A potential Mg-enriched biochar fertilizer: Excellent slow-release performance and release mechanism of nutrients

Highlights: • MBF was fabricated via pyrolysis of corn straw and reclaim of N and P from biogas effluent. • MBF exhibited excellent slow-release ability of both N and P nutrients. • MBF was a potential slow-release fertilizer to promote the plant growth. • The P slow-release behavior of MBF was enhanced by the 'P-trap' effect of MgO. • The N-release behavior of MBF was controlled by multi-effects of biochar carrier. A potential Mg-enriched biochar fertilizer (MBF) was successfully synthesized via pyrolysis of MgCl₂-enriched corn straw and high-efficiency reclaiming of N- and P-containing nutrients from biogas effluent. Mathematical modeling and column leaching method demonstrated that the MBF exhibited excellent slow-release performances of total P and N with sustainable release rates. Leaching experiment indicated that the final accumulative release ratios of N and P from MBF were 7 times and 6 times lower than those of chemical fertilizer (CF), respectively. The mechanism study reveals that the P-release performance of MBF was not only controlled by the low solubility of MgP precipitates formed on the biochar surface, but also enhanced by the 'P-trap' effect of MgO through re-precipitation process of PO₄³⁻. Meanwhile, the N-release behavior of MBF was dominated by the multi-effects of biochar carrier, including the confinement effect and electrostatic attraction for NH₄⁺, as well as the hydrogen bonds and pore-filling effect for N-containing organic matter. In addition, MBF significantly promoted the corn growth and enhanced the nutrient uptake efficiency of corn. These results suggested that MBF may therefore have promising potential in sustainable agriculture application with multiple environmental benefits.