into biochar offers a promising approach to mitigate nutrient loss while enabling nutrient recycling; however, nutrient retention and release behavior strongly depend on feedstock characteristics and pyrolysis conditions. In this study, biochars derived from swine, cattle, and poultry manure were produced at 350 °C and 450 °C and systematically evaluated for their adsorption-desorption behavior toward NH₄⁺, PO₄^3-, and K⁺. Results demonstrated that feedstock mineralogy, rather than pyrolysis temperature alone, governed nutrient-specific retention mechanisms, with distinct behaviors observed for NH₄⁺, PO₄^3-, and K⁺. Cattle manure biochar exhibited enhanced NH₄⁺ retention due to abundant Si-O functional groups, while moderate-temperature cattle manure biochar showed strong K⁺ retention associated with high cation exchange capacity. In contrast, poultry manure biochar displayed superior PO₄^3- immobilization driven by carbonate-rich phases promoting precipitation. Importantly, desorption experiments revealed that high adsorption capacity did not necessarily correspond to strong nutrient retention, highlighting the need to assess nutrient release behavior when evaluating environmental performance. Among the tested materials, cattle manure biochar produced at 450 °C achieved a favorable balance between nutrient retention and release reversibility. These findings demonstrate that manure-derived biochars can substantially alter nutrient retention pathways under controlled laboratory conditions, providing mechanistic insights and practical implications for mitigating agricultural nutrient loss and improving environmental nutrient management.