Retardation of phosphorus release from modified biochar through targeted Fe-N_x-P chemisorption bonding

Fertilizer runoff caused non-point agricultural phosphorus (P) pollution poses a severe threat to aquatic ecosystems. Biochar confers controlled-release properties, and its chemical modification further augments this capability. However, the relationship between modulated release kinetics and targeting chemical bond energies remains unclear. Here, a novel Fe-N_x-P bond-mediated co-pyrolyzed biochar (P-CB) was designed for balancing P release in soils. P release from P-CB reduced 36.3% during stage I (0-15 days), while increased 25.8% during stage II (15-36 days) compared to conventional P encapsulated fertilizer, which fits better with crop growth. Surface area, crystal structure, chemical bonds, and surface element characterization evidenced the release transitioned from physical desorption (stage I) to Fe-N_x-P chemical desorption (stage II), which was accompanied by a 12.7% increase in Fe-N_x. Density Functional Theory calculation revealed the Fe-N_x-P bond is 2.3-fold stronger than physical sorption, pointing to mesopore desorption identified by the Mantel test. The Boltzmann-distributed correlation between theoretical binding energies and experimentally derived partition coefficients k_d confirms that the strengthened N-P bond dictates the slow-release kinetics. Pot experiments exhibited a 63% increase in mung bean biomass of P-CB fertilizing compared to P encapsulated fertilizer (P-EF) with equivalent P. In addition, large-scale economics and environmental implication challenges were evaluated of P-CB in the article. These findings provide an innovative modulating strategy to control quantitative P release and reduce agricultural non-point pollution.